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Unmanned Spaceflight.com _ Uranus and Neptune _ Neptune Orbiter

Posted by: Rob Pinnegar Nov 10 2005, 03:51 PM

This seems like a good place to start off the Uranus and Neptune forum: with the next ice-giants mission.

I will admit to not knowing a whole lot about the Neptune Orbiter With Probes (NOWP), other than the fact that it's in the planning stages, and a few other details I've gathered from Wikipedia and various other Internet sources. Anyone care to get this one going with a bit more information?

Posted by: elakdawalla Nov 10 2005, 04:43 PM

I don't know much myself about what's possible either, but I know a good place to start would be to look up Thomas Spilker, who has done a lot of thinking about future Uranus and Neptune missions (he's also Cassini Deputy Project Scientist Linda Spilker's husband). He can talk your ear off very passionately about creative and mind-bending ideas for ways to tour giant planet systems, including one way to have an orbit that perpetually bounces on one side of a giant planet ring plane. I didn't understand the details -- I'd love it if someone could look into his publications and abstracts and figure out how this would work.

--Emily

Posted by: tedstryk Nov 10 2005, 05:26 PM

I would really like to see a Neptune orbiter with a small Triton lander. I have also wondered about flyby missions, to at least check up on changes. Maybe this is a crazy idea, but I always thought that a Neptune flyby craft could be carried like a probe on a Jupiter or Saturn mission, and separate before JOI or SOI, and instead use its engines in conjunction with the gravity assist to accelerate on to Neptune.

Posted by: Rob Pinnegar Nov 10 2005, 05:59 PM

QUOTE (tedstryk @ Nov 10 2005, 11:26 AM)
Maybe this is a crazy idea, but I always thought that a Neptune flyby craft could be carried like a probe on a Jupiter or Saturn mission, and separate before JOI or SOI, and instead use its engines in conjunction with the gravity assist to accelerate on to Neptune.

That doesn't sound crazy at all -- it seems like a perfectly sensible idea. So I guess there are two things to consider: (1) would a combined mission be cheaper and more fuel efficient that two separate launches, and (2) is it worth the risk of losing both if something goes wrong?

Posted by: elakdawalla Nov 10 2005, 06:20 PM

QUOTE (tedstryk @ Nov 10 2005, 10:26 AM)
I would really like to see a Neptune orbiter with a small Triton lander.  I have also wondered about flyby missions, to at least check up on changes.  Maybe this is a crazy idea, but I always thought that a Neptune flyby craft could be carried like a probe on a Jupiter or Saturn mission, and separate before JOI or SOI, and instead use its engines in conjunction with the gravity assist to accelerate on to Neptune.
*

Well, it may be crazy, but it's not out of the mainstream crazy smile.gif. In fact Sushil Atreya and Toby Owen are pushing for a mission concept called "Multiple Probes to Multiple Planets," which consists of a flyby spacecraft ("delivery truck") dropping two deep atmospheric probes at each giant planet. See
http://www.lpi.usra.edu/opag/feb_05_meeting/presentations/deep_atmospheres.pdf
However I'm not sure when there would be a launch opportunity that would permit such a repeat of Voyager 2's flyby "grand tour" -- that would be my first question.
--Emily

Posted by: JRehling Nov 10 2005, 07:09 PM

QUOTE (elakdawalla @ Nov 10 2005, 11:20 AM)
However I'm not sure when there would be a launch opportunity that would permit such a repeat of Voyager 2's flyby "grand tour" -- that would be my first question.
--Emily
*


Jupiter "laps" the outer planets every thirteen or so years, so opportunities will always keep coming around for J->U or J->N gravity assists.

I think a strategic plan is needed. Given the apparent rejection of NH2 as a Uranus flyby, it's a blank slate.

One nice bit of synergy is that Saturn, Uranus, and Neptune all have similar atmospheric profiles, so one probe design might accomodate all three. (Saturn's higher escape velocity may, however, mean that the Saturn probe would unavoidably arrive at higher velocity.) It would be nice to us Jupiter gravity assists to fling the two outer ones on their way, perhaps as add-ons, and get the synergy of unified manufacture and parallel investigations at three planets.

Proper flyby craft for remote sensing of the satellite systems (of Uranus and Neptune) are other options, but redundant if orbiters are planned, which for Neptune, at least, it should be.

Finally, there are an ever-increasing number of opportunities for KBO exploration and mini-Grand Tours. (Add in Sedna, which isn't a KBO!) I hope a broad view is taken in planning opportunities, because it could be a colossal waste to identify priorities, and pluck missions off of the top of the list, missing out on two-in-one possibilities.

Posted by: tedstryk Nov 10 2005, 07:13 PM

If an orbiter is a long way off, I think a Triton flyby would be very useful, as it would allow for change detection since Voyager and, of course, when the orbiter finally gets there, its data can also be compared. Unlike the other moons of these two planets Triton is a dynamic world. Of course, the other issue is that the coverage of the Uranian moon's souther hemispheres is going to get poorer and poorer the longer we wait.

Posted by: RNeuhaus Nov 10 2005, 07:38 PM

The trip to planets beyond than Saturn, I think the project would be most benefical to launch a big rocket along with three or four orbiters in which they are going to be dropped on each planet (Uranus, Triton, Neptune, or others) on its fast way toward a KBO... it might need a rocket which is capable to send around 10 TM to the space.

The trip to these planet is of very long time so it is very desirable that a rocket would be capable to send multiples probes, orbiters or landers in one shot.

Rodolfo

Posted by: ljk4-1 Nov 10 2005, 07:53 PM

QUOTE (tedstryk @ Nov 10 2005, 02:13 PM)
If an orbiter is a long way off, I think a Triton flyby would be very useful, as it would allow for change detection since Voyager and, of course, when the orbiter finally gets there, its data can also be compared.  Unlike the other moons of these two planets Triton is a dynamic world.  Of course, the other issue is that the coverage of the Uranian moon's souther hemispheres is going to get poorer and poorer the longer we wait.
*


We should drop a lander right into one of Triton's geysers. Talk about a relatively easy entry into the moon's subsurface.

Posted by: tasp Nov 10 2005, 09:49 PM

QUOTE (RNeuhaus @ Nov 10 2005, 01:38 PM)
The trip to planets beyond than Saturn, I think the project would be most benefical to launch a big rocket along with three or four orbiters in which they are going to be dropped on each planet (Uranus, Triton, Neptune, or others) on its fast way toward a KBO... it might need a rocket which is capable to send around 10 TM to the space.

The trip to these planet is of very long time so it is very desirable that a rocket would be capable to send multiples probes, orbiters or landers in one shot.

Rodolfo
*



Prometheus would make a great 'carrier' type vehicle. It could send a heavy orbiter, probe, lander, and retro stage on it's way to Neptune, then return to earth for a refuel, and another payload to 'fling to the nether reaches'.

Posted by: RNeuhaus Nov 10 2005, 10:00 PM

QUOTE (tasp @ Nov 10 2005, 04:49 PM)
Prometheus would make a great 'carrier' type vehicle.  It could send a heavy orbiter, probe, lander, and retro stage on it's way to Neptune, then return to earth for a refuel, and another payload to 'fling to the nether reaches'.
*

It seems that the reality would be become beyond the year 2030-2040...after the Earth's world economic becomes stronger and also the science, engineering and technology becomes highly feasible and capable to develop greater projects likes ones of Spaceship of Space Odyssey 2001 which is highly capable to ram among planets of our solar system.

Rodolfo

Posted by: JRehling Nov 10 2005, 10:06 PM

QUOTE (tasp @ Nov 10 2005, 02:49 PM)
Prometheus would make a great 'carrier' type vehicle.  It could send a heavy orbiter, probe, lander, and retro stage on it's way to Neptune, then return to earth for a refuel, and another payload to 'fling to the nether reaches'.
*


Prometheus is canceled, and the reasons for that ought to speak to the idea of putting multiple orbiters on a single launch -- extraordinarily unrealistic.

Prometheus offered lots of electricity, but remember, that's not unlimited thrust. You still need to have some sort of fuel to push off against, and when that mass becomes enormous, so does the requirement of how much chemical thrust is needed to put the thing into space (from Earth's surface) in the first place.

Prometheus was already wildly unrealistic.

I think there's plenty of useful reality-based discussion we can have before we plan the 24th century's missions.

Posted by: Decepticon Nov 11 2005, 12:42 AM

There where some ideas for Neptune orbiter in Astronomy magazine (Forgot When)

Posted by: tasp Nov 11 2005, 12:59 AM

To revise and extend my remarks,

Would a reusable 'upper stage' (like a Prometheus, or at least an uprated ion drive) be considered a little more palatable, perhaps to the funding committees in congress? If an orbiter, lander, and atmospheric probe were too much mass, in view of the reusablity, the mssion could be flown on multiple flights.

Combining a reusable upper stage 'tug' with the VEEGA type flybys would give us even more payload, and would have the advantage of an easier return to earth of the reusable stage. Add in aerobraking at earth (yoiks! the fur will fly in the media, aerobraking a nuclear stage in earth's atmosphere!!!!) and the payload this system could send to Jupiter and beyond, repeatedly, would keep JPL hopping for decades.

I guess I'm coming around to seeing the objections to this.

Sigh.

Posted by: tasp Nov 11 2005, 01:09 AM

Meanwhile, back at Neptune,

Has anyone considered the advantages and disadvantages of either a prograde or retrograde orbit for a Neptune orbiter?

I assume Triton will be employed similarly to Titan for orbit shaping, does it matter which way Triton goes 'round Neptune for this?

High flyby speeds for the prograde option at Triton can cause dificulty in photography, but then you reduce that problem at every other target. I'm not sure if ring plane crossings are more dangerous either way, hit something at either speed regime and the craft is toast anyhow.

Probe deploy and relay tasks seem easier if the orbiter is in a retrograde path. Perhaps Triton probe release could occcur far enough out, that orbiter and Triton (assuming we send a Huygens or better follow on) probe could each take the optimum path.

Posted by: Rob Pinnegar Nov 11 2005, 01:36 AM

QUOTE (tasp @ Nov 10 2005, 07:09 PM)
Has anyone considered the advantages and disadvantages of either a prograde or retrograde orbit for a Neptune orbiter?

I assume Triton will be employed similarly to Titan for orbit shaping, does it matter which way Triton goes 'round Neptune for this?

Hmm. That's an interesting point.

From what I remember, Triton is (probably just barely) massive enough to get an orbiter around Neptune via gravitational capture. (Titania and Oberon aren't big enough to do this at Uranus which is why aerobraking would be required for a Uranus orbiter.) Probably the probe _would_ have to enter a retrograde orbit around Neptune, unless aerobraking were used.

This isn't necessarily a bad thing, though. The situation at Neptune is different from Cassini's at Saturn. At Saturn, Titan is the main attraction, but there are several other bodies in the system (Enceladus, Iapetus, Hyperion etc.) that are also very interesting and worthy of plenty of study in their own right.

The Neptune orbiter's "Titan" is obviously Triton, but, at Neptune, there is no Iapetus, no Enceladus, and no Hyperion. Apart from some inner and outer gravel there is only Proteus which, apart from having a funny shape, seems more like a Mimas-in-waiting than a Miranda (though we could always be surprised). I wonder if there will be any chance of arranging a Nereid encounter or two? Might as well try if we're going all the way out there.

Triton isn't just the main event -- it's pretty much the only event. There is no "second stage" (a la Ozzfest) at Neptune. So if a retrograde orbit is preferable for studies of Triton, then we might as well make it a retrograde orbit -- even if aerobraking is used instead of capture.

Posted by: tedstryk Nov 11 2005, 02:28 AM

QUOTE (Rob Pinnegar @ Nov 11 2005, 01:36 AM)
Triton isn't just the main event -- it's pretty much the only event. There is no "second stage" (a la Ozzfest) at Neptune. So if a retrograde orbit is preferable for studies of Triton, then we might as well make it a retrograde orbit -- even if aerobraking is used instead of capture.
*


I don't know if I would go that far. For one thing, a flyby during approach of Neried (a la Cassini at Phoebe) would be nice. And Proteus and the others may prove to be fragments from the former Neptunian system before Triton's capture, which would be quite interesting.

Posted by: BruceMoomaw Nov 11 2005, 04:01 AM

There has actually been quite a lot of work done by a JPL group led by Tom Spilker on the design for a Neptune Orbiter that doesn't require nuclear-electric propulsion -- the latest mission design can be found at http://www.lpi.usra.edu/opag/jun_05_meeting/presentations/OPAG_Neptune_API1.pdf . (And, yes, it would use a retrograde orbit.) Aerocapture is an absolute necessity for this mission if you want a combination of acceptably short trip time and acceptably low mass.

Posted by: tasp Nov 11 2005, 04:11 AM

I'll digress to Uranus orbiting briefly.

Assuming Uranian orbit is achieved via aerobraking or whatever means, would orbit shaping form Oberon (for purposes of discussion) be possible?

I'm thinking an elliptical orbit around Uranus that grazes (safely) the rings at perigee, has an apogee way out past Oberon. And if inclined to equatorial plane, not inclined too much.

Would a series of orbital encounters with Oberon give us an interesting mission by modifying our orbit sufficiently?

Idea:

Always have the modification have the same effect on the craft orbit. I'm thinking every Oberon encounter could be oriented to raise the perigee of our orbit slightly. More by 'dumb luck' than design, as the perigee slowly (over many orbits) rises through the Uranian system, you will inevitably get a reasonably close flyby of everything interior to Oberon.

The big trick; every Oberon encounter needs to put the craft in an orbit that eventually encounters Oberon again. If you hit a period for the craft that doesn't divide into Oberon's period very well, you next flyby might not happen for a while.

Once you get your perigee above Titania, you can start using the Oberon encounters to change the plane of the orbit. Having the period at 2X or 3X Oberon's means every encounter can nudge your orbit inclination a tad more.

Then you can study the magnetic field and checkout the higher latitudes of Uranus.


Granted, Oberon isn't that massive, but its mass ratio to Uranus isn't terribly different from that of one of the Galileans to Jupiter, with out doing the math, (a trait of mine I am not likely to change) it seems that this might be feasible.

A possible advantage (snicker) of this idea is I think it would take a very long time to do the orbit shaping with Oberon, and therefore, you get to watch the Uranian system for a nice long arc around the sun, and maybe we eventually get to see some things that perhaps we might have thought would take a second probe.

Posted by: BruceMoomaw Nov 11 2005, 04:53 AM

I may have something to say on THAT subject in my "Astronomy" article.

Posted by: Rob Pinnegar Nov 11 2005, 06:22 AM

QUOTE (tedstryk @ Nov 10 2005, 08:28 PM)
I don't know if I would go that far.  For one thing, a flyby during approach of Neried (a la Cassini at Phoebe) would be nice.  And Proteus and the others may prove to be fragments from the former Neptunian system before Triton's capture, which would be quite interesting.

That's a good point about Proteus and the small inner moons -- I guess that their semi-major axes (with corrections for long term tidal effects since Triton's capture) should give a good idea of the lower limit for Triton's periastron right after its capture.

And, yeah, it's a good bet that Nereid will throw us some curveballs -- there will probably be _something_ there that we don't expect.

Triton'll still put 'em all to shame, though. tongue.gif

Posted by: JRehling Nov 11 2005, 02:23 PM

QUOTE (tasp @ Nov 10 2005, 09:11 PM)
I'll digress to Uranus orbiting briefly.

Assuming Uranian orbit is achieved via aerobraking or whatever means, would orbit shaping form Oberon (for purposes of discussion) be possible?

Idea:

Always have the modification have the same effect on the craft orbit.  I'm thinking every Oberon encounter could be oriented to raise the perigee of our orbit slightly.  More by 'dumb luck' than design, as the perigee slowly (over many orbits) rises through the Uranian system, you will inevitably get a reasonably close flyby of everything interior to Oberon.

*


If the craft arrives near solstice, it will be irrelevant: each world will be half in decades-long dark, and every flyby will illuminate the same half. An orbit that is modestly inclined WRT the moons' could eventually give a good look at each moon, leaving us equally (and eternally?) ignorant of their other hemisphere. And we would also miss out dramatic high-phase illumination of most of the hemisphere we see -- note that all of Cassini's long-range view of the icy satellites of Saturn with the terminator in different places is eventually going to help us build DEMs of their topography.

Arriving near equinox will allow the kind of illumination change that would be highly desirable. Unfortunately, equinox is coming up soon, and we're obviously not going to make it in time. So this plan won't possibly come off before 2050. At which time, I'll be on soft foods.

Posted by: tedstryk Nov 11 2005, 03:01 PM

QUOTE (JRehling @ Nov 11 2005, 02:23 PM)
Unfortunately, equinox is coming up soon, and we're obviously not going to make it in time. So this plan won't possibly come off before 2050. At which time, I'll be on soft foods.
*


That was my point. Get a flyby craft there while Uranus is near equinox. I doubt it is even possible to get an orbiter there that quickly and break it into orbit (not to mention design it).

Posted by: tasp Nov 11 2005, 03:12 PM

QUOTE (JRehling @ Nov 11 2005, 08:23 AM)
If the craft arrives near solstice, it will be irrelevant: each world will be half in decades-long dark, and every flyby will illuminate the same half. An orbit that is modestly inclined WRT the moons' could eventually give a good look at each moon, leaving us equally (and eternally?) ignorant of their other hemisphere. And we would also miss out dramatic high-phase illumination of most of the hemisphere we see -- note that all of Cassini's long-range view of the icy satellites of Saturn with the terminator in different places is eventually going to help us build DEMs of their topography.

