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?

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

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.

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?

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

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.

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.

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

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

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

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.

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

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.

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.

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.

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.

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.

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.

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

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.

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.

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).

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.

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.

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.

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.)

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!

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.

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.

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.

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.)

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.

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.

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.

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.

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

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.

A simple Google search reveled this!?

What Probe concept was this?

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

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!)

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.

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.

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.

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.

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.

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.

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