Uranus Orbiter, The other proposed ice-giant mission Options

[Guest_BruceMoomaw_*]

Well, shucks, how can anybody not jump at a bargain like that? However, they receive that journal at U.C.-Davis, which is within my driving range, so I'll photocopy it next time I'm down there. (That may be a while, though -- the advent of the Internet has tremendously reduced my need to go traipsing regularly to various university libraries for my astronautical information needs.)

[Guest_AlexBlackwell_*]

For $450, Amazon claims it can deliver a copy of the proceedings...

That's nuts.

I could post a scanned version of the Heaton and Longuski paper here (temporarily) but I'll let Doug decide since it's his forum. Of course, I'm assuming he's reading this thread.

[tasp]

That's nuts.

I could post a scanned version of the Heaton and Longuski paper here (temporarily) but I'll let Doug decide since it's his forum.  Of course, I'm assuming he's reading this thread.

I confess a burning desire to see the whole paper.

An amazing finding I hope is employed in future exploration plans of Uranus.

Comparing the Uranian staellites close up to all the other moons would synergistically expand our knowledge of all the objects.

[Guest_AlexBlackwell_*]

I confess a burning desire to see the whole paper.

I dropped a line to Doug but I don't think he saw he it before his sojourn to Spain. I don't want to post it here without his permission. I guess I could email it to interested parties; however, maybe we should wait on Doug's return, especially since I don't know what the potential demand is.

EDIT (11/23/05): Anyone (as long as the number remains reasonable) who wants a scanned PDF version can drop me an email via this board and I'll send it via my own personal email account. A few members already have taken this route.

This post has been edited by AlexBlackwell: Nov 23 2005, 08:07 PM

[tasp]

Darn, wouldn't you know that I don't have a copy of the paper available on my system at the moment 

That said, I'll review it when I get home but from what I recall, the main concept behind the end-of-tour plan to insert into orbit at Ariel is based on orbital pumping and cranking, which not only changes inclination from the initial Uranian insertion, but reduces relative velocity so that the deterministic delta-V for insertion at Ariel can be handled by the orbiter's propulsion system.  I can't remember whether the scheme also utilizes any "third-body" effects from Uranus or "fuzzy boundaries" but these might possible as was planned for the original Europa Orbiter insertion in europan orbit.

Whether this resembles, even in a broad sense, the MESSENGER trajectory through the inner solar system, is, I guess, in the eye of the beholder.

Have reviewed the Heaton/Longuski paper (Thanx!) now and have some comments.

The 'clever bit' of the Messenger trajectory, gravitational interaction with the target object to facilitate orbital insertion, isn't employed at Ariel. This technique is not addressed in the paper, but since utilizing it would take quite a while (if it were even possible at Ariel) and the paper was assuming 2 year long orbital missions the omission is not surprising. Also, the 1km/sec 'burn off' at Ariel doesn't seem excessive anyhow, so the utility of the technique at Ariel is less appparent. That the technique works at Mercury may be due more to Mercury's high density alowing the craft to approach closer to the planets center of mass to 'realize the math' than may be possible with an icy body like Ariel. It is not clear (to me) how the mass ratios and distances scale for this technique from sun/Mercury to Uranus/Ariel.

I am also amazed with the Heaton/Longuski trajectory in that in 2 years, 40 (!) encounters with Oberon, Titania, Umbriel, and Ariel are possible. A significant truth revealed in the paper is that since the Uranian system is 'scaled' smaller than the Galilean satelllites, useful distant flybys of Uranian moons also occur more frequently than they did for the Galileo spacecraft.

Inferring to Cassini, it seems the mass ratio of Saturn/Titan being so different from that of Jupiter or Uranus does allow Cassini much more flexibility in changing it's inclination around Saturn. I had wondered why this was not done with Galileo at Jupiter, and it seems many more satellite flybys would be required for a given inclination change than for Cassini at Saturn.

It does seem that an interesting mission could be orbited at Neptune following the Cassini style tour.