Arriving near equinox will allow the kind of illumination change that would be highly desirable. Unfortunately, equinox is coming up soon, and we're obviously not going to make it in time. So this plan won't possibly come off before 2050. At which time, I'll be on soft foods.
*



Can 'Uranus shine' help out illumination of moons? Cassini took some pix of Iapetus that way, might help when equatorial plane of Uranus is perpendicular to orbital path around sun. 20 year mission life at the target gets you 90 degrees around the sun, that would help global coverage of satellites, too.

Voyagers are coming up on 30 years longevity, so craft lifetime of 20 years at Uranus might be feasible. Way cheaper than two missions.

Posted by: JRehling Nov 11 2005, 04:45 PM

QUOTE (tasp @ Nov 11 2005, 08:12 AM)
Can 'Uranus shine' help out illumination of moons?  Cassini took some pix of Iapetus that way, might help when equatorial plane of Uranus is perpendicular to orbital path around sun.  20 year mission life at the target gets you 90 degrees around the sun, that  would help global coverage of satellites, too.

Voyagers are coming up on 30 years longevity, so craft lifetime of 20 years at Uranus might be feasible.  Way cheaper than two missions.
*


Uranus shine might help, although from the dark pole of a moon in the worst case scenario, a half-Uranus would be on the horizon. You'd get some good illumination of the areas near the terminator, gradually fading to black at the dark pole.

Uranus gets <25% of Saturn's illumination, it has about 25% the area, and a half-Uranus (vs a full one) would provide somewhere between 25% and 50% the illumination -- multiplying those out gives us a Uranus which is about 1/50 the brightness of a full Saturn. Also, Iapetus is in an inclined orbit, and so gets some ringshine that the uranian satellites won't.

However, Iapetus is also MUCH farther from Saturn than the uranians are from it. Thus, to make up that factor of 50, a uranian only need be 7 times closer to Uranus than Iapetus is to Saturn, and this is true of the inner four of Uranus's Big Five. Oberon is just a bit farther.

Counter: Iapetus has the highest contrast of any body in the solar system! But we should be able to see some topography, especially for the moons closer in. And cameras for a Uranus mission would be made more sensitive than ones for Saturn. So, yes, Virginia, there is a (useful) uranusshine!

90 degrees does help, though not as much if it's on either side of a solstice, in which case, you see the same extra real estate later as you did sooner, so a lifetime of 10 years would be almost as good.

Posted by: Rob Pinnegar Nov 11 2005, 05:11 PM

QUOTE (tedstryk @ Nov 11 2005, 09:01 AM)
That was my point.  Get a flyby craft there while Uranus is near equinox.  I doubt it is even possible to get an orbiter there that quickly and break it into orbit (not to mention design it).
*

Yeah (sob) Uranus reaches equinox in about two years' time. If we start planning an orbiter right this minute, it will probably get to Uranus just in time for solstice.

It's becoming fairly evident that we need a Uranus thread here to complement the Neptune one.

Posted by: Jeff7 Nov 11 2005, 05:20 PM

QUOTE (tedstryk @ Nov 10 2005, 02:13 PM)
If an orbiter is a long way off, I think a Triton flyby would be very useful, as it would allow for change detection since Voyager and, of course, when the orbiter finally gets there, its data can also be compared.  Unlike the other moons of these two planets Triton is a dynamic world.  Of course, the other issue is that the coverage of the Uranian moon's souther hemispheres is going to get poorer and poorer the longer we wait.
*


Well who knows. Saturn's system has shown interesting things on "inactive" moons - Enceladus' fairly young tiger stripes for instance. There always seems to be more to be seen.

Posted by: BruceMoomaw Nov 11 2005, 10:18 PM

That's another reason why, given the likely delay for the Neptune Orbiter, it might indeed be wise to previously fly a simpler ice-giant flyby mission with entry probes -- but to aim that mission at Uranus instead of Neptune. (Especially given the recent idea, which is very rapidly gaining momentum, that the first entry-probe mission to any of the three outer giant planets will be very much worthwhile scientifically even if it only penetrates to about the 20-40 bar level instead of 100 or more bars -- which would make it a much easier mission to fly. 100-bar Uranus and Neptune probes -- unlike such for Jupiter and Saturn -- don't have to contend with high temperatures at that level and so can be vented rather than armored in design, but they still have major communications problems compared with higher-altitude probes.)

Posted by: tasp Nov 12 2005, 02:26 PM

Alex Blackwell has a post in the Uranus Orbiter thread that refers to a paper outlining the feasability of a Galileo style tour of the Uranian system.

Far easier to do than I realized, which is a good thing!

Posted by: tedstryk Nov 15 2005, 11:59 PM

Here is an approach sequence of Proteus. All color is based on the view in the lower left (the "bad tooth" picture). The view on the left is the best - 1.3 km/pixel, but is so underexposed that it is hard to interperet.



It is interesting to see how irregular Proteus is, despite the fact that it is a bit larger than relatively-round Mimas. Perhaps it is because it is the re-assembled lumps of old Neptunian moons. Perhaps some other reason. Either way, I find it most interesting.


Posted by: Rob Pinnegar Nov 16 2005, 06:25 AM

QUOTE (tedstryk @ Nov 15 2005, 05:59 PM)
It is interesting to see how irregular Proteus is, despite the fact that it is a bit larger than relatively-round Mimas.  Perhaps it is because it is the re-assembled lumps of old Neptunian moons.  Perhaps some other reason.  Either way, I find it most interesting.

Could be that it's just really beat up from impacts -- which makes one wonder about whether Triton had anything to do with that. I don't have much trouble imagining that satellite-satellite collisions would have been epidemic in the Neptune system right after Triton came barrelling onto the scene.

If Neptune's early system was similar to Uranus' (with Proteus as the "Miranda" perhaps) the chaos would have been pretty incredible there for a while.

Posted by: hendric Nov 16 2005, 08:22 AM

QUOTE (elakdawalla @ Nov 10 2005, 12:20 PM)
However I'm not sure when there would be a launch opportunity that would permit such a repeat of Voyager 2's flyby "grand tour" -- that would be my first question.
--Emily
*


According to Wiki, about every 176 years

http://en.wikipedia.org/wiki/Planetary_Grand_Tour

But...We've learned alot about gravitational assists since Voyager, and I wouldn't be surprised if it is now possible to do a repeat tour or tours.

Posted by: Rob Pinnegar Nov 16 2005, 02:02 PM

QUOTE (hendric @ Nov 16 2005, 02:22 AM)
According to Wiki, about every 176 years

Yup. Uranus has now overtaken Neptune in its orbit, which means that we can't have a Grand Tour now. We have to wait for Uranus to catch up to Neptune again.

Since Uranus and Neptune are almost in a 2:1 orbital resonance, 176 years sounds about right. (That assumes Jupiter and Saturn will be well placed, of course.)

Posted by: tedstryk Nov 16 2005, 02:28 PM

It is no doubt battered, but so is Mimas. A possibility is that it is positioned to receive very little tidal energy, so it never even partially melted down. If it does turn out to be a collection of fragments from an ancient Neptunian system, this could be good, as it would give us much more insight into what it once was than, say, Miranda.

Posted by: Rob Pinnegar Nov 16 2005, 05:09 PM

QUOTE (tedstryk @ Nov 16 2005, 08:28 AM)
A possibility is that it is positioned to receive very little tidal energy, so it never even partially melted down.

In a way, that's a shame -- if Proteus had shown any evidence of long-term (but long-lost) tidal heating, this might have given some evidence that other large moons used to be nearby.

Of course, had such activity been found, Triton could also have been the culprit. I guess there's not much point musing about the might-have-beens.

Posted by: tasp Nov 16 2005, 06:31 PM

And even Miranda is somewhat oblongish.

Would I be presumptuous to request a mosaic of Proteus, Hyperion, Miranda, etc. (all the satellites around the transition zone from irregular to sphereical) ?

The moons to scale mosaics are fascinating, if my computer skills were better, I might try to put one together, but I defer to those who are skillful in such things.

Thanx for the consideration.

Posted by: David Nov 17 2005, 03:11 AM

QUOTE (tasp @ Nov 16 2005, 06:31 PM)
And even Miranda is somewhat oblongish.

Would I be presumptuous to request a mosaic of Proteus, Hyperion, Miranda, etc.  (all the satellites around the transition zone from irregular to spherical) ?
*


By way of reminder, those moons are Enceladus, Miranda, Proteus, Mimas, Hyperion, and Nereid; then there's a large gap in size before we get down to Amalthea and Phoebe, which I would say are no longer "transitional" -- that is, they might be vaguely roundish (as Phoebe is) but there's no particular reason for them to be so, other than chance. I would go so far as to say that Hyperion and Nereid are probably outside the "transition zone".

The Main Belt asteroids which are also in that "transition zone" are Pallas, Vesta, Hygiea (and - smaller than Hyperion - Interamnia and Davida); unfortunately none of them other than Vesta and Davida have images that show their shape.

The "transition zone" appears to be the 400km-600km diameter range. Below Mimas everything is pretty irregular. I don't know where the cutoff would be above 600km, since Vesta is itself rather irregular, and there's a big gap between it and Ceres.

If you count Iapetus as irregular, then there's a much larger transition zone, including Charon, Umbriel, Ariel, Dione, and Tethys, but Iapetus is really the odd one out there.

Posted by: tasp Nov 17 2005, 04:46 AM

I suspect the 'lumpiness' of Iapetus is a result of a 'leisurely' formation period, time to radiate impact and radio-nuclide heat, too distant to have been warmed significantly from Saturnian formation heat, and also a very slow tidal braking effect from the the distant Saturn.

All these effects would allow for maximum thickness and strength of its crustal materials via cryogenic rigidity.

Posted by: Bob Shaw Nov 17 2005, 08:01 PM

QUOTE (tasp @ Nov 17 2005, 05:46 AM)
I suspect the 'lumpiness' of Iapetus is a result of a 'leisurely' formation period, time to radiate impact and radio-nuclide heat,  too distant to have been warmed significantly from Saturnian formation heat, and  also a very slow tidal braking effect from the the distant Saturn.

All these effects would allow for maximum thickness and strength of its crustal materials via cryogenic rigidity.
*



I agree - it's not just a simple x-axis/y-axis graph, but a series of interconnecting variables, but with similar processes leading to similar outcomes...

Bob Shaw

Posted by: tasp Nov 26 2005, 02:55 PM

With a greater understanding or how orbital tours work (thanx to the Heaton and Longuski paper on a possible Uranian mission) it seems a very interesting orbital mission at Neptune is possible.

The favorable mass ratio of Neptune/Triton would allow for considerable flexiblity for an orbital tour.

Which ever is easier at Neptune orbit insertion, a prograde or retrograde path, doesn't seem so critical now, as it seems repeated flybys of Triton could be used to change either orbit into the other. A detailed examination of Neptunes magnetic field at virtually all orientations would be possible. Neptune might be the key to understanding the diverse magnetic fields of all gas giant planets.

While Neptune seems 'short changed' in the satellite department, the satellites that are present were revealed by Voyager 2 to be interesting objects nevertheless.

Posted by: Decepticon Nov 26 2005, 03:41 PM

A simple Google search reveled this!?

What Probe concept was this?

http://www.astro.univie.ac.at/~wuchterl/Kuffner/im_brennp/archiv2004/h_neptune_orbiter_probe_02.jpg

Posted by: BruceMoomaw Nov 27 2005, 02:43 AM

That's the nuclear-electric version of Neptune Orbiter, back when Sean O'Keefe's nuclear behemoth was still among NASA's official plans. It will, I think, be a long time before we see it -- not just because of the huge cost and environmental problems, but also because it turns out to significantly PROLONG the travel times of many spacecraft into the outer System, although it does allow a huge amount of cruising around from one moon to the next after you get there.

But the concepts for the Neptune entry probes and the possible Triton lander do still look just the way they're shown in that picture. As for the new "aerocaptured" Neptune Orbiter concept -- the one which is now overwhelmingly most likely to fly first -- you can see it still folded up behind its aerocapture heat shield on page 9 of http://www.lpi.usra.edu/opag/jun_05_meeting/presentations/OPAG_Neptune_API1.pdf . I can't find a picture of the Orbiter after it's been released from that shield and unfolded in Neptune orbit, but clearly it will look radically different from Galileo and Cassini -- and clearly it requires an unfolding high-gain dish. (Note how much it initially looks like the classic SF magazine-cover spaceship!)

Posted by: Decepticon Nov 27 2005, 12:31 PM

I hate anything that Unfurls. mad.gif

When Galileo's Antenna failed to open I was Beyond upset.

I even read a article in Astronomy magazine where the person being interviewed (Forgot who) One of his fears was Galileo's antenna failing to open.

Posted by: BruceMoomaw Nov 27 2005, 01:32 PM

Just keep in mind that Galileo's antenna was exactly the same design used on the TDRS satellites, each of which carried four of them. Out of (I believe) a total of 28 on them, not one has ever shown any trouble unfolding -- which is why the Galileo failure caught virtually everyone by shock. No one had ever anticipated truck vibrations as a cause. Now they do. And so, while I distrust moving parts in space as much as anyone, I see no reason to flee screaming from the idea of an unfolding antenna. It only requires making sure that the deployment springs have enough of a margin this time.

Posted by: vjkane2000 Nov 28 2005, 03:38 AM

QUOTE (BruceMoomaw @ Nov 27 2005, 06:32 AM)
Just keep in mind that Galileo's antenna was exactly the same design used on the TDRS satellites, each of which carried four of them.  Out of (I believe) a total of 28 on them, not one has ever shown any trouble unfolding -- which is why the Galileo failure caught virtually everyone by shock.  No one had ever anticipated truck vibrations as a cause.  Now they do.  And so, while I distrust moving parts in space as much as anyone, I see no reason to flee screaming from the idea of an unfolding antenna.  It only requires making sure that the deployment springs have enough of a margin this time.
*


Or make sure that reverse on the deployment motor is enabled. The cut the wires for reverse on Galileo before launch to make sure that the motor would never refold the antenna. From what I understand, the problem would have been simple to solve if they could have simply removed the tension from the pins.

Posted by: edstrick Nov 28 2005, 06:06 AM

In a conversation some 10? years ago, maybe at the MagicCon World SF convention in Orlando, I chatted with Gentry Lee and asked him something about the reverse motors on the Galileo antenna, having heard some references to that. *AS I RECALL*, he indicated they had needed that command capability for something else, maybe sun-shade deployment for revised mission that took Galileo inward to Venus where it wasn't designed to survive. He also stated that simple reversing the motor direction would not have backed the pins out of the position in which they'd jammed and would have in fact made the problem worse. I don't understand the design well enough to see how that would happen, but I can imagine design conditions where it would. Everything I'm told is that the failure was more complicated and less intuitively preventible (or fixable -- "if only") than everybody wants to imagine.

The Galileo antenna deploy failure was simply one of those "damn things" that should never force you to not use a proven design or concept because of one terrible example.

Posted by: mchan Nov 29 2005, 07:51 AM

QUOTE (BruceMoomaw @ Nov 27 2005, 05:32 AM)
Just keep in mind that Galileo's antenna was exactly the same design used on the TDRS satellites, each of which carried four of them.  Out of (I believe) a total of 28 on them, not one has ever shown any trouble unfolding -- which is why the Galileo failure caught virtually everyone by shock.  No one had ever anticipated truck vibrations as a cause.  Now they do.  And so, while I distrust moving parts in space as much as anyone, I see no reason to flee screaming from the idea of an unfolding antenna.  It only requires making sure that the deployment springs have enough of a margin this time.
*


Galileo's folding HGA design was used on the first series of TDRS satellites, each of which carried two antennas. There were 7 satellites built and launched, but one was destroyed with Challenger. The last 3 satellites launched after the Galileo deployment (or non-deployment as it was). An antenna of very similar design was also used on the US Navy FleetSatCom satellites, of which 6 were successfully launched and deployed before Galileo. A total of 12 unfurlings, all successful, occurred prior to Galileo.

Furled dish antennas of newer designs continue to be used in commercial comsats. NASA certainly has not backed away from furled structures. One need not look further than the James Webb Space Telescope.

Posted by: edstrick Nov 29 2005, 08:44 AM

Deployments have ALWAYS been one of the big mission-killers. Without the ability to actually test fly a spacecraft in orbit before boosting it to it's destination, there's no way to do a really adequate pre-flight test of in-space deployment. They've gotten DAMN good at faking things in 1 G and 1 Atmosphere, but the ghods of space still have nasty tricks up their sleeves.

Posted by: tasp Dec 5 2005, 05:23 AM

Just in case NASA/JPL finds themselves looking for a 'flagship' type mission idea (I hold no illusions regarding the probability of that), how about this:

Send a probe to Neptune that can aerobrake in Neptunes' atmosphere to acheive Neptune orbit. During the decel phase, have some instruments do some direct sampling and analysis of the Neptune atmosphere.

After achieving Neptune orbit, use Triton's gravity to modify the probe orbit to allow close examination of everything in the Neptune system deemed interesting.

Towards the end of the mission, use aerobraking in Triton's atmosphere to decel into Triton orbit (probably can't use the same heat shield for both Neptune and Triton, but some effort would be expended to make sure of that, handy if you can). Examine Triton from orbit, use more aerobraking to circularize the orbit (or, if possible, use a steerable ballute for orbital plane changes) to examine interesting landing locals on Triton.

Then, again using the ballute or aerocapture shell for re-entry, land on the best spot found on Triton. Hopefully with almost dry fuel tanks at touch down.



I'm thinking the advantages of this will be a great savings in fuel needed for a very demanding mission, utilization of various instruments in all phases of the mission, difficulty in approving 3 missions to Neptune can be avoided by having 1 mission do all 3, probe will potentially have a long life on the surface of Triton from the nuclear batteries needed for the probe.

