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Titan Rover
algorimancer
post Dec 31 2005, 02:05 AM
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Any showstoppers? I would envision something similar to the upcoming Mars Science Laboratory (RTG's are a given). Lot's colder on Titan... rough on lubricants, and parts would be brittle. Not sure whether the organics would pose a problem in the way of gumming up the works, but it doesn't seem likely at liquid methane temperatures. Smaller world, lower gravity, thick atmosphere... I'm inclined to think that a simple copy of the MSL, perhaps with some added insulation, would work just fine. The only alternative I've been hearing about is a balloon-borne probe, but that doesn't allow the same level of geologic prospecting.
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Steve G
post Dec 31 2005, 02:13 AM
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QUOTE (algorimancer @ Dec 30 2005, 07:05 PM)
Any showstoppers?  I would envision something similar to the upcoming Mars Science Laboratory (RTG's are a given).  Lot's colder on Titan... rough on lubricants, and parts would be brittle.  Not sure whether the organics would pose a problem in the way of gumming up the works, but it doesn't seem likely at liquid methane temperatures.  Smaller world, lower gravity, thick atmosphere...  I'm inclined to think that a simple copy of the MSL, perhaps with some added insulation, would work just fine.  The only alternative I've been hearing about is a balloon-borne probe, but that doesn't allow the same level of geologic prospecting.
*


At the ludicrously low ambient temperatures, woudn't any heat form the probe's RTGs or other spacecraft parts just create havoc with the local environment?
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David
post Dec 31 2005, 03:15 AM
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QUOTE (algorimancer @ Dec 31 2005, 02:05 AM)
Smaller world, lower gravity, thick atmosphere... 
*


And considerably longer turnaround times for commands from Earth.
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Guest_BruceMoomaw_*
post Dec 31 2005, 04:27 AM
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It's being seriously considered -- in fact, for a detailed design based largely on MER (but with big tires), see http://www.lpi.usra.edu/opag/jun_05_meetin...s/opagtitan.pdf . But the Outer Planet Assessment Group's Titan study subgroup is leaning strongly against it for two reasons. First, Titan's surface appears to be rugged and nasty enough (very steep-walled arroyos and cliffs, quagmires, etc.) that it would give even a MER with huge inflated tires a rough time. Second, we want multiple sampling over really long distances, because Cassini seems to be confirming that the major compositional differences in Titan's surface that we're interested in are spread out over long distances rather than short ones.

So the inclination is still strongly toward an inflatable airship that drops down periodically to within 100 meters or so of the surface, ejects tiny sampling harpoons, and then reels them back on cables into the gondola. The main debate now seems to be whether this should be a powered blimp (far more control, but also far more expensive), or a hot-air balloon that just gets blown along by Titan's slow near-surface winds. That's assuming, of course, that the next Titan mission is a sampling balloon at all -- the Titan study group also suggested that it might on balance be preferable to make the next Titan mission an orbiter combined with some kind of small fixed lander or non-landing balloon, before we move on to the next stage of exploration. (By the way, I AM giving away some secrets from my "Astronomy" article to tell you guys this.)
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ermar
post Dec 31 2005, 04:52 AM
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QUOTE
it might on balance be preferable to make the next Titan mission an orbiter combined with some kind of small fixed lander or non-landing balloon, before we move on to the next stage of exploration.


Wouldn't an orbiter in conjunction with any ground/atmospheric Titan mission be preferable regardless? In addition to observing Titan and other moons from a distance, it could serve as a relay for the surface mission, reducing the size of the probe's dish and meaning that it could be contacted during those looong 8-day nights...
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Guest_BruceMoomaw_*
post Dec 31 2005, 05:29 AM
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Obviously a Titan orbiter has massive utility as a com relay -- in fact, it was once thought impossible to fly any kind of Titan lander or balloon without it. But it also adds a hell of a lot to the cost of a mission, and so there is now great interest in the possibility of flying a sampling balloon mission without it.