Also, granted the enormous delta vee needed to effect orbit insertion around Pluto, once that is achieved, an interesting mission at Pluto is possible utilizing Charons gravity for orbit shaping. Close observations of Pluto and Charon at a variety of inclinations seems quite feasible, and arbitrarily close approaches to the 2 new satellites seems 'easy' now. The big problem remains though, the difficulty in achieving orbit around Pluto due to the high approach speed of any reasonable spacecraft.

[Guest_BruceMoomaw_*]

The best way to orbit -- or land on -- either Pluto or Triton seems to be Angus McRonald's suggestion for a big, heat-resistant ballute dragged behind the spacecraft on a cable to serve as an aerobrake in their extremely faint atmospheres. His preliminary studies show this to be quite workable. There was a little bit of discussion on it at COMPLEX, but as they said it will require quite a bit of technological work as compared to simple heat-shield aerocapture, which is almost ready to go right now. McRonald himself has become a victim of Ames Research Center's rather unselective layoffs, but some groups at Knoxville and Purdue U. are studying the question further.

As for a Neptune orbital tour, the plan already worked up for Tom Spilker's JPL design team involves a lot of orbital flexibility, but no actual switch from retrograde to prograde orbit at any point in the tour. (By the way, a lightweight but scientifically useful Triton lander that does most of its braking by ballute is seriously considered for addition to the Neptune Orbiter mission, if NASA hits the really big money.)

[tasp]

The best way to orbit -- or land on -- either Pluto or Triton seems to be Angus McRonald's suggestion for a big, heat-resistant ballute dragged behind the spacecraft on a cable to serve as an aerobrake in their extremely faint atmospheres.  His preliminary studies show this to be quite workable. 

Wow, imagine watching the ballute go overhead while standing on the surface of Triton!

Would it radiate enough heat to flash the surface frost to vapor as it goes by?

Interesting to watch the surface after that if it did.

I assume that an accelerometer on the main body of the craft would cut the cable to the ballute when the craft slowed enough to enter orbit, but if the ballute didn't get deep enough into the thin atmosphere, the system wouldn't decel enough. Sounds stress provoking for the mission team at JPL.

Amazing concept, hope to live long enough to see one fly.

[Guest_BruceMoomaw_*]

McRonald's best paper on the subject seems to be at http://trs-new.jpl.nasa.gov/dspace/bitstre...6/1/99-0422.pdf .

[tasp]

McRonald's best paper on the subject seems to be at http://trs-new.jpl.nasa.gov/dspace/bitstre...6/1/99-0422.pdf .

Doesn't look like it will burn off too much Tritonian frost.

But what an amazing technique for getting to interesting places in the solar system!

Not sure a human payload will ever try this technique (shades of 2010), at least not at Pluto.

It is quite hard to imagine decel at up to 40 gees from such a thin atmosphere at hundreds of km above Pluto's surface. Still, the technique has great potential for orbiters and landers at a variety of objects.

[Guest_BruceMoomaw_*]

Just keep in mind that -- because Pluto's gravity is so weak -- its atmosphere, thin though it is, towers up and up and up above the surface, with a huge scale height. In fact, there is speculation that some of it may get exchanged with Charon through tidal forces! High-altitude aerocapture in this situation is quite practical, if you have a moderately-sized ballute. (Ditto for Triton.)

[tasp]

Just keep in mind that -- because Pluto's gravity is so weak -- its atmosphere, thin though it is, towers up and up and up above the surface, with a huge scale height.  In fact, there is speculation that some of it may get exchanged with Charon through tidal forces!  High-altitude aerocapture in this situation is quite practical, if you have a moderately-sized ballute.  (Ditto for Triton.)

I noted with interest in the referenced paper on the ballutes the one with a lens shape.

If this is not an original idea with me, my apologies; could such a near disk shaped ballute be angled during decel ? That advantage might be that a craft could manuever during the deceleration phase. The force applied by the ballute to the craft would not have to pass through the center of mass of the craft. The effect would be usable lift. The decel phase could be prolonged at a higher altitude while the craft arced further around the target object. Regions that perhaps would not be accessable from the 'standard' ballute might be reachable with a steerable one.