Difficulties would be great complexity of the probe, cost and technical risk.

Posted by: dvandorn Dec 5 2005, 07:35 AM

Does Triton *really* have enough of an atmosphere to allow for efficient aerobraking? At least, without a gazillion passes before you're significantly slowed down?

-the other Doug

Posted by: Toma B Dec 5 2005, 08:34 AM

QUOTE (dvandorn @ Dec 5 2005, 10:35 AM)
Does Triton *really* have enough of an atmosphere to allow for efficient aerobraking?  At least, without a gazillion passes before you're significantly slowed down?

-the other Doug
*


NO IT DOES NOT!
The atmospheric pressure at Triton's surface is about 15 microbars , 0.000015 times the sea-level surface pressure on Earth...that's not enough for any kind of aerobraking...
Mars has average presure of 7 milibars, that is why MRO will have to spend many months aerobraking and don't forget that it uses only upper parts of atmosphere where presure is much less than that...
The average pressure on the Earth surface (sea level) is 1000 millibars....
In order to use that little atmosphere on Triton for aerobraking our unlucky spaceprobe would have to fly verry,verry,verry dangerously close to surface...
Edit:
Some approximate calculatins based on these facts:
Mars atmosphere is 143 times less efficient in aerobraking then atmosphere of Earth is...
Triton atmosphere is 467 times worst then Mars... sad.gif
So it would take gazillon passes through it to slow down significantly... sad.gif
Solution : USE ROCKET ENGINES!!! smile.gif

Posted by: chris Dec 5 2005, 10:49 AM

QUOTE (Toma B @ Dec 5 2005, 08:34 AM)
Solution : USE ROCKET ENGINES!!! smile.gif
*


Which could get exciting if you landed on a patch of methane or nitrogen ice.

Posted by: edstrick Dec 5 2005, 11:07 AM

Note that the surface of Triton may be very rough on spacecraft scales, in some places.

When a moon's been cratered "till the rubble bounces", the surface's largely pulverized and while large scale topography is rough, churned regolity or ice-rubble is locally smooth. A moon like Enceladus or Europa, with active geology, can be very very rough, with sharp local topography and lots of broken ice.

Add glacial activity possible in nitrogen and/or methane ices, and sublimation eroding processes, you may get a complicated and very rough surface indeed.

Posted by: Toma B Dec 5 2005, 11:42 AM

QUOTE (chris @ Dec 5 2005, 01:49 PM)
Which could get exciting if you landed on a patch of methane or nitrogen ice.
*


What did you meen by "exiting" ?
What's exiting about landing on a patch of nitrogen or methane ice?

Edit:
Also what does it have to do with rocket engines?

Posted by: helvick Dec 5 2005, 12:02 PM

QUOTE (Toma B @ Dec 5 2005, 09:34 AM)
Mars atmosphere is 143 times less efficient in aerobraking then atmosphere of Earth is...
*


Not really. Despite having only a few millibars surface pressure Mars' atmosphere is denser at high altitudes than the earth's and extends further so it is not any worse than the earth's for orbital aerobraking. The cross over point is at around an altitude of around 70km if I recall correctly and orbiter aerobraking takes place much further out than that. The problems arise for landing craft as the lower density near surface atmosphere leads to much higher terminal velocities. Again I'm just going by recollection here but I think martian terminal velocities are about 5-10x Earth ones so from a lander aerobraking perspective I reckon it would be more accurate to say that Mars was about 10 times less efficient.

Triton's atmosphere does seem to be just too thin to be of much use but it would be worth running some calculations on it to be certain. More on this later when I can find my drag calculations spreadsheet.

Posted by: chris Dec 5 2005, 12:52 PM

QUOTE (Toma B @ Dec 5 2005, 11:42 AM)
What did you meen by "exiting" ?
What's exiting about landing on a patch of nitrogen or methane ice?

Edit:
Also what does it have to do with rocket engines?
*


If you land on a patch of such ice, and the rocket exhaust is hot enough to vaporise the ice, then you might get an explosive release of gas, which would be dangerous to the lander. The surface of Triton, at -235 Centrigrade, is only 20 degress lower than the freezing point for nitrogen, so it wouldn't be that hard.

Chris

Posted by: Toma B Dec 5 2005, 01:27 PM

QUOTE (chris @ Dec 5 2005, 03:52 PM)
If you land on a patch of such ice, and the rocket exhaust is hot enough to vaporise the ice, then you might get an explosive release of gas, which would be dangerous to the lander. The surface of Triton, at -235 Centrigrade, is only 20 degress lower than the freezing point for nitrogen, so it wouldn't be that hard.
*

Question:
Will methane explode without oxygen?
NO IT WILL NOT !!!
...there's not much oxygen on Triton isn't it?
About Nitrogen...
How long would rocket engine work when verry close to surface...2-3 seconds perhaps?
Without any atmospheric presure there might be some sublimation but I'm sure nothing exiting....

Here's a deal:
Why don't we make a bet and we will see when NASA send that probe in the next 20 to 150 years... smile.gif

Posted by: paxdan Dec 5 2005, 01:47 PM

QUOTE (Toma B @ Dec 5 2005, 01:27 PM)
Question:
Will methane explode without oxygen?
NO IT WILL NOT !!!
*

Err yes it will, the mechanism is descibed in the post you quoted. Explosive does not just refer to combustion. Careful with the caplocks key and the exclamation marks when making such bold statements.

Posted by: ljk4-1 Dec 5 2005, 02:57 PM

Drop a probe right into one of the Triton geysers. That way we get a quick and relatively easy access to the moon's subsurface.

I presume we can make a probe tough enough to survive such a journey long enough to relay back useful data?

Posted by: tasp Dec 5 2005, 03:05 PM

Check out the last half of the Uranus orbiter thread. Bruce Moomaw has posted reference to a paper outlining use of ballutes in thin atmospheres.

Seems very exciting missions involving Trtion and Pluto are more feasible than almost anyone dared hope for. Decels of up to 40 to 50 gees are possible in the thin upper atmospheres of these objects for orbit mods and landing missions.

Stunning information, really. {love this site for that very reason}

cool.gif

Posted by: chris Dec 5 2005, 03:07 PM

QUOTE (Toma B @ Dec 5 2005, 01:27 PM)
Question:
Will methane explode without oxygen?
NO IT WILL NOT !!!
...there's not much oxygen on Triton isn't it?
About Nitrogen...
How long would rocket engine work when verry close to surface...2-3 seconds perhaps?
Without any atmospheric presure there might be some sublimation but I'm sure nothing exiting....

*


Toma,

I wasn't talking about combustion. The explosion I am postulating would be purely mechanical, driven by gas pressure. Imagine if the exhaust warmed some water ice with pockets of nitrogen gas in it.

Secondly, remember the geysers on triton - small in temperature lead to some pretty dramatic effects - from http://en.wikipedia.org/wiki/Geyser:

"It is thought that the surface of Triton probably consists of a semi-transparent layer of frozen nitrogen, which creates a kind of greenhouse effect, heating the frozen material beneath it until it breaks the surface in an eruption. A temperature increase of just 4 K above the ambient surface temperature of 38 K could drive eruptions to the heights observed."

(That height being 8Km!)

Chris

Edit: added Wikipedia info

Posted by: JRehling Dec 5 2005, 04:23 PM

QUOTE (Toma B @ Dec 5 2005, 12:34 AM)
Mars atmosphere is 143 times less efficient in aerobraking then atmosphere of Earth is...
Triton atmosphere is 467 times worst then Mars... sad.gif
So it would take gazillon passes through it to slow down significantly... sad.gif
*


The proposal is certainly complicated, but your calculations are not relevant. Aerobraking would not take place at the surface, but high up, and Triton would have a high scale height. Whereas aerobraking at Earth would involve a short skip through the upper atmosphere, aerobraking at Triton would involve about 1000 km through gas not that much less dense than at Triton's surface.

Of course, if you want a source of gas to really slow down through, Neptune is right next door. The problem is, if aerocapture is used on the way in, the resulting orbit would probably be one that would make it very difficult to later pass through Neptune's atmosphere again -- unless a low peri-neptune were maintained throughout the mission.

A more feasible combo mission might be an orbiter/Triton lander duo. One heatshield for braking in Neptune's atmosphere would be utilized. The craft would be stacked as follows:

Heatshield
Triton lander
Neptune orbiter

On arrival, the stack would aerocapture into a Neptune orbit with peri-neptune very near the cloudtops. The orbit would be highly eccentric, and intersect Triton's, allowing several flybys to perform initial reconnaisance and landing site selection. Then, on one apo-neptune, the orbiter's thruster would point the stack onto a path into Neptune's atmosphere. Then the orbiter would separate, leaving the heatshield on the lander. The lander would decelerate through Neptune's atmosphere, and emerge on an "orbit" that would just barely make it to Triton at apo-Neptune, at a low velocity relative to Triton. Then a ballute system might be able to bring the lander down. The orbiter would continue on indefinitely.

Seems inordinately difficult. Also, I don't see a Triton lander being part of the next mission to that part of the solar system. I'd bet on a Neptune entry probe first.

Posted by: Toma B Dec 6 2005, 08:48 AM

QUOTE (chris @ Dec 5 2005, 06:07 PM)
I wasn't talking about combustion. The explosion I am postulating would be purely mechanical, driven by gas pressure. Imagine if the exhaust warmed some water ice with pockets of nitrogen gas in it.
*


All right Chris...maybe you are right...I'm no expert in Triton's surface nor atmosphere...

But I do know this:
It's not like we are going to land 10 ton "lander-rover", using some surplus Saturn-5 rocket engines that burn ferociously for 10,15 seconds near surface so that they can melt some nitrogen ice andtherefore cause violent sublimation...
It's more like small lander 100-200 kg with small engines working at low power near the surface for 2-3 seconds...so if there would be some melting/sublimation it would not be verry hazardous to the unfortunate lander in question...

BTW it's not like they are really planing any Neptune/Triton mission so it's not so big deal... sad.gif

Posted by: chris Dec 6 2005, 10:41 AM

QUOTE (Toma B @ Dec 6 2005, 08:48 AM)
All right Chris...maybe you are right...I'm no expert in Triton's surface nor atmosphere...


Me neither - my expertise comes straight from Google smile.gif

Posted by: ermar Dec 10 2005, 02:23 AM

As a long-time lurker, sorry for spamming links, but I couldn't help but notice:

http://news.bbc.co.uk/2/hi/science/nature/4515752.stm

Of course, if such a mission were ever to fly, they'd likely ditch the probes/landers... (not that I am advocating that, but how many times have mission add-ons been sacrificed before the bottom line?)

Posted by: BruceMoomaw Dec 10 2005, 06:42 AM

They definitely won't fly the nuclear-propelled version. Not only would it be gigantically expensive -- Project Prometheus, O'Keefe's brain child (if that's the word for it), has now been cancelled -- but it would actually take considerably longer to reach Neptune than the tremendously cheaper alternative version of this mission, which will reach Neptune with a Jupiter gravity-assist and/or solar-powered ion engines, and then brake into orbit around Neptune with aerocapture. JPL has worked out the design for that one in great detail already, and I think it likely to fly some time before 2030 -- although it will still cost $2 billion or more. (Its main rival for an expensive Solar System launch in the 2020s seems to be a large Europa lander.) At a minimum, it would carry two Neptune entry probes (unless NASA has already taken them to Uranus or Neptune on a cheaper flyby mission) -- but a Triton lander is definitely an optional accessory for it, depending on funds.

Posted by: Toma B Dec 10 2005, 06:56 AM

QUOTE (ermar @ Dec 10 2005, 05:23 AM)
As a long-time lurker, sorry for spamming links, but I couldn't help but notice:
http://news.bbc.co.uk/2/hi/science/nature/4515752.stm
Of course, if such a mission were ever to fly, they'd likely ditch the probes/landers... (not that I am advocating that, but how many times have mission add-ons been sacrificed before the bottom line?)
*

Welcome ermar! Nice catch!
I particularly like this part:
QUOTE
"The probe would have a mass of about 500kg - 65% of that is a propulsion system to slow you down so you don't crash," he explained.
"There is a very thin atmosphere on Triton but there's not enough for parachutes to slow you down. You've got a lot of engineering overhead just to deliver the science package."

So if my calculation is right lander would have "dry" mass of 175kg - That's like Spirit or Opportunity...Maybe they should think about attaching some wheels on it... smile.gif
Seriously I don't believe that thing is ever going to fly...
BTW BruceMoomaw is there any way you could post link to that JPL Neptune project...?

Posted by: BruceMoomaw Dec 10 2005, 08:46 AM

Sure: http://www.lpi.usra.edu/opag/jun_05_meeting/presentations/OPAG_Neptune_API1.pdf

http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/19345/1/98-0675.pdf

Regarding the possible addition of a Triton lander:
http://www.aas.org/publications/baas/v37n3/dps2005/504.htm

There's also a description of a Triton lander design that could work for this mission in
http://solarsystem.nasa.gov/multimedia/downloads/Standard_RPS_Report_Final_011205.pdf (pg. 22-36).

Posted by: tedstryk Dec 10 2005, 01:29 PM

QUOTE (BruceMoomaw @ Dec 10 2005, 08:46 AM)
There's also a description of a Triton lander design that could work for this mission in
http://solarsystem.nasa.gov/multimedia/downloads/Standard_RPS_Report_Final_011205.pdf  (pg. 22-36).
*


I noticed they used my combined HST view of Neptune and Triton in that pdf.

Posted by: AlexBlackwell Jul 17 2006, 07:51 PM

I apologize for reviving a dead thread; however, Frank Morring Jr., reporting from the Farnborough 2006 Air Show, has an interesting article ("Improving Solar Cell Efficiency Enables NASA's Solar-Powered Jupiter Probe") in the July 17, 2006, issue of Aviation Week & Space Technology.

While the bulk of the article relates to the http://newfrontiers.nasa.gov/missions_juno.html, there is an interesting passage relating to a Neptune orbiter concept:

QUOTE
While Juno will be pushing the state of the art for deep-space use of solar power, scientists have analyzed the use of sunlight to power spacecraft at planets even more distant from the Sun. NASA has studied a Neptune orbiter that would use inflatable structures to deploy the ultra-large solar arrays that would be needed there and concentrators to focus the dim sunlight on the collectors (AW&ST Dec. 13, 2004, p. 56).

[Paul] Stella [principal engineer for space power systems at JPL] is skeptical. Even without considering the inflatable structures that would be needed to hold the solar concentrators and underlying cells, there would be difficult problems to overcome with the power-generating system itself. NASA flew a solar concentrator based on Fresnel lenses on its Deep Space-1 technology testbed, which he says "worked very well." But array pointing is more critical with concentrators than with conventional planar solar arrays, and the concentrator aperture will be "about the same size" as a planar array, even if the number of cells beneath it is reduced. Ultimately, it becomes a question of whether an array large enough can be built and delivered.

"Those are pretty advanced studies," Stella says. "A solar cell at Neptune will probably generate some power, but . . . the amount of power is going to be dropped down by literally hundreds, close to a thousand, and that makes for a very big solar array. The question is, if you need 30 watts at Neptune, which is a small amount, how many thousands and tens of thousands of watts do you have to build on Earth? Our biggest communications satellites are probably on the order of 20 kw., and that's lot of solar array, very complex, and again we might need even more than that. We're getting into uncharted territory."

Engineers at JPL also are working on boosting solar-cell efficiency by increasing the number of junctions, with promising results. But there is a limit to how much sunlight can be converted to electricity.

"I would think that numbers approaching 50% are probably at least analytically feasible," says Stella. "We've done some work here. We looked at going from three junction cells to four to five to six to seven. And there's a point of diminishing returns . . . . I think we ended up probably in the order of 50%, and that doesn't mean you're going to get there."

Posted by: Rob Pinnegar Jul 18 2006, 03:36 PM

QUOTE (AlexBlackwell @ Jul 17 2006, 01:51 PM) *
I apologize for reviving a dead thread... <<snip>>

Yeah, but a thread like this is by its very nature of the type that can be expected to go dormant for long periods. After all, it isn't as if we're hearing Neptune Orbiter news every day.

Posted by: JRehling Jul 18 2006, 03:38 PM

I would think that at some point the weight of the solar panels would be prohibitive.

I guess another concept for a Neptune orbiter would be to have solar panels spend 90%+ of a highly elliptical orbit charging batteries, with periapsis spent using that battery power to run instruments and transmit data home. In principle, there's almost no limit to how high the apoapsis is -- and such an orbit would also require less propellant. If the Neptune encounters are far apart in time, that would increase operations costs, but there'd be a lot of savings in engineering.

Posted by: Chmee Jul 18 2006, 03:47 PM

QUOTE (JRehling @ Jul 18 2006, 11:38 AM) *
I would think that at some point the weight of the solar panels would be prohibitive.

I guess another concept for a Neptune orbiter would be to have solar panels spend 90%+ of a highly elliptical orbit charging batteries, with periapsis spent using that battery power to run instruments and transmit data home. In principle, there's almost no limit to how high the apoapsis is -- and such an orbit would also require less propellant. If the Neptune encounters are far apart in time, that would increase operations costs, but there'd be a lot of savings in engineering.


By why even consider a solar array when RTG's are a proven, safe, and stable power source for deep space missions? It seems such large, heavy, and complex solar arrays are solution for a problem that does not exist.

Posted by: AlexBlackwell Jul 18 2006, 04:42 PM

QUOTE (Chmee @ Jul 18 2006, 05:47 AM) *
By why even consider a solar array when RTG's are a proven, safe, and stable power source for deep space missions? It seems such large, heavy, and complex solar arrays are solution for a problem that does not exist.