The trouble is that -- for such a mission landed in the late 2010s -- you have to land near the north pole, or the vehicle will indeed be out of touch with Earth for periods of several days at a time. In the case of a powered blimp, during an extended mission you can later cruise southward out of the constant-contact zone, and have the blimp programmed to fly itself back later. But of course you can't do that with a passive balloon (and the Titan study group seems to be leaning toward recommending that at this point; the cost cut seems to be a lot bigger thn the expected science loss).
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ermar
post Dec 31 2005, 06:27 AM
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QUOTE
you have to land near the north pole, or the vehicle will indeed be out of touch with Earth for periods of several days at a time.


Of course, the atmosphere will be circulating in the meantime. I know that Titan's atmosphere is a hazy subject, but how would we expect to see an drifting polar balloon move near solstice - south, stuck in a stationary polar cell, or too slowly in any direction to matter much?
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tasp
post Dec 31 2005, 06:29 AM
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IIRC, one Voyager style RTG dissipates about 9000 watts of waste heat continuously.

Not sure you would want to deal with the consequences of this near the surface.

--However--

would 9000 watts (or a multiple thereof) be enough to buoyantify a hot air balloon with a useful payload of instrumnents in the Titanian gravity?

Kinda handy to have a near perpetually fueled hot air balloon for a lengthy traverse of the Titan aerodrome.
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Guest_BruceMoomaw_*
post Dec 31 2005, 09:53 AM
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That's exactly the plan. (Interestingly, a blimp requires a more sophisticated and efficient Stirling-type RTG that utilizes less plutonium and produces less waste heat that will interfere with the hydrogen-filled blimp's maneuverability -- whereas the passive hot-air balloon concept requires the less sophisticated and less efficient current RTG design to produce enough waste heat to inflate the balloon.) As for the heat produced by either RTG, studies have shown that -- even after the vehicle finally loses buoyancy and actually falls onto the suface rather than hovering 50 meters or more above it -- the Stirling RTG won't melt into the ice, while the lightweight cage around the hot-air balloon's RTG will keep it from melting into the ice either.

The pattern that a passive balloon might follow, however, does present a problem. The plan is to have a passive balloon hover 3 km or so up, as against only 1-2 km for the blimp -- thus allowing the balloon's controllers more advance view of the unpredictable upcoming surface features so that they have more time to decide whether or not to land the balloon at a particular spot. But Huygens detected winds below 10 km that completely reversed direction from the normal eastward winds of the superrotating atmosphere, presumably due to local terrain features -- so it may be impossible to predict just where a balloon at that low altitude would end up getting blown.
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algorimancer
post Jan 1 2006, 02:34 PM
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I'm afraid I don't have a lot of confidence in the practicality of balloons. Historically, even competently manned balloons & derigibles have had a nasty habit of crashing (look at the repeated failures involved in circumnavigating the earth in a balloon, for instance). Mix-in automated control in a novel enviroment hours away from intelligent reaction, and it seems to me that we're just setting ourselves up for an expensive disaster.

One nice thing that the MER's have established regarding rovers is that, if something goes awry you can just leave the rover parked for a few weeks or months while you determine a solution, then continue the mission. Hard to do that with a balloon.

I'm intrigued by the notion of roving an ice world for the first time. Comparison/contrast of geological processes under those conditions vs. those on rocky worlds ought to yield lot's of surprizes. I'd settle for Europa or Ganymede, but I think the broader interest will be Titan.

As to the waste heat issue, I would think that placing radiators on the top, shielding the ground with reflectors, ought to ameliorate the problem to a large extent.
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Guest_BruceMoomaw_*
post Jan 1 2006, 10:04 PM
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Titan balloons have a huge advantage over balloons to any other world in the Solar System -- including our own -- for the same reason that Titanian aircraft have one: Titan's unique combination of dense atmosphere and low gravity means that you need a much smaller balloon (or much smaller wings and propellers) to stay aloft, which in turn means that you can afford to make the balloon envelope out of thicker and tougher material than for a balloon on any other world.

As for the automated control problem: this is relieved in these proposed missions by the fact that the vehicle isn't supposed to come closer than 50-100 meters of the surface -- it will instead carry 5-10 tiny sampling harpoons (the design for these has been worked out in some detail, but they clearly require and will get more work) to drop down onto the surface and then reel back in on thin cables. This system should allow sampling to be done at heights of up to 200 meters above the surface, which provides the balloon a lot more latitude in avoiding dangerous terrain features. (The harpoon heads are little tripods which will hit the surface and instantly fire a small core tube downward into the surface, with the core tube then being released from the tripod and reeled back up to the gondola after less than a second on the surface.)

