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Neptune Orbiter, Another proposed mission
Rob Pinnegar
post Nov 11 2005, 01:36 AM
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QUOTE (tasp @ Nov 10 2005, 07:09 PM)
Has anyone considered the advantages and disadvantages of either a prograde or retrograde orbit for a Neptune orbiter?

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

Hmm. That's an interesting point.

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

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

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

Triton isn't just the main event -- it's pretty much the only event. There is no "second stage" (a la Ozzfest) at Neptune. So if a retrograde orbit is preferable for studies of Triton, then we might as well make it a retrograde orbit -- even if aerobraking is used instead of capture.
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tedstryk
post Nov 11 2005, 02:28 AM
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QUOTE (Rob Pinnegar @ Nov 11 2005, 01:36 AM)
Triton isn't just the main event -- it's pretty much the only event. There is no "second stage" (a la Ozzfest) at Neptune. So if a retrograde orbit is preferable for studies of Triton, then we might as well make it a retrograde orbit -- even if aerobraking is used instead of capture.
*


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


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Guest_BruceMoomaw_*
post Nov 11 2005, 04:01 AM
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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.
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tasp
post Nov 11 2005, 04:11 AM
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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.
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Guest_BruceMoomaw_*
post Nov 11 2005, 04:53 AM
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I may have something to say on THAT subject in my "Astronomy" article.
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Rob Pinnegar
post Nov 11 2005, 06:22 AM
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QUOTE (tedstryk @ Nov 10 2005, 08:28 PM)
I don't know if I would go that far.  For one thing, a flyby during approach of Neried (a la Cassini at Phoebe) would be nice.  And Proteus and the others may prove to be fragments from the former Neptunian system before Triton's capture, which would be quite interesting.

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

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

Triton'll still put 'em all to shame, though. tongue.gif
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JRehling
post Nov 11 2005, 02:23 PM
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QUOTE (tasp @ Nov 10 2005, 09:11 PM)
I'll digress to Uranus orbiting briefly.

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

Idea:

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

*


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

Arriving near equinox will allow the kind of illumination change that would be highly desirable. Unfortunately, equinox is coming up soon, and we're obviously not going to make it in time. So this plan won't possibly come off before 2050. At which time, I'll be on soft foods.
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tedstryk
post Nov 11 2005, 03:01 PM
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QUOTE (JRehling @ Nov 11 2005, 02:23 PM)
Unfortunately, equinox is coming up soon, and we're obviously not going to make it in time. So this plan won't possibly come off before 2050. At which time, I'll be on soft foods.
*


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


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tasp
post Nov 11 2005, 03:12 PM
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QUOTE (JRehling @ Nov 11 2005, 08:23 AM)
If the craft arrives near solstice, it will be irrelevant: each world will be half in decades-long dark, and every flyby will illuminate the same half. An orbit that is modestly inclined WRT the moons' could eventually give a good look at each moon, leaving us equally (and eternally?) ignorant of their other hemisphere. And we would also miss out dramatic high-phase illumination of most of the hemisphere we see -- note that all of Cassini's long-range view of the icy satellites of Saturn with the terminator in different places is eventually going to help us build DEMs of their topography.

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



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

Voyagers are coming up on 30 years longevity, so craft lifetime of 20 years at Uranus might be feasible. Way cheaper than two missions.
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JRehling
post Nov 11 2005, 04:45 PM
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QUOTE (tasp @ Nov 11 2005, 08:12 AM)
Can 'Uranus shine' help out illumination of moons?  Cassini took some pix of Iapetus that way, might help when equatorial plane of Uranus is perpendicular to orbital path around sun.  20 year mission life at the target gets you 90 degrees around the sun, that  would help global coverage of satellites, too.

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


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

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

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

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

90 degrees does help, though not as much if it's on either side of a solstice, in which case, you see the same extra real estate later as you did sooner, so a lifetime of 10 years would be almost as good.
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Rob Pinnegar
post Nov 11 2005, 05:11 PM
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QUOTE (tedstryk @ Nov 11 2005, 09:01 AM)
That was my point.  Get a flyby craft there while Uranus is near equinox.  I doubt it is even possible to get an orbiter there that quickly and break it into orbit (not to mention design it).
*

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

It's becoming fairly evident that we need a Uranus thread here to complement the Neptune one.
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Jeff7
post Nov 11 2005, 05:20 PM
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QUOTE (tedstryk @ Nov 10 2005, 02:13 PM)
If an orbiter is a long way off, I think a Triton flyby would be very useful, as it would allow for change detection since Voyager and, of course, when the orbiter finally gets there, its data can also be compared.  Unlike the other moons of these two planets Triton is a dynamic world.  Of course, the other issue is that the coverage of the Uranian moon's souther hemispheres is going to get poorer and poorer the longer we wait.
*


Well who knows. Saturn's system has shown interesting things on "inactive" moons - Enceladus' fairly young tiger stripes for instance. There always seems to be more to be seen.
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Guest_BruceMoomaw_*
post Nov 11 2005, 10:18 PM
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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.)
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tasp
post Nov 12 2005, 02:26 PM
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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!
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tedstryk
post Nov 15 2005, 11:59 PM
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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.



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