It depends on one's definition of "problem." Non-nuclear power sources are politically and programmatically more palatable, even if in some cases such solutions are more difficult to implement from an operational/engineering standpoint.

Posted by: ljk4-1 Jul 18 2006, 05:34 PM

I wonder if a very large solar sail could be designed not just for getting a
probe to Neptune but also to serve as a solar collector/reflector?

Would it be less heavy than any currently conceivable solar panel for such a
mission?

This could solve the nuclear "problem" - though I personally have no issues
with nuclear power as the energy source for craft in deep space. Or on this
planet, for that matter.

Posted by: Analyst Jul 20 2006, 10:54 AM

Before the "nuclear problem" comes the money problem. RTGs may be expensive, but they are very reliable, long lasting with predictable slow degration. Solar arrays at Neptune? There you have 1/36 the light level compared to Jupiter, this means 36 times larger arrays (or collector space). I guess this will be more expensive than RTGs (at least to develop), much less reliable and a problem for navigating/pointing.

Save the money problem first.

Posted by: antoniseb Jul 21 2006, 11:52 PM

QUOTE (Analyst @ Jul 20 2006, 04:54 AM) *
Before the "nuclear problem" comes the money problem. RTGs may be expensive, but they are very reliable, long lasting with predictable slow degration. Solar arrays at Neptune? There you have 1/36 the light level compared to Jupiter, this means 36 times larger arrays (or collector space). I guess this will be more expensive than RTGs (at least to develop), much less reliable and a problem for navigating/pointing.

Save the money problem first.


Your mention of the factor of 36 difference in Sun light compared to Jupiter seems to have ignored the previous post. It would be fairly cheap and reliable to have thousands of square meters of mylar reflecting light back to a small PV array.

Posted by: Stephen Jul 25 2006, 04:35 AM

QUOTE (antoniseb @ Jul 21 2006, 11:52 PM) *
Your mention of the factor of 36 difference in Sun light compared to Jupiter seems to have ignored the previous post. It would be fairly cheap and reliable to have thousands of square meters of mylar reflecting light back to a small PV array.

But is it a practical solution?

Once you got to Neptune and started orbiting the planet you would need to ensure that all that mylar stayed at a fixed attitude with respect to the Sun. If it functioned simply as a solar sail the easiest solution would be to eject the sail. If you have to keep it to power your PV array then you would surely be complicating matters.

For which attitude would you want to use? To have the mylar facing the Sun would be best for reflecting light back into PV array. However, doing so would probably have all that mylar acting like a solar sail again, trying to drag the probe in directions its masters back on Earth would (probably) not want it to go in. The latter could be minimised by having the mylar face edge-on with respect to the Sun, but doing that would also minimise the amount of light being reflected into the PV array.

======
Stephen

Posted by: Greg Hullender Jul 25 2006, 05:01 AM

But surely at the distance of Neptune the force applied by light to a mirror can't amount to much -- especially when the point of the mirror is to collect about as many photons as the solar panels on a Mars probe receive all the time! (Or do Mars probes have a big problem coping with the force of the photons falling on their solar cells?)

Posted by: helvick Jul 25 2006, 05:34 AM

QUOTE (Greg Hullender @ Jul 25 2006, 06:01 AM) *
But surely at the distance of Neptune the force applied by light to a mirror can't amount to much -- especially when the point of the mirror is to collect about as many photons as the solar panels on a Mars probe receive all the time! (Or do Mars probes have a big problem coping with the force of the photons falling on their solar cells?)


That's true enough but a more fundamental problem would be deploying and then keeping a 400m^2 mirror tensioned and pointed to a sufficient accuracy.

One thing to remember is that while concentrator type solar panels allow you to increase the effective area of a panel cheaply the system becomes significantly more sensitive to pointing accuracy. The main reason we don't generally see small (<1m^2) panels + 10x+ refractor\reflector concentrators on EO or MO spacecraft is that the concentrator accuracy\pointing problem is not trivial.

Posted by: ljk4-1 Jul 25 2006, 02:58 PM

Would a Neptune orbiter be able to utilize the planet's magnetic field in
some way for power and manuevering, or would that just complicate
matters even more than a solar sail?

Maybe it could utilize the geothermal energy from Triton's geysers. cool.gif

Posted by: TritonAntares Jul 25 2006, 11:17 PM

QUOTE (ljk4-1 @ Jul 25 2006, 03:58 PM) *
Maybe it could utilize the geothermal energy from Triton's geysers. cool.gif

Or maybe Triton's natural resources could be used -
let's built up a chemical plant there to synthesize hydrazine... blink.gif tongue.gif

Posted by: qraal Jul 26 2006, 01:40 PM

Hi All

Solar concentrators are such a cool concept for powersats, and I've seen Neptune probe designs with concentrators, but the pointing issue is pretty much mission critical, especially since batteries are too damned heavy to use when the collectors aren't pointing at the Sun. Supercapacitors are getting better all the time and the claimed energy density of some are better than batteries too.

Yet nukes are a proven technology and not the potential catastrophe that excessively strident environmentalists carryon about. To me a proper nuclear reactor is a better option than RTGs. Thermoelectric conversion technology is getting better all the time - the old Russian Topaz reactors had a mere 3% efficiency, yet 18% efficiency is being developed, and even higher efficiencies are possible in theory. So why throwaway the option to avoid a minor risk? And a reactor is even safer if it is deactivated until high orbit is achieved.

It just seems insane to me that NASA dumped the original reactor program so long ago, and then to have a half-revival with Prometheus, to then see that program in limbo... Nuts!

Adam

Posted by: mimile Feb 11 2007, 11:38 AM

There have been many good answers to the question of prograde / retrograde orbit for the spacecraft. Let me add one point :

If the spacecraft is in a retrograde orbit (same direction as Triton), it will fly by Triton with a smaller relative velocity. Thus Triton is able to deflect it by a larger angle, and there is a larger maneuvering capacity : Triton can send the spacecraft on a wider set of orbits.

On another point, visiting Nereid, I think there is no need to do it only once before aerobraking. Nereid has a VERY eccentric orbit around Neptune (0.75) : its minimum approach to Neptune is about 1.4 million km, only 4 times the size of Triton's orbit. this can easily be reached using Triton flybys. For example, at Saturn, recall Iapetus orbit radius is 3.6 million km and Titan's 1.2 million. And there are Two flybys, one distant already done, one close this year. True : it is easier in this case because Iapetus orbit is circular. But I'm pretty sure at least one close flyby of Nereid could be arranged.

Posted by: Rob Pinnegar Feb 11 2007, 05:04 PM

Those are good points, but it may be worth keeping in mind that the relevant distance from Neptune for a Nereid pass won't be Nereid's periapsis. It'll be more likely Nereid's distance from Neptune at the two points when Nereid crosses Triton's orbital plane. This will make it more than 1.4 million kilometres (though probably not by an enormous amount).

Of course, a Neptune orbiter would not be completely constrained to Triton's orbital plane, but it would tend to make things easier. Iapetus has only a 15-degree inclination to Titan's orbital plane, and this is still cited as being a substantial problem in setting up Iapetus flybys. I don't know what the angle is between the orbital planes of Triton and Nereid, but it wouldn't surprise me if it was larger than 15 degrees.

Posted by: nprev Feb 11 2007, 05:37 PM

Nereid: 27.6 deg with respect to Neptune's equator, orbital period 360 days, eccentricity 0.75(!)
Triton: 157.35 deg

Not easy, but not impossible if it's done as a one-time-only good deal at system entry like Cassini's Phoebe encounter, I think; arrival timing would be everything. Getting out there again after entry into orbit even when Nereid is near periapsis (esp. if Triton is the prime focus, which is probable) seems unlikely in the extreme, esp. because the relative velocity would probably be pretty bad (Kepler strikes again!)...image smearing would be a significant concern, for example. Besides, Proteus is larger, probably more interesting, and far more accessible; I'd expect Nereid to be a lot like Phoebe.

Posted by: JRehling Feb 12 2007, 01:42 AM

It's odd that people brought the topic of Nereid up this week -- I was just thinking about the same issues.

Here's the key fact about Nereid: It rotates. You don't need to have multiple flybys to see most of its surface.

If geometry permitted, the ideal situation would be to find Nereid near its apoapsis while the craft were still en route to Neptune. It could make a leisurely approach while Nereid also approached Neptune, and could make close observations over a span of 12 hours. That alone would reveal most of Nereid's surface. Then, why fly by again?

With a periapsis four times Triton's, there's no need to have the Neptune orbiter ever venture out so far again. The other "major" satellites are all well inside of Triton's orbit. Unless the energy to keep the orbit in so tight cannot be spared, it should head inside and never venture out again. Most likely, the non-Triton satellites are minor priorities themselves. Gettings lots of looks at Triton (active over time), Neptune (active over time), and the rings (active over time) would comprise 85% of the mission.

Posted by: nprev Feb 12 2007, 04:13 AM

Good point, JR. In fact, Nereid spins pretty fast: 11.5 hrs per http://www.journals.uchicago.edu/cgi-bin/resolve?id=doi:10.1086/377067&erFrom=-355696328195837116Guest, so if a low relative velocity inbound encounter could be arranged, you'd get a very good global view of it during one visit.

Posted by: tasp Feb 12 2007, 06:24 AM

OK, I am not smart enough to visualize this in my head, but if Triton's orbital plane is tilted 157 degrees, and Nereid is tilted 27, then aren't the orbital planes only tilted from each other by 5 degrees?



I must be missing something . . . .

Posted by: mchan Feb 12 2007, 08:05 AM

Possibly that the node crossings of the two orbits are offset from each other. E.g., imagine if Saturn's A rings were inclined 20 degress and B rings were inclined 160 degress. The A and B rings would be coplanar if the node crossings were the same. (If going by convention of using the ascending node, then node crossings are offset by 180 degrees.) But if the node crossings were offset, you would get the B rings twisted out of the plane of the A rings. The two rings will still come close at two points, but this is because their orbits are circular. With two elliptical orbits, particularly if they do not have resonant periods, objects in the two orbits could potentially never get close to each other.

Posted by: tasp Feb 12 2007, 03:14 PM

Wikipedia has stats on Nereid and reports its' orbital plane is tilted to the ecliptic ~5 degrees. This would make an encounter for an incoming spacecraft easier. I am still having trouble visualizing the orientation of the entire Neptune/Triton/Nereid system.

With Triton 157 degrees retrograde, and Nereid 27 degrees, we wind up at 185 degrees total, so aren't they 5 degrees off of coplanar?

{you can tell I don't have Celestia on my PC . . . .}

Posted by: JRehling Feb 12 2007, 04:12 PM

QUOTE (tasp @ Feb 12 2007, 07:14 AM) *
Wikipedia has stats on Nereid and reports its' orbital plane is tilted to the ecliptic ~5 degrees. This would make an encounter for an incoming spacecraft easier. I am still having trouble visualizing the orientation of the entire Neptune/Triton/Nereid system.

With Triton 157 degrees retrograde, and Nereid 27 degrees, we wind up at 185 degrees total, so aren't they 5 degrees off of coplanar?

{you can tell I don't have Celestia on my PC . . . .}


The math doesn't work there because there are three dimensions in which the axes can be tipped.

Imagine two drunk men with hula hoops around their waists. One man leans forward at 45 degrees. The other man leans to his left at 45 degrees. The number 45 is equal, but their hula hoops are not coplanar.

Just playing around with Solar System simulator, I see that Nereid's next close approach to Triton will be mid-April, and will bring it within about 1.25 million km -- no closer. Other Nereid periapsises may bring the two closer, but they can't get too much closer. Think about Cassini imaging Iapetus from a million km away -- and Iapetus is rather large.

Posted by: tasp Feb 13 2007, 03:43 AM

Thanx, appreciate the clarification very much.

Some days I just shouldn't try to do anything without coffee . . .

But . . . .

Over time, it seems the orientations of these orbits might change (while maintaining their respective inclinations) and at various epochs (not ours, unfortunately) the orbital planes might turn in to relatively coplanar alignment.

Do we have any idea how fast this might happen at Neptune?

Posted by: Rob Pinnegar Feb 14 2007, 01:57 AM

The orbits will certainly precess -- but at that distance from the Sun, tidal effects have got to be tiny. The changes would happen over a very long period of time.

Posted by: TritonAntares Feb 17 2007, 09:16 PM

Hi,
any ideas for an illustration, report, paper, etc. of Triton's seasonal changes during its 176 year trip
together with Neptune around the sun combined with its 157.35 deg retrograde orbit?
Would be very interesting to visualize how polar night and day are changing and which latitudes are affected... wink.gif
Triton should be the strangest world in the solar system concerning seasonal changes, even if the seasons are quite long out there... blink.gif smile.gif

THX & Bye.

Posted by: mchan Feb 17 2007, 11:42 PM

There is a graph in "The New Solar System" by Beatty, et al, which shows, IIRC, Triton's sub-solar latitude over a thousand years or so.

Posted by: TritonAntares Feb 18 2007, 02:10 PM

Hi again,
here a link to a french website dealing with the question of latitude changes of Triton's subsolar point over historical time :
http://bugle.imcce.fr/fr/observateur/support/Triton/

There seems to be some disagreement on the calculation of those variations... huh.gif ?
Anyhow, Triton's seasons don't look that simply predictable as Earth's.... blink.gif

Bye.

Posted by: tedstryk Feb 22 2007, 02:13 PM

It also has some of the strangest seasons, due to its tilt, its odd orbit, and Neptune's tilt and long orbit. By the way, I am glad to see new work is still being done at Pic du Midi. The French nearly made a tragic decision to close it a few years ago, due to the fact that their observatories in other parts of the world have larger telescopes, and, given the nature of this sharp peak, there really isn't anywhere else to build on Pic du Midi. Still, it is unbeaten in its steady skies for planetary astronomy. Fortunately, I think the French powers that be realized that.

Posted by: Rob Pinnegar Mar 1 2007, 02:30 PM

Yeah, looking at some of the references cited above, the precession period for Triton's orbit around Neptune is on the order of 600 years or so -- a lot shorter than I would have thought.

So basically Neptune's equatorial plane has about a 28-degree tilt relative to Neptune's orbit around the Sun, and Triton's orbit is inclined 22 degrees to that (retrograde of course). The result of this is that Triton's axial tilt, relative to Neptune's orbit around the Sun, can be as large as (28+22) degrees, or as small as (28-22) degrees; generally it's somewhere in between. (If we take things relative to the Earth, there's also the matter of the tilt of Neptune's orbit relative to the ecliptic, but that's only a couple of degrees or so.)

Posted by: tasp Mar 2 2007, 05:30 AM

Just throwing out some ideas here:

Modify a follow on New Horizons craft to orbit Neptune. Take advantage of Neptune's huge Hill sphere by having the craft execute big, leisurely orbits of Neptune, perhaps keeping the orbital period around Neptune at ~6 months or so. The advantage of this is commonality with the current NH design, gather data during periapsis and transmit during the 5 months or so when your not near the planet. NH is apparently anticipated to be a long lived craft, having a follow on craft study Neptune for a significant portion of it's arc about the sun would be useful. The large orbit might also allow observations of the interesting apparent KBO style outie satellites. We can have a Triton pass every orbit and time the encounters to give us a close pass by an inner ring satellite and/or an outie too.

I'm still thinking having the craft start in either a prograde or retrograde orbit and then orbit shaping to the other is possible and useful in studying Neptune's magnetic field.

Posted by: djellison Mar 2 2007, 08:13 AM

Where do you get the delta V to break into orbit around Neptune? We're talking a SIGNIFICANT difference in spacecraft design to add that capacity. Also - NH is designed for short, brief encounters - not a long-life orbiting mission. You would want to move away from the RCS to reaction wheels.

What about downlink? you would want more than the 1k/sec that NH will probably manage from that range.

Once you've done that - you have very little that is still NH.

Doug

Posted by: nprev Mar 3 2007, 03:13 PM

Good points, Doug.

It's sure easy to forget the downlink bitrate in particular when talking about these deep outer system missions. Lasercomm really needs to be developed for future Flagship-class forays beyond Saturn. Cassini may well be the last one of these that relies on RF...

Posted by: tasp Mar 3 2007, 04:18 PM

The ~6 month orbit about Neptune is driven by the low data rate. Taking advantage of Neptune's huge Hill sphere, we can have the craft loiter for months to send back data between periapsis at Neptune. If the 6 month orbit is still to short for the data rate, bump it up to a year. I don't consider the low data rate a problem, it is the driver for the longest possible mission time at Neptune.

Advantages to this include :

* potentially very long probe life at Neptune, studying Neptune over a significant arc of its' orbit about the sun is one of the mission goals.

* reuse of the NH design, thus saving $ and increasing odds mission can be flown

* long excursions away from Neptune may allow study of Nereid (apparently it is ~coplanar to the ecliptic, and at least a few probe orbits about Neptune should be ~ in the ecliptic to facilite ring studies) and/or some of the other (potentially KBO derived) outie satellites (the NH design is , after all, optimized for studying such objects, and 'parking a NH follow on craft in Neptune's vicinity might get us several flybys of these objects.

* not mentioned in my post, but adding a Dawn style ion thruster and some RTG's to power it, could still result in a probe that, with a variation of the VEEGA trajectory, still reach and orbit Neptune as cheaply as possible



Cheap, interesting missions seem to have the best chance of making launch. I am just trying to come up with something that is as off the shelf as possible, and can do an interesting mission for the bare minimum $.

Posted by: helvick Mar 3 2007, 04:50 PM

Tasp - NH is a flyby craft not an orbiter and its design is totally unsuited for an orbiter mission. There is no way to get something like it into orbit around anything without so much rework that it would end up being something completely different.