Given the apparent extent to which Titan's surface composition varies over long distances, this system (including using the gondola's aerial cameras for detailed reconnaissance of the types of Titanian surface features in the area) is regarded as a workable substitute for the need we would have otherwise to launch several fixed landers to different places picked out by previous orbital reconnaissance more detailed than Cassini will probably be able to do. However, clearly this whole subject is still open to lots more debate, and will get it in the near future.

As for the heat from the RTG on a lander or wheeled rover affecting the surface, this can indeed be solved by putting a cone-shaped thermal shield around the RTG to divert the air which it heats straight upward, keeping it from hitting the surface.
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dvandorn
post Jan 2 2006, 01:24 AM
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QUOTE (BruceMoomaw @ Jan 1 2006, 04:04 PM)
As for the heat from the RTG on a lander or wheeled rover affecting the surface, this can indeed be solved by putting a cone-shaped thermal shield around the RTG to divert the air which it heats straight upward, keeping it from hitting the surface.
*

...turning your rover into the font of a trundling column of superheated air, rising rapidly up from any position the rover takes. Pulling cooler air in from the surrounding terrain, which is sucked up into the rising hot-air column.

Hmmm -- in fact, this could have some positive aspects. Your rover would act as a vacuum cleaner, sucking small fines and such toward itself wherever it went. You could also put turbines into the airflow and generate additional electricity that way.

And finally, since you're generating a truly anomalous (to the natural conditions) updraft, you might even create weather as you wander along.

Of course, you'd have to correct for these impacts in your observations...

-the other Doug


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“The trouble ain't that there is too many fools, but that the lightning ain't distributed right.” -Mark Twain
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ljk4-1
post Jan 2 2006, 05:23 AM
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QUOTE (David @ Dec 30 2005, 10:15 PM)
And considerably longer turnaround times for commands from Earth.
*


By the time we get a rover on Titan, AI and robotics will be advanced enough that the rover will be able to handle most issues on its own and not need to wait for help from Earth.

How about a hovercraft?


--------------------
"After having some business dealings with men, I am occasionally chagrined,
and feel as if I had done some wrong, and it is hard to forget the ugly circumstance.
I see that such intercourse long continued would make one thoroughly prosaic, hard,
and coarse. But the longest intercourse with Nature, though in her rudest moods, does
not thus harden and make coarse. A hard, sensible man whom we liken to a rock is
indeed much harder than a rock. From hard, coarse, insensible men with whom I have
no sympathy, I go to commune with the rocks, whose hearts are comparatively soft."

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ermar
post Jan 2 2006, 06:11 AM
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QUOTE
How about a hovercraft?


I remember someone asking if anyone had read Stephen Baxter's Titan...

In summary, the protagonists try using a hovercraft, only to find that it's hopelessly unstable and throws them into the slush, repeatedly. "He went to the airlock. Once inside and de-suited he started to clean off the gumbo still sticking to his EMU. Fifty million bucks, he thought."

Aside from that... I don't see any reason why it shouldn't work, provided you can land it in a flat enough area. And if the surface really is the consistency of wet sand, it would seem to have an advantage over a rover in that it wouldn't be as likely to get stuck.
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Guest_BruceMoomaw_*
post Jan 2 2006, 09:53 AM
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A hovercraft, however, has one major problem that a rover doesn't: it's almost impossible for it to climb up or down even midly steep slopes -- and Huygens made it clear that these are a major feature of Titanian terrain.

Ralph Lorenz is an enthusiast for the idea of a Titan helicopter -- which, thanks to that combination of dense air and low gravity, would require fully 37 times less energy to hover than it would on earth -- but the control problems seem alarming to me, and in any case it lacks that really long range that one gets out of a balloon or blimp and which they badly want for Titan. The biggest goal of surface sampling, after all, is a region which has been exposed to liquid water -- or water-ammonia lava -- and it may take quite a lot of flying to find one.
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