Should Pluto (or whatever) not be favorably aligned at arrival, this technique might still get you were you want to go.

Perhaps the decel could start over the far side (as seen from earth) and wind up on the fore side for the actual landing phase.

Another application might be (again using Pluto as an example) to enter the deceleration phase where convenient from the arrival trajectory, and manuver during decel and then exit the atmosphere on a path that leads to Charon (at a low enough speed that perhaps you could orbit or land there).

An application at Triton might be to decel in its' atmosphere, but not so much as to be captured by it, and wind up in an orbit favorably inclined about Neptune.

Aerocapture into Saturn orbit via ballute passage through Titan's atmosphere seems doable. Perhaps the manuvering technique would put the craft into an equatorial orbit about Saturn (for example) from a greater variety of incoming trajectory angles than any other technique?

[dvandorn]

The biggest problem with using steerable aerocapture / landing trajectories is that you need to have a *very* good model of the atmosphere you're using to slow down with.

It's bad enough to have only a vague idea of the temperature/pressure profile of an atmosphere you're using to do straight-line drag braking. If you're designing into your spacecraft and trajectory a need for predictable changes in direction based on lift, *any* significant variation in the atmosphere's pressure, density and temperature profile from your model will result in completely different results.

They're talking about needing a Mars orbiter for day-to-day monitoring of the upper atmosphere in order to successfully land future probes -- apparently, Mars' upper atmosphere was so much thinner and extended than predicted during Spirit's EDL that she was very nearly dashed against the surface a la Beagle II. Cassini is seeing that Titan's upper atmosphere also seems highly variable in pressure/altitude. So, in order to successfully use Titan aerobraking for a Saturn orbiter, you'd almost need to already have a working Saturn orbiter there to tell you the detail you need about Titan's atmosphere at that moment...

Like many other elegant ideas, this one tends to fall apart when you look at it in greater detail.

-the other Doug

[tasp]

Like many other elegant ideas, this one tends to fall apart when you look at it in greater detail.

-the other Doug

{grasping at straws}

Can the craft itself scan ahead with a lidar device (or whatever would work) and adjust path 'on the fly' ?

Do we expect the Titanian weather reports to repeat precisely every Saturnian year?

Are Pluto and Triton far enough out that their atmospheres will model out quite stable in the the relavent altitudes?

Would a more 'structured' decel device be worth while? Maybe an RCC wing with some movable aerodynamic surfaces on the trailing edge. (I'm thinking of a shuttle type re-entry with hypersonic manuevuering capability, the craft flies to the appropriate density/altitude, executes the manuvuer wherever that layer happens to be).

I'm thinking the amount of gas over a particular surface area will be constant, but its' distribution vertically will vary. Set up something that tolerates the vertical gas distribution range expected and GO!

[centsworth_II]

What about a small,simple atmospheric probe that would fly ahead, similar to the way Huygens arrived at Titan before Cassini?

[Guest_BruceMoomaw_*]

You don't need to grasp at straws -- Tasp's fears about the unusability of aerocapture due to uncertainties about the planet's atmospheric structure are, according to Jeffrey Hall of JPL, very seriously overstated. Hall's team has run almost 2000 simulated aerocapture entries into the atmospheres of both Titan and (more difficult) Neptune, inserting random changes in atmospheric structure to see if the written autopilot programs can cope with them -- without a single failure. (I can give you the URL for this actual report, once I track it down again in my records.)

He said flatly at COMPLEX that -- once a single New Millennium mission (maybe ST-9 in 2010) is flown to confirm the overall validity of the aerocapture autopilot programs -- the technology will be ready for use immediately at the smaller worlds (for instance, MEPAG was wrong in saying that an engineering test of aerocapture will be necessary at Mars) And he also went so far as to say that -- while aerocapture at the giant planets requires a radically different kind of boat-shaped heat shield, to provide the higher lift/drag ratio that will allow the autopilot to make corrective maneuvers faster, we won't even need another flight test for those aerocaptures; just some more ground work. Indeed, his cheerleading for this technology was one of the major pieces of news at the COMPLEX meeting, and seemed to effectively change the minds of the assembled members.