Posted by: djellison Mar 3 2007, 05:12 PM

RTG powered ion propulsion, that's VERY VERY off the NH design - as would be the need for reaction wheels. It's not a re-use of the NH design at all. It's something very very different.

Doug

Posted by: vjkane2000 Mar 3 2007, 05:53 PM

It is so difficult to get a craft to Neptune in a reasonable about of time (very high speeds are needed) and then slow it down enough to enter orbit (aerocapture appears to be required). You can probably solve the first problem by waiting to use a Jupiter gravity assist or a solar electric ion "boost stage". However, that aerocapture means that whatever you do will be a new design and new technology. At that point, there's probably good reason to spend more on the orbiter and instruments to get more bang from the bucks. As marvelous as the NH instruments are, heavier instruments built with the same level of technology would provide a higher science return.

If you want an inexpensive mission that could fly sometime in the reasonable near future, you could certainly take the core elements of NH and attach an atmospheric probe for a flyby mission. From what I understand of Triton's orbit, though, you can probably either have a close flyby of Triton or Neptune, but not both. Perhaps someone here knows. I also don't know if the relay from the atmospheric probe can be reasonably done if you go for the Triton flyby.

Posted by: tasp Mar 3 2007, 07:22 PM

QUOTE (helvick @ Mar 3 2007, 10:50 AM) *
Tasp - NH is a flyby craft not an orbiter and its design is totally unsuited for an orbiter mission. There is no way to get something like it into orbit around anything without so much rework that it would end up being something completely different.



Yet New Horizons seems amply capable of Neptune encounters at ~1 year intervals. All the Jupiter data is coming back in less than a year, from its' recent encounter. It seems that characteristics of the Neptune system (such as the lower number of sizeable moons, Triton's favorable mass ratio with Neptune and the huge Hill sphere) might guide us towards a mission profile and probe design that would not be a 'flagship' mission. Cheap missions get flown far more often.

As I recall, the speed decrease for Cassini at Saturn for orbit insertion was ~1500 KPH, I don't have a figure for the Dv total expected for the Dawn mission, but that would be an item for discussion if someone knows. SEP at Neptune's distance seems iffy, I don't know how many RTGs it would take for a mission like this, but once the orbit insertion is over, residual power from the RTGs could power a NH derived clone
for decades.


Trade offs between aerocapture and ion propulsion are topics for discussion, off course. Engineering trade offs is the whole point here. Can a low cost, off the shelf derived, Neptune orbiter deliver 80% of the bang for 20% of the price?

Posted by: djellison Mar 3 2007, 07:28 PM

You're talking about a flagship mission, which ever way you look at it.

Doug

Posted by: mchan Mar 5 2007, 10:34 AM

QUOTE (tasp @ Mar 3 2007, 11:22 AM) *
Can a low cost, off the shelf derived, Neptune orbiter deliver 80% of the bang for 20% of the price?


Considering Cassini cost around $3.3 billion, and proposals for the Europa orbiter have project costs of $1.5 billion or more, a guess for the fagship Nepture orbiter costs could be ~$2 billion. Assuming this guess is realistic, 20% of the flagship Nepture orbiter cost would be $400 million which would not be enough to even buy another build of the original New Horizons spacecraft.

Posted by: tasp Mar 5 2007, 02:46 PM

{For 'just wanting to throw out some ideas', it seems this topic has become a real grinder for me.}

A Cassini class flagship orbiter might go for 5 billion (over a decade of inflation mostly, and more rocket to get there, and upgraded dish and com system for Neptune's distance and a scan platform) so I'm thinking the 'cheap' Neptune orbiter might be <$1bil.

Posted by: Rob Pinnegar Mar 5 2007, 03:15 PM

QUOTE (djellison @ Mar 3 2007, 12:28 PM) *
You're talking about a flagship mission, which ever way you look at it.

Not only that, but a flagship mission that (1) would have a good chance of failure due to the long travel time (and aerobraking risks) and (2) wouldn't make it to Neptune until after any political supporters would be retired.

This is going to be a tough nut to crack.

Posted by: J.J. Mar 5 2007, 04:35 PM

Honestly, I'd just be happy with a cheap NH-type flyby of Uranus and Neptune, utilizing the advantages of equipment optimized for the light levels at those distances, and of course 30+ years of technological improvement. I'd dig seeing the other hemispheres of the Uranian moons, as well. wink.gif

For instance, telescopic observations since Voyager have shown that Uranus's atmosphere shows much more detail in the near-IR than it does in the visual band--near-IR capability that neither spacecraft had. Sending a spacecraft so equipped to Uranus and Neptune could yield outstanding results, regardless if it orbited or not.

Posted by: JRehling Mar 5 2007, 06:58 PM

I think to a considerable extent, Uranus and Neptune "meteorology" is eventually going to be more amenable to continuous survey from Earth than from pricey orbiters that provide outstanding spatial resolution but spotty temporal coverage.

This shows the best image that HST can provide now of either of these planets:

http://hubblesite.org/newscenter/archive/releases/1998/35/image/a/format/web_print/

If this resolution could be, say, doubled, either via a space telescope or future ground telescopes, then a lot of scientific goals regarding Uranus, Neptune, and Io (among other places) could be met by conducting temporal surveys on a "loose" basis: observing each of those worlds, say, at least three times a week (to see all latitudes as they rotate) and on an occasional "tight" basis, observing hourly or so. Certainly other places would fit on the same agenda, but imagine just those three worlds getting such a telescope's attention, with each of them observed weekly for years on end, and numerous "tight" surveys of each to get an idea of the fine-grained dynamics.

Note that Neptune and Uranus have essentially no variation in their distance from Earth (though each goes through a solar conjunction blackout), while Io's distance varies by 50%, so its tight surveys should obviously be done around Jupiter conjunction.

HST has a resolution of about 280 km/pixel at Uranus, 450 km/pixel at Neptune, and 65 km/pixel at Io. If that could be doubled (or even if not) an awful lot of the science you'd like to do with an orbiter could be done from terra firma (or LEO or L2). I think a single good flyby of each of Uranus and Neptune to "ground truth" the excellent temporal coverage with some close-ups would produce some good answers to basic questions. Obviously, magnetometers and satellite flybs would be part of the package.

Overall, I think there's a bias in the system against finding best-bang-for-the-buck options that do a pretty good job of exploring several targets without specializing for any one of them. Ultimately, a long-distance survey for time and close-up flybys for spatial resolution would accomplish a lot without the cost and "bother" of orbiters that have to sprint to the outer-outer solar system and then slam on the brakes to orbit a giant planet. The flyby craft for Uranus and Neptune would have obvious opportunities to also provide close-up views of other worlds along the way, including Io as a possibility for one or both of them, and perhaps KBO destinations like Sedna after their ice-giant flyby.

The real "victim" of this approach would be Triton, which can't be seen very well from 29 AU away and which deserves excellent coverage. A hedge to the approach would be to send two flybys to Neptune to capture opposite hemispheres of Triton and to catch two snapshots of its time-varying activity. A total of three flyby craft (and here we could talk in terms of NH analogues, if not clones) could visit Io three times, Uranus once, Neptune and Triton twice, and certainly get some KBO exploration done as well, possibly visiting some of the bigger KBOs.

I suspect this would be cheaper than a good Neptune orbiter and would let the next mission to the Neptune system be a Triton lander that could even spend some time looking skyward at Neptune and should be able to last a long time on the surface.

Posted by: AlexBlackwell Mar 5 2007, 07:08 PM

QUOTE (vjkane2000 @ Mar 3 2007, 07:53 AM) *
It is so difficult to get a craft to Neptune in a reasonable about of time (very high speeds are needed) and then slow it down enough to enter orbit (aerocapture appears to be required). You can probably solve the first problem by waiting to use a Jupiter gravity assist or a solar electric ion "boost stage". However, that aerocapture means that whatever you do will be a new design and new technology.

For those with access, there is a pretty interesting paper http://www.sciencedirect.com/science?_ob=PublicationURL&_tockey=%23TOC%235738%239999%23999999999%2399999%23FLA%23&_cdi=5738&_pubType=J&view=c&_auth=y&_acct=C000050221&_version=1&_urlVersion=0&_userid=1349829&md5=57db6913ea8286f295d30d32547217cb with Advances in Space Research:

A Study of Trajectories to the Neptune System Using Gravity Assists
Adv. Space Res., In Press, Accepted Manuscript, Available online 1 March 2007
C.R.H. Solórzano, A.A. Sukhanov and A.F.B.A. Prado

Abstract
QUOTE
At the present time, the search for the knowledge of our Solar System continues effective. NASA's Solar System Exploration theme listed a Neptune mission as one of its top priorities for the mid-term (2008-2013). From the technical point of view, gravity assist is a proven technique in interplanetary exploration, as exemplified by the missions Voyager, Galileo, and Cassini. Here, a mission to Neptune for the mid-term (2008-2020) is proposed, with the goal of studying several schemes for the mission. A direct transfer to Neptune is considered and also Venus, Earth, Jupiter, and Saturn gravity assists are used for the transfer to Neptune, which represent new contributions for a possible real mission. We show several schemes, including or not the braking maneuver near Neptune, in order to find a good compromise between the ΔV and the time of flight to Neptune. After that, a study is made to take advantage of an asteroid flyby opportunity, when the spacecraft passes by the main asteroid belt. Results for a mission that makes an asteroid flyby with the asteroid 1931 TD3 is shown.

I believe this paper was presented at the recent http://www.space-flight.org/AAS_meetings/2007_winter/2007_winter.html in Sedona, Arizona.

Posted by: dvandorn Mar 6 2007, 02:08 AM

I hate to say this, but I really think that extensive exploration of the outer Solar System is going to have to wait for a new generation of propulsion technologies. Uranus and Neptune orbiters, in particular, are very, very difficult to do with our present propulsion technologies.

We need some form of constant-thrust propulsion that doesn't require tons and tons of propellant in order to get out to the outer System in months, not decades, and that allows us to brake into orbit around the outer planets. Frequent energetic maneuvering, and even breaking out of orbit to travel to another planet, are also capabilities that we're going to need to develop eventually.

As long as we're stuck with either big-push-then-lots-of-coasting chemical propulsion and constant-but-really-miniscule-thrust ion propulsion, we'll never be able to afford more than three or four flagship missions to the outer planets in any one person's lifetime. That's just not going to get it done.

-the other Doug

Posted by: Stephen Mar 8 2007, 07:17 AM

QUOTE (nprev @ Mar 4 2007, 02:13 AM) *
It's sure easy to forget the downlink bitrate in particular when talking about these deep outer system missions. Lasercomm really needs to be developed for future Flagship-class forays beyond Saturn. Cassini may well be the last one of these that relies on RF...

Hmm. Is Cassini a "beyond Saturn" foray or an "at Saturn" foray? If the latter, then as yet there have been no Flagship-class missions beyond Saturn (unless you count Voyager).

But I digress...

Wasn't there going to be a trial a laser communications system on the Mars Telecommunications Orbiter that was due to be flown in 2009?

Now MTO has been cancelled there will probably not be a test flown until at least the mid-2010s (hopefully on the putative MSO-MTO mission). But that in turn means that Flagship-class missions are unlikely to be able to start using it until at least the later 2010s.

Which in turn suggests that any Flagship-class missions to or beyond Saturn which go into the pipeline between now and then will be unlikely to be using laser communications, at least as its primary communications systems.

I guess that's just one more reason why a Titan mission is unlikely to be jumping the queue in front of a mission to Europa. sad.gif

======
Stephen

Posted by: JRehling Mar 9 2007, 10:39 PM

QUOTE (Stephen @ Mar 7 2007, 11:17 PM) *
I guess that's just one more reason why a Titan mission is unlikely to be jumping the queue in front of a mission to Europa. sad.gif


There's one outstanding reason which is that the data from Cassini hasn't been digested yet. As a rough sketch of the exploration process, I'd say that it tends to take a few years to understand the data from the previous mission before you can solidly know which investigations to undertake with the next one. (At least, when a world's exploration is in the early stages. We're farther along with, eg, Moon, Mars, Venus, Jupiter.) If Cassini were to last til 2010, it would be fair to estimate that it won't be any sooner than 2015 before you could count on the community having pretty well modeled Titan from that data. A 2020 launch would be sort of a rush job.

I think Europa is ready for planning the mission (the better part of a decade after the most productive Galileo flybys), and Titan isn't even in the same point in the queue for the two of them to be "competing". I think it's a dead issue. Even if Titan were intrinsically more desirable to visit next, Europa's time in the spotlight can come and go before the next Titan mission should be launched, unless we don't care about whether or not it's well-designed...

Posted by: PhilHorzempa Mar 17 2007, 11:05 PM

QUOTE (vjkane2000 @ Mar 3 2007, 02:53 PM) *
It is so difficult to get a craft to Neptune in a reasonable about of time (very high speeds are needed) and then slow it down enough to enter orbit (aerocapture appears to be required). You can probably solve the first problem by waiting to use a Jupiter gravity assist or a solar electric ion "boost stage". However, that aerocapture means that whatever you do will be a new design and new technology.



I believe that NASA has considered the development of aerocpature to be one
of the enabling technologies for Outer Planet Orbiters. However, if you examine
NASA's proposed FY 2008 budget, then you will notice some more bad news for
Unmanned Spaceflight from Mr. Griffin. Take a look at p.130 in the Full Budget,
(accessible on NASA's budget page) where there is a list of proposed budgets for
the New Millenium Program for the next few fiscal years.
Griffin plans to reduce funds for NMP from $90 million to $65 million in FY08.
This is followed by "going-out-of-business" budgets for FY90 and FY10. NMP receives
only about $35 million in each of those years. Apparently, Mike Griffin is planning to
eliminate yet another part of our country's efforts to explore the Solar System.
In addition, in the FY08 Budget Proposal, the plan to delay the next NMP mission,
ST-9, by 2 years is also mentioned. This is bureaucratic talk that means that NASA
is essentially cancelling ST-9, just as it has cancelled SIM, TPF and MSR.
The cancellation of ST-9 is important in this thread because one of the technologies
that was a candidate for that mission was an AEROBRAKING demonstration.
You can now kiss that one bye-bye, courtesy of Griffin and the VSE. Our missions
to Neptune and Uranus and Titan and Mars just slipped that much further into the future.

Another Phil

Posted by: algorimancer Mar 27 2007, 03:52 PM

Of course, Nasa administrators commonly change with presidential administrations. We have only a couple of years of Michael Griffin to worry about, then hopefully we'll get someone in charge who appreciates the science of space exploration (no surprise that an ex-astronaut doesn't). Personally I think that unmanned exploration really ought to be getting at least fifty percent of the Nasa budget, and that the science budget ought to be immune from scavenging to fund the manned exploration program. Things change, budgets, priorities, technology, people. Just have to be patient smile.gif

Posted by: nprev Mar 28 2007, 01:30 AM

True enough...just wish there was some way to program & lock in funds long enough to completely cover missions like a Neptune orbiter. We're talking about a probable 30+ year program duration from concept development to EOM, and even if we disregard the Galileo & Cassini delay debacles, don't forget that there was a serious proposal to shut down Voyager 2 while still enroute to Uranus to "save" a few bucks.

Sorry for the OT excursion into policy, but we've already seen that this does become an extremely serious consideration for missions to the outer planets. Wish that NASA program funding could be blocked out like military construction (10-year planning horizon vs. annual appropriations...in other words, they have ten years to spend their pot of money for a given project & in the interim it's not supposed to be touched). Maybe TPS should do some digging in this area & try to influence some policy changes; obviously, there's a precedent under current US law.

Posted by: PhilHorzempa Mar 28 2007, 04:06 PM

[quote name='algorimancer' date='Mar 27 2007, 11:52 AM' post='86914']
Personally I think that unmanned exploration really ought to be getting at least fifty percent of the Nasa budget, and that the science budget ought to be immune from scavenging to fund the manned exploration program.


Here, here! I totally agree with those comments. Space Science should be at 50% of
NASA's budget, not the 30% that it gets now. Griffin acts as if that 30% level is written
in legislation. In fact, in recent comments by him, he bemoans the fact that manned spaceflight
gets ONLY 60% of the budget, double that of Space Science.
I don't see any reason why Manned flight MUST gobble up that much of NASA's budget.

With a 50-50 split, the astronauts would learn to live with fewer missions, while we supporters
of UMSF would begin to see some of our dreams come true -

Mars Sample Return, Saturn Ring Observer, Europa Orbiters and Landers and Submersibles,
Planet-Detecting Space Telescopes,
Comet Sample Return, Venus Balloons and Landers, High-Resolution Venus Radar Mapper,
Neptune Orbiter, Triton Lander, Enceladus Lander, Titan Orbiter and Rover, Venus Sample Return,
Mercury Sample Return, Main Belt Asteroid Sample Return.

Thiose are all missions that have been studied/proposed by NASA.
We have a whole Solar System (and new ones) to explore.
What are we waiting for?


Another Phil

Posted by: djellison Mar 28 2007, 05:20 PM

Ahem.

Politics people - stop dancing with the rules.

Posted by: JRehling Mar 28 2007, 06:09 PM

BAD QUOTING - STOP IT - Doug

I think the turn of topic serves to signify that a Neptune mission is as far out in the future as it is in kilometers. The only advantage Neptune has over the other flagship mission candidates of the outer solar system is that the approach that is required is less mysterious. While the nature of the orbit, and the possibility of entry probes for Neptune or a sounder of some kind would be up in the air, a Neptune mission will obviously be a kind of mini-Cassini. Exploration strategies for Europa, Titan, or Enceladus are much harder to pin down, and will be a kind of Twenty Questions game. Triton may get to that point eventually, but it is obvious that whatever mission follows up the NEXT mission to Neptune will be very far out in the future.

