Neptune Orbiter, Another proposed mission Options

[Rob Pinnegar]

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?

[elakdawalla]

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

[tedstryk]

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.

[Rob Pinnegar]

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?

[elakdawalla]

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 . 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_meetin...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

[JRehling]

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.

[tedstryk]

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.

[RNeuhaus]

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

[ljk4-1]

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.

[tasp]

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'.

[RNeuhaus]

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

[JRehling]

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.

[Decepticon]

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

[tasp]

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.

[tasp]

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.

[Rob Pinnegar]

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.

[tedstryk]

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.

[Guest_BruceMoomaw_*]

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_meetin...eptune_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.

[tasp]

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.

[Guest_BruceMoomaw_*]

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

[Rob Pinnegar]

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.

[JRehling]

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.

[tedstryk]

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).

[tasp]

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.

[JRehling]

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.

[Rob Pinnegar]

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.

[Jeff7]

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.

[Guest_BruceMoomaw_*]

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.)

[tasp]

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!

[tedstryk]

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.

[Rob Pinnegar]

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.

[hendric]

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.

[Rob Pinnegar]

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.)

[tedstryk]

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.

[Rob Pinnegar]

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.

[tasp]

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.

[David]

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.

[tasp]

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.

[Bob Shaw]

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

[tasp]

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.

[Decepticon]

A simple Google search reveled this!?

What Probe concept was this?

http://www.astro.univie.ac.at/~wuchterl/Ku...er_probe_02.jpg

[Guest_BruceMoomaw_*]

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_meetin...eptune_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!)

[Decepticon]

I hate anything that Unfurls.

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.

[Guest_BruceMoomaw_*]

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.

[Guest_vjkane2000_*]

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.

[edstrick]

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.

[mchan]

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.

[edstrick]

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.

[tasp]

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.

[dvandorn]

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

[Toma B]

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...
So it would take gazillon passes through it to slow down significantly...
Solution : USE ROCKET ENGINES!!!

[chris]

Solution : USE ROCKET ENGINES!!!

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

[edstrick]

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.

[Toma B]

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?

[helvick]

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.

[chris]

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

[Toma B]

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...

[paxdan]

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.

[ljk4-1]

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?

[tasp]

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}

[chris]

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 Wikipedia:

"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

[JRehling]

Mars atmosphere is 143 times less efficient in aerobraking then atmosphere of Earth is...
Triton atmosphere is 467 times worst then Mars...
So it would take gazillon passes through it to slow down significantly...

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.

[Toma B]

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...

[chris]

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

[ermar]

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?)

[Guest_BruceMoomaw_*]

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.

[Toma B]

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:

"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...
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...?

[Guest_BruceMoomaw_*]

Sure: http://www.lpi.usra.edu/opag/jun_05_meetin...eptune_API1.pdf

http://trs-new.jpl.nasa.gov/dspace/bitstre...5/1/98-0675.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/dow...inal_011205.pdf (pg. 22-36).

[tedstryk]

There's also a description of a Triton lander design that could work for this mission in
http://solarsystem.nasa.gov/multimedia/dow...inal_011205.pdf  (pg. 22-36).

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

[Guest_AlexBlackwell_*]

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 Juno New Frontiers-class mission, there is an interesting passage relating to a Neptune orbiter concept:

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."

[Rob Pinnegar]

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.

[JRehling]

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.

[Chmee]

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.

[Guest_AlexBlackwell_*]

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.

[ljk4-1]

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.

[Guest_Analyst_*]

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.

[antoniseb]

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.

[Stephen]

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

[Greg Hullender]

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?)

[helvick]

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.

[ljk4-1]

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.

[TritonAntares]

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

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

[qraal]

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

[mimile]

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.

[Rob Pinnegar]

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.

[nprev]

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.

[JRehling]

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.

[nprev]

Good point, JR. In fact, Nereid spins pretty fast: 11.5 hrs per this paper, so if a low relative velocity inbound encounter could be arranged, you'd get a very good global view of it during one visit.

[tasp]

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 . . . .

[mchan]

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.

[tasp]

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 . . . .}

[JRehling]

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.

[tasp]

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?

[Rob Pinnegar]

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.

[TritonAntares]

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...
Triton should be the strangest world in the solar system concerning seasonal changes, even if the seasons are quite long out there...

THX & Bye.

[mchan]

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.

[TritonAntares]

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... ?
Anyhow, Triton's seasons don't look that simply predictable as Earth's....

Bye.

[tedstryk]

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.

[Rob Pinnegar]

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.)

[tasp]

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.