Budget and top-level priorities come up in discussion of Neptune because Neptune's as low as a priority could be and still be on the list. It hasn't been said yet in X vs. Y terms, but Enceladus (by matching the highest expectations of how interesting a place it seems to be) will probably knock Neptune out of contention for a very long time to come. Picture the competition for outer solar system flagship missions as a horserace. Whatever place Neptune was in before, it has seen Titan surge to rival Europa for the lead, seen Enceladus move out of a pack to almost challenge the others for the lead, and such places as Io, Ganymede, and Uranus aren't quite completely dead. In comparison with any of these, Neptune fares terribly in terms of distance from Earth -- even Saturn is over three times closer! And that's only accounting for outer solar system competition for funding, to say nothing of a (prioritywise) colossus like Mars or dollar-gobbling one-off possibilities like a Venus Sample Return.

Pondering Neptune missions is therefore probably a mere delight for our imaginations. I strongly believe that only an unforeseen mission of opportunity (like a physics / solar magnetosphere mission that flies by Neptune just for what-the-heck, or an entry probe that sails off of a jovian gravity assist) could possibly get there before 2040. I don't think it's very likely a Neptune Orbiter will fly before the ground rules of outer solar system exploration change in some unforeseeable way (other nations taking a role; new kinds of propulsion, etc.)

I'd be overwhelmed if Europa, Titan, and Enceladus each receives a followup before 2040.

Posted by: Greg Hullender Mar 28 2007, 11:36 PM

Here's a question: why is the distance such a big deal? The energy difference between getting to Saturn and getting to Neptune shouldn't be that great. I may have computed this wrong, but it looks to me like the Saturn-to-Neptune potential energy difference is just 1/12 of the Earth-to-Saturn difference.

Clearly it takes a lot longer to get there, but how much of the cost of a mission is the cruise time? I'd expect that to be pretty small compared to the up-front probe cost and the data analysis cost at the end.

In fact, it seems that if we really did want to study all the outer planets, it'd be most cost-effective to make 4 of the same probe and send them off to all four gas giants more or less at once, then process the results over a period of years as they arrive.

But even if (as I know) we can't do that, I'm still puzzled why a Neptune probe would be more than (say) 20% more than an equivalent Saturn probe. Error in my math? Or some other factor I'm missing?

--Greg

Posted by: JRehling Mar 29 2007, 12:15 AM

BAD QUOTING - STOP IT - Doug

For a flyby like NH, the distance is not as much of a factor as it is for a putative orbiter, simply because the orbiter has to stop. The minimum-energy path that would make that easy would be prohibitively slow for Neptune (ie, it would be similar to the trajectory of Halley's Comet). Faster trajectories mean more resources (aero- or propulsive) to brake on the other end. Cassini's flyby velocity at Phoebe (before a lot of Saturn's gravity well had come into play) was 6.4 km/s, and a delta-v of only one tenth that was needed for SOI. A cruise to Neptune like New Horizons is taking to Pluto would mean almost triple Cassini's arrival velocity. Even though Neptune's gravity well is not as deep as Saturn's, and we could tolerate a more elliptical orbit (more time spent far from Triton, etc.), that's an expensive engineering constraint, and could lead to a sacrifice in terms of instrumentation, etc.

Someone is surely more qualified to speak to the operational costs in terms of ground crew (NH being a good model?) and the quantitative engineering costs of the aforementioned problem.

Posted by: algorimancer Mar 29 2007, 01:07 PM

What I'd like to see is a paradigm shift over to taking advantage of microelectronics and microfluidics to develop really small (light) yet capable spacecraft. Heck, my cell phone has a more powerful computer than my desktop had less than ten (five?) years ago. Combine that with microfluidics & ion drive, and the only real remaining challenges are a compact nuclear power supply and communications (improve the DSN's sensitivity to allow lower power comm?). I can envision a pretty competent little spacecraft massing on the order of a kilogram or several. We could send swarms of these things all over the solar system and beyond for the cost of a single mission today, a Neptune orbiter would be trivial.

Posted by: JRehling Mar 29 2007, 05:23 PM

BAD QUOTING..STOP IT - THREE BY YOU IN ONE PAGE - Doug

Rodney Brooks (robotics/AI, MIT) proposes things like this.

One problem is that while many critical subsystems have been miniturized into invisibility (or to massive capacity) others can't be. A good telescope still has to have a big mirror or lens. A radio transmitter still has to crank out some power for reception 30 AU away. A nuclear power source can't be tiny. Firing a thruster to get orbital insertion -- you just can't do that with a pocket-sized craft.

I think there's some future to this idea, though. Maybe centralizing what needs to be centralized on one comm-craft with lots of little drones in other locales. I could especially imagine the space weather around the Sun and Jupiter being explored by a large number of little orbiters spread out in different orbits. That would be tremendously valuable. Exploring a world in situ would be the other great application: Imagine lots of little Surveyor rovers in hundreds of locations around Mars. The asteroid belt or the outer jovian/saturnian satellites might benefit from this approach.

An orbiter devoted to surveying static phenomena would seem to be immune to this approach, though. Why not just have one big orbiter? Lots of little spaced-out (no pun intended) orbiterlets don't seem to offer much as an alternative. Eventually, the one orbiter will get all of the spatial perspectives anyway.

Posted by: helvick Mar 29 2007, 06:10 PM

Danger Will Robinson - Serious arm waving ahead.

Anyway, assuming we can just linearly scale things down to this size which is almost certainly untrue but ..

A ~250g Plutonium RTG would yield around 1 watts sustained for many years. If we were to shrink Deep Space 1's Ion drive\Fuel load by a factor of 500 we would have a drive that required ~164 grams of fuel which combined with a reliable 1 watt power supply could provide about 3600 m/sec delta V to a 1 kg probe. I seem to recall from somewhere that we'd need about 15km/sec delta-V to get to neptune so we're looking at slightly more than four times that fuel load - let's say 650g.

So you are left with ~ 100g with which to build the micro-probe's chassis, fuel tank, sundry scaffolding and all the comms / sensor equipment. Now that would be tricky but basically that's more than enough mass to build a cell phone with a camera today and the DSN can certainly pick out a signal broadcast at cell-phone range power (~1 watt) from Jupiter type distances although the data rate would probably be pretty poor. OK space rating all this is a monstrously untrivial task but we're not talking about magic here just some steady technical progress.

Plausible within the realms of mad arm waving at least.

Posted by: tty Mar 29 2007, 07:49 PM

Being an aerospace person I feel I have to do some counter arm-waving. It takes a looong time for new technology to become mature enough and robust enough for flight-rating, much less space-rating. Components that doesn't even exist yet probably won't be flying until the 2020's at the earliest.

Also not everything can be miniaturized. Maybe DSN can handle a 1 watt signal from Jupiter - but not a non-directional one or communications with Galileo would not have been a problem. So You need a directional antenna, and good directivity means that the antenna has to be large compared to the wavelength of the signal.

Ion engines are just dandy for the long haul but they are utterly useless for orbit insertion, so the flight profile would either mean having to decelerate for nearly half the distance to avoid arriving with too high speed or adding a conventional rocket for the insertion burn. In either case much of the savings in weight/time is lost.

Posted by: djellison Mar 29 2007, 07:53 PM

Basically - you're not going to the outer solarsystem with a cube sat smile.gif

Doug

Posted by: Greg Hullender Mar 30 2007, 12:11 AM

I wonder if someone can double-check my figures here. I figure a minimum-energy orbit for Neptune from Earth needs a delta-V of 11.65 KPS and takes 30.6 years. (Technically that's from Earth's orbit to Neptune's orbit -- I'm ignoring the gravity of both objects for the moment.)

I agree -- 30 years is a long time to wait.

However -- and this is what surprises me -- if you use 12 kps instead of 11.65, it cuts the time to 15.68 years. That's a big improvement for one third of one kps, I think. Add another third, and it's just under 13. (And your orbit is parabolic). At 13 kps of delta-v, you get 10.38 years.

I stipulate that we could wait ten years. I know extra delta-v costs a lot because of the exponential in the rocket equation, but it doesn't seem like this should be enough to sink the boat.

Is there some other factor? Or (again) do I have the math wrong somehow?

--Greg

Posted by: helvick Mar 30 2007, 01:49 AM

Greg - I think your numbers are correct but I'm certainly no expert. My understanding was that 15km/sec delta-V gives the flexibility for a fast (10-12 year) mission including orbital insertion.

TTY - As far as oribital insertion is concerned I understand where you are coming from but as I understand it a DS1 type system with a DS1 type Ion drive engine is able to hit 3.6km/sec delta-V eventually. DS-1 consumed 100g per day under minimum thrust so a full load of of 82Kg of fuel would require 820days to expend in getting that 3.6km/sec delta-V. Extrapolating that out to my 4x DS-1 model we need to spend four and a half years accelerationing initially followed by a year (or three) of cruise and then a four and a half year deceleration into Neptune capture orbit. If ~15 km/sec delta-V is the magic number what's wrong with that approach?

With regard to Antenna size. The Messenger phased array Ka band http://ieeexplore.ieee.org/Xplore/login.jsp?url=/iel5/7416/20159/00931694.pdf and is good for 3-4kbps comms over 100million km http://messenger.jhuapl.edu/webcam/annotatedimages/annotated-20031005.html. Yes there are significant constraints in reducing that by (slightly more than) an order of magnitude in terms of both mass and power consumption that I can't solve but it seems to be in the ball park and it doesn't feel totally impossible at first glance.

While it is true that high res optics such as carried by MRO are simply physically impossible on a platform like this what you choose to carry surely depends on what you are attempting to achieve. You can certainly build a very poor camera for less than 20g, I have a 1.2 megapixel example in my phone in front of me. I reckon you could put some effective sensors in there one way or another although they might not be cameras.

Worst case - scale the whole thing up by a factor of 10 and you really should be able to design and build it today. Look at the Beagle 2 build specification and what they managed to cram into that, the idea of a midget deep space probe is not so mad.

The point being that if you _could_ build such a thing within a 1kg (or 10kg) mass budget then you could potentially build and launch a large number of them at once, as Algorimancer suggested. And you would definitely get some efficiences of scale if you were to build tens or hundreds and scatter them off a launch similar to New Horizons (ie with oodles of delat-V up front from those nasty chemicals).

Posted by: tasp Mar 30 2007, 03:58 AM

Imagine, if you will, an ion engined probe, already in Neptune orbit. Further, in view of Neptune's large Hill sphere, imagine the probe to be in an extremely elliptical orbit with a period of ~ 1 year. At apogee, even relatively minute adjustments to the orbit, made with the ion thruster perhaps over a period of a couple of months, will result in large controllable variations in the perigee portion of the orbit, ~6 months later. At perigee, orbit adjustments can be made via gravitational interactions with Triton. IIRC, large changes in period and inclination can be made easily at the very fringe of the Hill sphere. The long orbital period about Neptune is also conducive for using a relatively low power, slow data rate from Neptune to send back the perigee data.

Ignoring the challenge of the initial Neptune orbit insertion for the moment, it seems Neptune might be rather hospitable to a probe with an ion engine, as long as the probe was taking advantage of the large Neptune Hill sphere, and we would be content with a series of Neptune encounters at roughly 1 year intervals, for as long as the probe could remain functional. Considering the success with the Voyagers, we might anticipate a probe duration at Neptune of 10 years, possibly substantially more.

Posted by: algorimancer Mar 30 2007, 01:15 PM

Let me address a few issues related to the micro-probe concept.

Communications. Need a light-weight directional radio antenna? How about a half-silvered mylar balloon, inflated with a minimal pressure slightly above vacuum. I'm sure this could be improved upon, so that the balloon material hardened over time and the internal pressure was no longer needed, allowing for degradation of the material over time (inevitable). Better yet, use a diode laser like that in a laser pointer - I've seen cheap kilowatt versions of these lately.

Ion Drive. These things have progressed a ways since DS1, and are a fair bit more efficient these days, meaning less propellant needed or more available delta-v from the same propellant.

Telescope. While I know there are inherent weight limits behind telescope optics (though see above under Communications for a light-weight solution), there is an old fashioned solution to the lack of zoom-lens problem... move closer to the target and take more pictures.

And don't forget that aero-capture still works for something at this scale.

As to space-rating, heck at the price of these things you can afford to waste a few dozen working-out the bugs. Start with low earth orbit, do the moon, asteroids, mars, then head for Neptune. If someone in the field got serious about it, these things could be flying within a few years. You could easily piggy-back a few at a time on conventional launches, especially those to geostationary orbit. Opt to use Windows Mobile as an OS and you might even be able to get Microsoft to fund the development for the publicity.

Posted by: tty Mar 30 2007, 06:54 PM

QUOTE (tasp @ Mar 30 2007, 05:58 AM) *
Imagine, if you will, an ion engined probe, already in Neptune orbit. Further, in view of Neptune's large Hill sphere, imagine the probe to be in an extremely elliptical orbit with a period of ~ 1 year. At apogee, even relatively minute adjustments to the orbit, made with the ion thruster perhaps over a period of a couple of months, will result in large controllable variations in the perigee portion of the orbit, ~6 months later.


I fully agree. The problem I wanted to point out is that orbit insertion means essentially to convert an orbit with apogeum at infinity to one with a finite apogeum. This has to be done at the perigeum where orbital speed is necessarily quite high and You have only a very short time to do it in - ergo a chemical rocket is needed. Theoretically You could do it by aerobraking, but doing it by a single short aerobraking means going deep into the atmosphere which means very high temperatures and probably some kind of an ablative heat shield.
Some kind of a balute might work, but it would weigh a fair amount too. Also I strongly doubt that we know enough about the Neptunian atmosphere to bring this off on a first try.

QUOTE
As to space-rating, heck at the price of these things you can afford to waste a few dozen working-out the bugs. Start with low earth orbit, do the moon, asteroids, mars, then head for Neptune. If someone in the field got serious about it, these things could be flying within a few years.


The trouble with that method is that You only get to know that you have bugs, not what they are, so you can't fix them. Or do you plan to include advanced diagnostics and telemetry in those dead-cheap cubesats?

Believe me, nobody wants to spend years and millions flight-rating and space-rating equipment, but (expensive) experience shows that it's necessary.

QUOTE
Better yet, use a diode laser like that in a laser pointer - I've seen cheap kilowatt versions of these lately.


Laser communication over such distances will require quite exact pointing of the laser. Not easy to do cheaply and "weightlessly". You would need either a very precise attitude control system for the whole probe or a very good pointing system for the laser.

Posted by: helvick Mar 30 2007, 09:22 PM

Clearly the challenges are pretty serious but I'm not seeing that the problems are insurmountable within a reasonable time frame ( a decade or so ).

tty - you clearly know much more about transfer orbits than me so how is the problem you describe solved by Dawn which is managing it's Vesta orbit insertion and departure without a massive "insertion burn".

I share your scepticism with regard to optical comms - the pointing\attitude control requirements get more severe as you scale down too so I really think that is too much of a leap. I remain convinced that a phased array antenna (or two, build em of micromachined silicon) can meet or exceed the capabilities of traditional high gain antennas and their solid state nature will also solve the comm signal pointing problem without needing attitude control. There is a ton of work going on in this area right now as highly power efficient low weight antennas are going to be a key enabling technology for the next wave of handheld wireless comms. Bumping up the DSN's gain at this end, ideally by building large earthside arrays on the scale of the SKA, is also an option although that is obviously a mega expensive approach. I have to admit that thttp://www.acm.caltech.edu/%7Ebruno/Antsos_IEEE_Aero_2006.pdf doesn't really share my opinion of phased array antennas but it does make a few other equally interesting potential alternatives to solving the bulky HGA problem.

As far as getting diagnostics out of the systems if they fail your point is true if they completely fail but if you follow Algorimancer's approach of building lots, test them close in first and have an improvement cycle built into your design and manufacture process then you should be able to make them work. Algorimancer's idea of using Windows Mobile (well it would be Windows CE Embedded probably) is definitely plausible - it is a highly manageable OS that is pretty damn good these days and cheap and easy to get and develop. As an OS - ignore the toy GUI nonsense you see when tapping on a PDA or smartphone - but under the hood it puts all the mainstream popular OS's to shame. Anyway - it really would be a great PR coup for a company like Microsoft - and a much better spend of Paul Allen's billions than SpaceShip One in my book.

The current approach of building $100's of millions of dollar satellites, probes and rovers _requires_ the hiper reliable space rating approach which does take years (decades) and multiplies costs even more. The suggestion here is to try to an alternative track with small light and _cheap_ systems and accept that they will fail a lot. Cubesats to Neptune is exactly the idea.

Some interesting info onhttp://homepages.cae.wisc.edu/%7Eblanchar/res/BlanchardKorea.pdf - page 20 implies that a ~300g micromachined RTG is good for about 40mW - thats a factor of 20 below my estimate which is a bit of a problem however the very next page indicates that Betavoltaics is already hitting 10x the conversion efficiency of micro-RTG's so that could (in theory) be within spitting distance of 1 watt within a mass budget of 250g.

Posted by: tty Mar 31 2007, 07:22 PM

QUOTE
you clearly know much more about transfer orbits than me so how is the problem you describe solved by Dawn which is managing it's Vesta orbit insertion and departure without a massive "insertion burn".


That is because Vesta is a small body. Think of it like this: if you start an (unpowered) orbit at infinity with zero speed you will arrive at a planet (or other body) with the velocity needed to escape to infinity again. For Earth this would be about 11 kms-1 (or a little less since You aim slightly to the side). There is no way to arrive any slower since this would imply a negative speed at infinity, i. e. going the other way.

Since Vesta's gravity field is almost negligible it is possible to shape an orbit where you arrive with practicelly zero relative speed. If Neptune was as small as Vesta it would be perfectly possible to use a ion drive to brake to orbital speed.

As for phased array antennas, yes this is a mature technology (at least the passive array variety) and it can indeed be used to steer a beam without the necessity of physically slewing the antenna. However it is inherently a rather complex piece of equipment requiring a large number of transmitting elements to get good directivity.

As a matter of fact I think that Messenger has a (passive) phased array antenna.

Posted by: nprev Mar 31 2007, 10:25 PM

Very, very innovative thinking here...love it! smile.gif

One thought/question: Phased arrays are probably constrained in size largely by the desired freq (higher the smaller as far as individual element dimensions). Assuming we were to go with freqs 'way up in the 20-30Ghz range or higher if feasible, what is the minimum number of elements needed to assure at least 1 kbps both ways from Neptune?

Posted by: Greg Hullender Mar 31 2007, 10:43 PM

I was going to argue the point, but doing my own calculations, I have to agree. Here's my example:

Suppose you take a minumum-energy orbit from Earth to Neptune. You'll arrive at the edge of Neptune's Hill Sphere will a relative velocity of 4.05 kps. If you wanted to orbit that far out (ignoring the Sun, of course), you'd need to kill 3.59 kps.

If you let yourself fall all the way to the cloud deck (but still assuming no friction) :-), you'll crank up to 23.84 kps, but now you need only 0.356 kps to get into an ellipse with periapsis at the cloud deck and apapsis at the Hill Sphere.

That's a huge saving! After the 30.6-year trip, you might not appreciate it so much, though.

So if you invest in just 0.35 kps more delta-v leaving Earth, you'll get to Neptune in 15.7 years, and you'll need (using the same cloud-deck maneuver) an extra 0.59 kps at the end. Less that one extra kps, and you cut the flight time almost in half!

To cut it to ten years, though, we need almost exactly 4.0 extra kps.

So what's an extra few kps cost? Well, if we use the specific impulse of the Cassini engines (308 sec, I think), then the 15-year trip needs 36% more mass at the start and the 10-year trip needs 3.7x more. Oops.

Every extra 2 kps roughly doubles the required launch mass. Ouch!

I haven't done the calculation to see what you could accomplish via a Triton fly-by (gravitational-assist-only), but I'm not optimistic.
I'll get to it this weekend maybe.

As before, these are my best effort (using Calculus, Wikipedia, and Excel), so unless someone checks these, don't use them to launch your own space probe. :-)

--Greg

Posted by: Greg Hullender Apr 1 2007, 03:54 AM

Okay, I figure the best you can do with Triton (short of aerobraking) is lose 0.11 KPS -- hardly worth the candle. The problem is that we're going very fast and it's very small. At most, it can deflect us about 6 degrees.

So coming back to ion drives, I note that one KPS is as good as another. If the ion drive can get you to Neptune's Hill Sphere with a relative velocity of ~4 KPS, you can use a conventional ~300-sec-specific-impulse rocket to do the final orbital insertion. It'll only need to be about 11% of the payload.

Posted by: tty Apr 1 2007, 05:07 PM

I agree with practically all you say. As for using Triton I couldatoldya it won't work. It's too small and too high in Neptune's gravity well. Using gravity assists for orbit insertion requires a big body close to the primary to be worthwhile. Not even the Moon is much help when getting into an orbit around Earth.

Probably the best strategy would indeed be a conventional rocket for orbit insertion and a ion engine for the long haul out and for subsequent orbital adjustment. Perhaps the best solution might even be an NH-type orbit using a Jupiter gravity assist, and then running the ion engine the last few years before arrival to lose speed.

Posted by: Greg Hullender Apr 1 2007, 07:27 PM

Oh I knew it wouldn't work, but that's different from being able to measure just how much it doesn't work. Someone had asked (weeks ago), and everyone said "it won't work," but (as far as I know), no one actually ran the numbers.

What did surprise me is just how much more delta-V it does take to get to the outer planets.

Minimum-energy transfer orbit to Jupiter takes right at 9 kps (not counting escape velocity from Earth) and just over 2.5 years.

Same thing to Saturn needs an extra 1.75 kps, so it costs about 1.8 times as much (assuming, naively, that fuel is the only cost). It also takes 6 years.

To Uranus, we need an extra 2.8 kps, for 2.5x the cost, waiting 16 years in the bargain. We can cut that to ten years, but the cost is then 3.1x the cost of sending the same probe to Jupiter.

To Neptune, we need 3 kps more than a minimal Jupiter probe, so the base cost is 2.7x, and it takes 30.6 years to get there. We can cut that to just 10 years for an extra 4 kps, but the cost is then 10.2x sending the same probe to Jupiter!

That said, how much of the cost actually is the fuel? What might be fun to compute would be how much mass the various SpaceX boosters could put into a minimum-energy transfer orbit to Jupiter. Then see how much that'd leave for probes to the various outer planets.

Of course I'm still leaving out the benefit you could get from using Jupiter for a gravitational assist. I wonder if you do better to burn any fuel at Jovian periapsis, or if (like New Horizons, Cassini, Galileo, etc.) the best bet really is to do a big burn up front and then one more at the end. (Well, NH won't do the one at the end, of course.)

--Greg

Posted by: Greg Hullender Apr 1 2007, 08:18 PM

Now I see why Nasa is so big on gravity assists; otherwise, the biggest Space X booster (Falcon 9 heavy, which is just slightly smaller than the Titan that sent Cassini to Saturn) can send about 230 kg to Jupiter. Cassini weighed over 20 times that much . . .

http://www.sstd.rl.ac.uk/news/cassini/mission/traj.html

This guy's numbers seem to agree with mine, so that's some confirmation. (Wikipedia seems to get it wrong http://en.wikipedia.org/wiki/Gravitational_slingshot by a factor of 1.6, which makes me wonder if someone did a metric conversion on an already-metric number.)

Anyway, the thing that keeps killing us is the relatively-low specific impulse of our engines. That nasty exponential is awful. No wonder people are excited about things like ion drives and space elevators -- even scramjets.

I realize this is all probably old hat to most folks here, but, as I said earlier, lots of people TALK about the limitations, but I never seem to see any concrete numbers.

--Greg

Posted by: tasp Apr 1 2007, 10:03 PM

I appreciate your doing the numbers on this. I have no talent for that, I just have to work with things that have been accomplished, and try new combinations of things.

Would a 2 month period at the far end of our Neptune orbit be sufficient time for a reasonable follow on Dawn type ion thruster to thrust a 'modest' probe into an interestingly different orbit?

Like an orbit plane change of 20 degrees, or speeding up or delaying the next closest approach to Neptune by 2 weeks or so?

Just enough to be 'useful'.

If we get far enough from Neptune, can the orbit go from prograde to retrograde ?

Posted by: Greg Hullender Apr 2 2007, 12:26 AM

Well, if we used one of the NSTAR ion engines that Dawn is using:

http://www.boeing.com/defense-space/space/bss/factsheets/xips/nstar/ionengine.html

It only adds 8 kg of mass other then fuel, and it has a spectacular 3100 seconds of specific impulse. For comparison, suppose we started with 28 metric tons of spacecraft in LEO (what SpaceX says their Falcon 9 heavy can do). With the 308 s we usually have to work with, that'll get a 250 kg craft to Jupiter. (Note that New Horizons is 480 kg.) But if we had 3100 sec, we'd get 17.3 TONS of spacecraft there. Put differently, if we wanted to send a New Horizons probe to Neptune in 5 years, we'd only need one ton of fuel for it.

But this is all based on assumptions that all our burns are essentially instant. That NSTAR engine only provides 90 mN of thrust. If we attached it to New Horizons, it'd take 60 days to make a 1 kps change in speed. I haven't quite figured out how to calculate how much this costs you.

I still don't think that sinks the boat; heck, at 8 kg per engine, why not attach 9 of them and get a 1 kps change per week? The initial burn for Jupiter would take 8 or 9 weeks, and that doesn't seem TOO terrible.

As for the arrival problem, in my super trip to Neptune, you arrive with 26 kps to kill. If you could do it in a quick burn, you could cut that to 11.5, but you haven't got 11 weeks, so you do it the hard way -- slowly losing 22 of them over the 22 weeks before the Neptune encounter. The last 4 you can handle with Hydrazine. This appears to add another ton of propellant to your cost.

This looks too good -- something must be screwed up. ;-)

--Greg

Posted by: mchan Apr 2 2007, 02:39 AM

Err, I am not up on reviewing the numbers in the recent posts. However, is NEP (nuclear electric propulsion, e.g., Prometheus) taken out of consideration here (due to cost)? I recall NEP outer planet mission studies from the 80's that don't require a large chemical rocket for orbit insertion. The approach is that of Dawn: spiral out under constant thrust until a reasonable transfer trajectory is reached, then cruise, then spiral in under constant thrust to essentially match orbits and rendezvous at the target. The drawback is that trip times are long. IIRC, it would take 12-16 years to reach Neptune.

Posted by: Greg Hullender Apr 3 2007, 01:35 AM

Oops! I included the mass of the engine, but not of the power source. Have to look into that. So I guess I'd say I was agnostic about solar vs. NEP, except that for Neptune, I can't take solar seriously.

As for the orbits, it's that "spiral" part that makes it hard. Haven't seen a solution for it, and I'm speculating it needs calculus of variations to solve it. I'll check with a classmate who became a real physicist. :-)

--Greg

Posted by: dvandorn Apr 3 2007, 02:47 AM

The problem with a low-thrust spiral-out from the Earth's orbit is that you spend an extraordinarily large amount of time within the van Allen radiation belts. Even hardened electronics can only take so much of that.

You run the risk of burning out your spacecraft before it gets very far away from Earth.

-the other Doug

Posted by: mchan Apr 3 2007, 03:50 AM

For a NEP mission, the reactor startup needs to be at altititude sufficient to allow most of the radioactive decay to occur before atmospheric decay should the something go awry. At least 1000 km which is starting into the most intense part of the inner belt. Moving the start out to 5000 km puts it beyond the inner belt. The outer belt intensities are an order of magnitude lower than the inner belt. But of course it costs gas to boost it to 5000 km from LEO.

Posted by: tasp Apr 3 2007, 02:43 PM

To refine this a bit further;

Would it save ion thruster propellent to use the manuvers at our farthest from Neptune to primarily modify our orbit to adjust the timing for the close Neptune approaches, and to utilize Triton encounters for inclination changes? Anything we can do to conserve consummables will let us achieve a longer arc following Neptune around the sun to study seasonal effects.

Additionally, we might want to look at possibly orbiting some of the 'outie' satellites of Neptune. We have experience to draw upon from the Eros encounter in orbiting very low mass objects. Seems like for the really small objects, an ion thruster might be a rather practical gadget to use to make/break/modify our orbit.

Posted by: Greg Hullender Apr 3 2007, 03:30 PM

I think the trick is to refrain from trying to use the ion thruster the same way we use a chemical one. Instead of diving into Neptune's gravity well with a hyperbolic excess of over 4 kps, we want to gently kiss the Hill Sphere, and almost immediately go into a huge, slow circular orbit, from which we gently spiral down.

Trouble is, the math for that is a bit challenging -- sort of just doing a numerical simulation. Not sure anyone has actually worked this out.

Anyway, I think that disposes of most of the diference between Neptune and an asteroid. If the numbers work out, anyway.

--Greg

Posted by: remcook Apr 3 2007, 03:37 PM

I've seen transfer orbits to Mars with low and constant thrust, I think even from the '60s. It boiled down to first accelerating, then decelerate, and then accelerate again, and you'll end up with about zero relative velocity. Not sure how long it will take though, but maybe on long distances like Neptune the low thrust option may be quicker.

Posted by: Cugel Apr 4 2007, 01:13 PM

Bruce Moomaw is currently writing a very interesting series of articles on the exploration of the non-Martian Solar System (his words). His latest article has some news about the Neptune Orbiter as the proposed 4th NASA flagship mission.

http://www.spaceblogger.com/reports/NASA_Contemplates_Future_Trips_To_Europa_And_Neptune_999.html

Posted by: algorimancer Apr 4 2007, 05:26 PM

>Trouble is, the math for that is a bit challenging

One approach that might simplify this is to look at it in terms of changes in energy rather than orbital mechanics, at least for relatively circular orbits (or locally circular spirals). Assume that the energy expended by ion drive exhaust translates to changes in potential energy (equivalent to orbital radius) with regard to the body being orbited, assuming that the thrust of the ion engine is relatively orthogonal to the gravity vector. I recall seeing some elegant solutions to a variety of problems dealt with in this manner.

Posted by: helvick Apr 4 2007, 08:05 PM

Bruce's article describes a Neptune orbiter mission with a 15 year transit time to Neptune based on a similar idea to the one that was developing here, although his example does deal with more realistic mass scales and tech that is very close to being available. If you take a relatively low mass craft that you launch outwards with a significant initial chemical kicker, then use a Radioisotope Electric Propulsion system (ie something like an RTG+Ion drive) to slow you down in the latter half of the mission and then use a small chemical engine for insertion braking that would require only about 10% of the final orbit mass for braking fuel. The model he describes is a 500kg dry weight orbiter, ~400kg of Ion fuel most of which will be used up in the approach deceleration phase and a 750watt Stirling RTG power source (weight about 250kg), leaving you ~250kg to build the chasis, comms, command systems and the instrumentation.

_Assuming_ the components could be scaled down, the orbital equations should scale linearly with mass and this would indicate that ~1kg system could deliver a 100g "probe" to that range in ~15 years without breaking any fundamental laws. I absolutely accept that it is fantasy for now but I can only hope that something like this will eventually be built.

Posted by: tty Apr 4 2007, 08:28 PM

QUOTE (Greg Hullender @ Apr 3 2007, 05:30 PM) *
I think the trick is to refrain from trying to use the ion thruster the same way we use a chemical one. Instead of diving into Neptune's gravity well with a hyperbolic excess of over 4 kps, we want to gently kiss the Hill Sphere, and almost immediately go into a huge, slow circular orbit, from which we gently spiral down.


The difficulty with that idea is that it is just as hard to get down into a gravity well from a high circular orbit as to is get out of it up to the orbit. On the other hand if you get into a highly elliptic orbit you get to see both the near and far parts of the circum-neptunian area and you can still use the ion thruster to adjust the perineptunium and the orbit generally.

Posted by: Greg Hullender Apr 4 2007, 09:16 PM

True, and it actually requires a good bit more delta-v to do it via a spiral, BUT, it doesn't take 10x as much, and the ion drive does have 10x the specific impulse of a chemical thruster.

Using the ion drive to get into a highly elliptical orbit on purpose is a bit more challenging. (Mathematically, I mean.) Better would be if you could use aerobraking to kill 1/2 kps or so at Neptune and then let the Ion drive raise your periapsis enough before the next pass, but figuring out a reasonable amount of aerobraking to expect is another hard problem.

On the Van Allen Belt front, I note that using a chemical rocket (309 s specific impulse) to go from LEO to 5,000 km up, you'll lose roughly half your payload. So if you started with 28 tons in LEO, you'll only have 14 at 5000. If you used an ion drive, on the other hand, you'll keep essentially all of it. So my question is, would you really need 14 tons of shielding if you opted for a lesiurely trip through the belts?

--Greg

Posted by: volcanopele Apr 4 2007, 09:43 PM

I don't know if this was mentioned earlier, but there is a good description for a Neptune orbiter mission in the Decadal Survey white paper on the Exploration of the Neptune System:

http://www.aspbooks.org/publications/272/neptune_final.pdf

Posted by: helvick Apr 4 2007, 11:01 PM

To return to a question nprev asked earlier about data rates at Neptune.

Thttp://etd.caltech.edu/etd/available/etd-09302005-024349/unrestricted/thesis_xiang_final.pdfis a long, long way from developing into a space rated system that could be used for S/C comms but it does describe 24Ghz and 77Ghz solid state phased array antenna systems with total gains between 41 and 61dB from solid state SiGe arrays with total physical dimensions < 1 cm^2.

For the sake of argument lets take MRO's X-Band comms as a baseline and work down and out.
At max Earth/Mars range (400m km) MRO gets about 650kbps using a 100 watt amp and a 3m parabolic antenna (~ 47dB gain).

(Assuming that overall gain and efffective data rate are linearly related which is more or less the case)

Drop the Tx power to 1 watt, 100x loss => 6.5kbps.
Change the antenna from 3m Parabolic (47dB) to a solid state phased array (41 dB gain), ~4x Loss => 1600bps.
Change due to the range at Neptune (4600m km) ~132x Loss => 12bps.
Change to Ka Band. ~4x gain => 48bps.

So my 100bps WAG wasn't far off. A spacecraft antenna gain improvement to ~60dB would raise that to 5kbps and if the DSN was updated to arrays of 400 x 12M antennas we would see a further 10x gain which would bring things to 50kbps. All of that from a base transmitter at Neptune that is about as powerful as a cellphone.

Comparing this approach to New Horizons comms seems like a smart idea since we know what those should be. NH will give us 300-600bps at Pluto using a 12watt Tx amplifier and a 2.1m antenna.
So start with MRO's 650kps, 100watt, 3m parabolic at 400m km.
Drop the Tx power to 12 watt, 8x loss => 78kbps.
Change the antenna from 3m to 2.1m Parabolic, 3dB/ ~2x Loss => 39kbps.
Change due to the range at Pluto (~4600m km) ~132x Loss => 295bps.
Looks right to me.

Posted by: algorimancer Apr 5 2007, 05:27 PM

Back to the propulsion question, one of the CubeSats was to have tested a Micro Vacuum Arc Thruster, which might be relevant, though I think not as efficient as the larger Xe ion drives as used in Smart 1, etc., see

http://cubesat.atl.calpoly.edu/media/Documents/Papers/arc_thruster_paper.pdf

If you can pull-up reference 3 of that paper (in Review of Scientific Instruments, if you have access), it states that "scaling existing electric propulsion engines such as Xe ion engines ... down to 1-10 W power levels is not practical. The unavoidable overhead of propellant storage tank, flow controls, and plumbing in Xe ion engines ... makes their overall efficiency unacceptably low at these power levels." (but they don't cite a reference).

These scaling issues strike me as directly addressable via MEMS/Microfluidics. I'm not sure about a fuel tank for Xenon - I gather this is normally stored as a liquid under high pressure. Am I incorrect in recalling that it becomes easier to store something at high pressure as the volume becomes smaller? I would envision something like a 10 cm carbon fiber composite tank as a feasible option, or perhaps several smaller tanks for redundancy.

It sounds like helvick has addressed the power & communications issues. Seems to me like this notion is sounding progressively more feasible, particularly if the probe can piggyback to geosynchronous orbit with a comsat launch. Construction + launch ought to be in the $100K to $200K range, which sounds amenable to donations.

Posted by: Greg Hullender Apr 6 2007, 12:07 AM

I was surprised by Bruce's implication that New Horizons was comparable to Cassini in terms of its scientific capabilities. I seem to remember that Cassini weighed about 6 tons at SOI, half of which was fuel (and half the fuel was spent on SOI), while NH weighs a bit under half a ton. I know technology has gotten better, so I'd expect us to be able to make equally capable probes for less, but is it THAT much less?

Do any of our experts have a ballpark guess as to how light a Cassini-equivalent probe would be if we built it with modern technology?

--Greg

Posted by: helvick Apr 6 2007, 01:16 AM

At the risk of going waaay off topic here I just have to wax lyrical on just how dramatic the changes have been over the 10 years that elapsed between the launches of Cassini and New Horizons. To put the capability delta in perspective it is worth looking at just how much other things have changed -

General purpose consumer CPU's

1997 - Averge CPU speed 200Mhz single core. Pentium II andPentium Pro.
2006 - Average CPU speed 3Ghz, dual core. Dual Core P4 CPU's
Delta - 30x.

Consumer PC Hard Drive capacity.
1997 16.8GB (initial introduction of 16.8GB IBM Desktstar using Giant Magenetoresistive hHeads)
2006 500Gb
Delta 30x.

Solid State Memory - Compact Flash
1997 - 64MB
2006 - 4GB
Delta 62x

Digital Cameras
The very first megapixel resolution consumer cameras arrived at the very high end of the market in 1997 ($1000+). Just think for a second how much that technology has changed and then think how fantastic the Cassini pictures still are given the fact that the cameras predate those 1997 megapixel clunkers by a few years.

Cellular Telephony
The progress in the cellular telephony market should also give you some idea of the scale of the change in the sophistication of technologies in the last decade. The improvement in the basic capabilities of cellular handsets in the intervening period is hard to describe - in 1997 most countries had a cellular market penetration well below 20% however by 2006 it was at 90% or higher for more than half of the planet including much of the so called 3rd world with annual handset sales exceeding 1 billion units for the first time. In 1997 SMS messaging was a novel technology that few people had ever even heard of but by 2006 it was a technology in daily if not hourly use by the majority of the population in Europe and Asia with the Americas catching up fast. In 1997 a handset's "standby" battery life was typically 2-3 days but by 2006 10+ days standby time was not unusual for similar devices. In 1997 reliability was suspect at best and expected connectivity SLA's were in the region of 95% for first time connection success, by 2006 that had exceeds 99.9+% for most mature markets. Between 1996 and 2007 costs per unit call have dropped by a factor of 10 or more while the scope of services has increased dramatically (now voice + high speed data costs << voice alone cost in 1997).

Digital Music
In 1996 very few people realised that you could actually record\copy\process\listen to music using a computer, personal digital music players _did not exist_. The MP3 format was first patented in the US in 1996. By 2006 there were more than a billion personal digital music players in daily use.


Conculsion
Personally I'm surprised that New Horizons is so big.

( smile.gif Only joking. John\Alan and the team, I think she's the bomb really).

Posted by: helvick Apr 6 2007, 01:37 AM

QUOTE
The unavoidable overhead of propellant storage tank, flow controls, and plumbing in Xe ion engines ... makes their overall efficiency unacceptably low at these power levels." (but they don't cite a reference).

This doesn't make sense to me. It's decades since I did any solid materials\fluid dynamics work but I'm certain that a linearly scaled smaller pressure vessel will be able to handle progressively higher pressures.

That said it is true that it is harder to build a 1cm diameter fluid management system of a given complexity than (say) a 10cm diameter version of the same thing but the reason for that is that mechanical precision gets harder as machining sclaes diminish not that pressure systems become problematic as scales diminish.

MEMS\Microfluidics should be able to provide a solution to all of these problems someday. The general impression that I get is that folks think this is all pie in the sky (at leat for now) but it is worth noting that the microfluidics capabiliies that have been perfected over the past 15 years of Inkjet system development are more than capable of managing a fuel delivery system with the precision and finesse that we require for this sort of engine. Single nozzle control in the pico-litre ( 1E-12 ) scale is a reality today for example. The problem is that all our current technology is geared at doing this for un pressurized room temperature fluids at the earths surface and not for pressurized low temperature fluids in space\vacuum.

Posted by: nprev Apr 6 2007, 01:43 AM

QUOTE (helvick @ Apr 5 2007, 06:37 PM) *
The problem is that all our current technology is geared at doing this for un pressurized room temperature fluids at the earths surface and not for pressurized low temperature fluids in space\vacuum.


Boy, does THAT sound like a fruitful grant proposal topic for one or more people in our forum (hint, hint.... smile.gif ) This might be a key enabling technology for UMSF that would have extremely broad applicability...too bad I ain't smart enough to invent it & get embarrassingly rich off of the patent...

Posted by: Greg Hullender Apr 6 2007, 04:07 AM

Algorimancer and nprev: What am I missing here? Why is it desirable to scale the ion engine down below 10W? It seems that the engines only weigh about 8 kg now. Or is the issue with what the power source weighs?

On that topic, I do think there's way too much angst over using nuclear power for this, largely sparked by the kooky anti-Cassini protests of a few years ago. I note that New Horizons didn't draw any protesters to speak of, and I doubt any subsequent missons will either.

--Greg

Posted by: nprev Apr 6 2007, 04:20 AM

I might be off-base here entirely, but based on the discussion & Helvick's usual incredible technical insight it seems as if it might be possible to design microthrusters with low propellant consumption that have very high specific impulses which could propel microsat payloads (enabled by quantum leaps in electronics technology) to outer system targets within reasonable timeframes with consequent very significant cost savings.

Such devices might also find a considerable commercial market for boost to GEO from LEO as well as station-keeping applications. With respect to the latter, propellant consumption is now the chief limiting factor for satellite longevity due to improvements in payload/bus electronics reliability, so obviously increasing this margin would have a tremendous positive effect.

Posted by: djellison Apr 6 2007, 08:30 AM

Where are you going to get the power from...AND....where do you put the instruments - not seen a single mention of those yet in terms of mass, volume or power. I've not seen anyone, realistically, add up the figures here yet - just immediately jumping to "well - if the tank can be only 10cm then the whole thing will be 50 pence and return amazing science". You're jumping about 25 steps too far forward. You're talking about MRO's X-Band. That's 100 Watts. Where are you going to get 100 Watts from at Neptunian distances? That's just about 1/2 of NH's power budget gone right there. The mass of that power supply alone is going to be an order of magnitude above what you're talking about. So your delta V is going to be an order of magnitude less.

If you're going to do the outer solar system, as small, light and cheap as possible - the only logical place to start is New Horizons, the most advances, lightest, cheapest outer planetery spacecraft to date, and work from there.

Doug

Posted by: edstrick Apr 6 2007, 09:06 AM

(evil grin)
I got it all figured out...
A dilithium crystal ore body is what's powering all the thermal activity and plumes on Enceladus.
We go there and mine it so we can explore the rest of the solar system.
(you can shoot me now)

Posted by: helvick Apr 6 2007, 11:23 AM

Doug,

Just to be clear - I don't think any of this is possible today. What I'm trying to work out for myself is whether this is closer to reality or fiction and whether R&D efforts aimed at maturing these types of technologies would be worthwhile or not. Certainly any deep space probe that is going to be planned and built today or in the near term (5 years) is going to be based on proven technologies that are mature on the ground today and will result in something in the 100-2500kg mass range.

This works but it results in having rare (once per decade) and very expensive missions and more importantly (to me) it is very inefficient at developing\maintaining general purpose solar system exploration capabilities. The reliance on mega budgets and the rarity of missions means that there is always a cancellation risk and a single cancellation can result in an entire generation of scientists missing out on any significant deep space exploration experience. That is why Algorimancer's suggestion appeals to me - if it could be done then it could change the economics of this endeavour dramatically.

I think you are not being totally fair about my arm waving numbers. smile.gif You are right though that there should be less arm waving and more presenting things with precision so they can be properly judged and torn apart if they are wrong.

However they aren't as weak as you describe - let me re-state my telecoms example since you specifically call that out.
I used MRO's X-Band system as a baseline and then worked out from that what the practical bit rate for a highly power constrained transmitter around Neptune would be. I used their published data rates because that allows me to work out the comms performance of the DSN without knowing anything about how the DSN itself performs.
I then confirmed that my method made sense by applying the same transformations to the New Horizons comms design and came up with a number (~300bps) that agrees with the number published by the NH team for their bit rate at Pluto.
So I am certain that a 1 watt Ka-Band transmitter with a fairly small antenna (1m parabolic) at Neptune could communicate with the DSN at 50bps today if we could put it there. That is poor but it is as good if not better than the crippled rate Gallileo had to work with.
Edited to add: http://spreadsheets.google.com/pub?key=pNkQ_0mEEOPo_sM9B-9ZDWgwith the two sets of comms rate estimates. http://spreadsheets.google.com/pub?key=pNkQ_0mEEOPo_sM9B-9ZDWg&output=xls format for those who do want to tear it apart.

Posted by: algorimancer Apr 6 2007, 02:22 PM

QUOTE (Greg Hullender @ Apr 5 2007, 11:07 PM) *
Algorimancer and nprev: What am I missing here? Why is it desirable to scale the ion engine down below 10W? ...

We've been discussing developing a moderately competent spacecraft capable of exploring the outer solar system which masses on the order of a few kilograms. This implies low power availability, as well as a few other challenges w.r.t. propulsion and communication. Current ion engines have good performance, but require too much power and are too massive for a vehicle of this scale. Otherwise they're great smile.gif

Doug, I'm in agreement with helvic in that the motivation for this discussion is a transformational change in the way we develop spacecraft, which would make deep space exploration trivially inexpensive and accessible. I've been attending a lot of biomechanics/bioengineering conferences these past few years where I've seen a lot of the current work in the MEMS/Microfluidics realm, and I am genuinely convinced that these capabilities are within reach. I think that if we could motivate the right group of people working with this technology (or if we could come up with the money to fund them) we could be manufacturing these sorts of vehicles in the next five years, and while we might not send a CubeSat to the outer solar system, we could certainly use it as a container vehicle during the launch. There are still details to be sorted-out (obviously) before we're in a position to firm-up specific masses for the various components, but considering the capabilities of my Windows Mobile driven cell phone, I don't see a problem with including such instruments as a several megapixel (telephoto even) camera and particles/fields instruments within the mass constraints. It appears thus far that hardware scales linearly, while performance (delta-v capability) remains constant.

I'm looking forward to working on the spacecraft software smile.gif

Posted by: centsworth_II Apr 6 2007, 02:47 PM

Sorry I'm late, but I've got to know:
While waiting on new technologies
(and a budget) for a Neptune orbiter,
why not send NH2 on a flyby?

I can't think of another mission that the
craft would be better suited for. They aren't
going to let it just rot on the shelf, are they?

Posted by: djellison Apr 6 2007, 03:02 PM

QUOTE (algorimancer @ Apr 6 2007, 03:22 PM) *
don't see a problem with including such instruments as a several megapixel (telephoto even) camera and particles/fields instruments within the mass constraints


QUOTE
So I am certain that a 1 watt Ka-Band transmitter with a fairly small antenna (1m parabolic) at Neptune could communicate with the DSN at 50bps today if we could put it there


4 megapixels, R, G, B, 12 bits, 20:1 compression. 40 hours of downlink. I see a problem.

Lets talk local - no one's put multiple megapixels on any cubesat in LEO, when you've got (comparatively speak) orders of magnitude more power and downlink that what you speak of.

Helvick - your 650kbps - is that for the 100w Xband or the 35 Ka Band on MRO? Are you scalling apples with apples here? And have you visited http://descanso.jpl.nasa.gov/DPSummary/summary.cfm?force_external=0 - it says that at Max range, the Ka band can actually manage only 331kbps, and the Xband, just over 500kbps ( so 600 seems reasonable ) - That document also tells us that the 35W Ka band on MRO requires 81W of actual spacecraft power. The 100W Xband, 172W of actual power - your 1W Ka band is now looking at more like 1.5 watts of consumed power, with a downlink of 25bps. That one colour frame now takes 80 hours.

I grant you - something smaller than NH is possible with near-future technology....but in the Microsat ( 50-100kg 50-100W ) bracket, not the multiples of cubesat ( 1-3kg and a few watts ). Pick a target of 100kg and then decide what power you can get, by what means, from that. Even then you're talking about technology found in PDF's - not in ATLO. The Stirling RTG is something like 34kg, for 110W - that would be my starting point.

Doug
(PS - NH2 was proposed by Alan et.al. http://www.boulder.swri.edu/pkb/NH2_community_info.ppt - but sadly turned down sad.gif A real pity as the Voyager/Pioneer/Viking/MER/Mariner syndrome of getting two for less than the price of two would have kicked in here )

Posted by: Greg Hullender Apr 6 2007, 08:23 PM

When I first read a proposal for "Grand Tour," which later became Voyager, one of the options was to use a laser for the comm link instead of radio. The biggest objection was that you had extra cost because you'd need a satellite on this end to receive the laser and then beam it to the ground with regular radio waves.

Forty years later, I never seem to see this discussed. Is there some other reason that makes it nonviable? I'd think we could put a receiving satellite in Geosynchronous orbit and design it to cope with up to a half-dozen missions at once. Would that really be so expensive amortized over a few missions?

Note: I'm NOT proposing running a really long fiber-optic cable out there! :-)

On a separate note, I just discovered this report on NASA's advanced propulsion efforts.

http://books.nap.edu/execsumm_pdf/11432.pdf

I thought it was very apropos, and not too out of date (copyrights between 2004 and 2007).

--Greg

Posted by: nprev Apr 6 2007, 10:28 PM

The cancelled Mars Telecommunications Orbiter would have tried to demonstrate lasercomm DTE...think that's looking more like a real loss in terms of technology development every day.

Every so often I drag up that idea I had (which was actually first described in the 1930s) for putting DSN-augmenting data relay sats at the Earth/Sun L4 & L5 points. Maybe we need another set at Jupiter's Lagrange points for outer system support. Although that doesn't shorten the range all that much, at least we can avoid some significant atmospheric attenuation...plus, if they happen to carry a couple of cameras to check out any Trojan asteroids they may pass near from time to time, that'd be cool, too... wink.gif

Posted by: JRehling Apr 6 2007, 10:42 PM

QUOTE (nprev @ Apr 6 2007, 03:28 PM) *
Maybe we need another set at Jupiter's Lagrange points for outer system support. Although that doesn't shorten the range all that much, at least we can avoid some significant atmospheric attenuation...


That doesn't shorten the distance at all when eg, Neptune is on the other side of the Sun from Jupiter. They're not even that close to Jupiter.

Posted by: helvick Apr 6 2007, 10:48 PM

Algorimancer sugggested the multi-megapixel camera not me, Boss.! smile.gif

That's a really interesting link - it does force me to correct errors in my baseline assumptions and that will push the data rate down to ~25bps. I really will have to dig out more info on those phased array antennas.

The Ka band numbers are strange - especially the min data rate of 331kbps. Oviously they must be correct but I would have thought that the S/N ratio would be much better even after reducing the Tx power to ~35%. Lots of reading.

Ah well we just have to ask the VLA to lend a hand like they did for http://deepspace.jpl.nasa.gov/dsn/history/dsn67.html That apparently gave V2 the same downlink rate at Neptune as it had at Saturn - which would be a 10x or more gain.

Posted by: nprev Apr 6 2007, 10:55 PM

QUOTE (JRehling @ Apr 6 2007, 03:42 PM) *
That doesn't shorten the distance at all when eg, Neptune is on the other side of the Sun from Jupiter. They're not even that close to Jupiter.

True, of course; the maximum possible advantage would only occur when Earth, one of Jupiter's Trojan points & Neptune were all at opposition to each other, and even then we'd only get a repeater boost to compensate for the last half-billion miles...lots of constraints there, might not even happen more than once every few thousand years or so. (Hidden agenda revealed...wanted to get closeup looks at the Trojan asteroids, looking for 'ride-on' extra mission justifications! tongue.gif )

Posted by: Greg Hullender Apr 7 2007, 03:49 AM

Frankly, I proposed a geosynchronous Earth orbit simply because it'd only need one ground station to support it. Might even be a step towards replacing the Deep-Space Network, assuming it were feasible to switch to all-laser missions after some point.

Anyway, one good question would be "ignoring the other issues, could a laser communications link be substantially lighter than a radio one for the same bandwidth?" Does anyone either know or know a good reference?

--Greg

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