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?

And considerably longer turnaround times for commands from Earth.

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.)

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...

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).

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?

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.

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.

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.

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.

...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

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?

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.

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.

Bruce:

A Titan helicopter need look very little like one designed for this planet, and could have a really stable flight regime. Look at the toy helicopters and other electric flying machines out there, and you can see a remarkable diversity - like four-engined ones, contra-rotating affairs and even a couple of honest-to-goodness saucers. Add in the experience from some of the smaller military UAVs and you really have the potential for a winner.

One thing that's certainly true about helicopters vs lighter-than-air is that the former would simply flit about, while the latter would be up and about for the longer term. Perhaps helicopters should be seen as an adjunct to a rover, or a lander - small, light and pretty much throwaway.

Bob Shaw

In my opinion this helicopter idea is utterly impractical. Helicopters are power-hungry, mechanically complex, highly stressed and dynamically unstable.

Yes, I know that a twin-rotor model like the CH-47 is stable, but at the cost of even greater mechanical complexity and the need to either synchronize the rotors, or have them far enough apart not to interfer with each other.

Just constructing a rotor head that will work at cryogenic temperatures would be an engineering nightmare. Metals will be brittle, standard lubricants useless, normal elastomers rock-hard etc. Building a rotor blade (which is highly stressed and has to move in a complicated way around all three axes during each rotation) wouldn't be easy either.

It is true that a helicopter for Titan would be very unlike an earth helicopter. It would also be impossible to test-fly on Earth. Temperatures and atmosphere composition it might just be possible (but very expensive) to simulate in a large refrigerated chamber, but never the gravity.

This also applies to the software. You could never test the flight control software (which must of course be completely autonomous) in a realistic way either. And remember, it must never re-boot and never safe itself. Also it had better be pretty good, so it doesn't get itself into a vortex ring situation, turns downwind at low altitude, overtorques or any other of the no-nos of helicopter flying.

Power: a RTG seems to be the only practical alternative. Unfortunately they're heavy and doesn't produce that much power. This is the best part of the balloon/blimp concept: the lift is produced by the waste heat, leaving all the electric power for propulsion and control/science/communications.

tty

FWIW, I believe that the proposed Titan helicopter concept would use a small RTG and storage using capacitors to give Ooomph! at takeoff. Think not so much of traditional helicopters here, so much as an electric boost-glide hopper, a bit like Zubrin's Mars Plane (which has actually flown), but using a rotary wing. We're NOT talking about sustained flight.

Bob Shaw

I'm still skeptical. Compared to winged aircraft helicopters have very bad gliding characteristics (steep glide, high rate of sink) and an unpowered (autorotative) landing is quite challenging.
You have very limited choice of landing site because of the steep approach and you must pull collective pitch at exactly the right moment or you're in big trouble.
The company where I work made a lot of money repairing the helicopters the army used to bend when training autorotative landings.

tty

All the power-laws are in your *favour* on Titan! Less gravity, greater density... ...and no PIO (Pilot Induced Oscillation - the bane of autorotation!).

Bob Shaw

Forget wheels: What about a Dante type crawler - you know, legs.

http://ranier.hq.nasa.gov/telerobotics_page/projects.html

Titan is full of methane, so why not bring some oxygen there as "oil" to make a normal chemistry engine?

How accidented is Titan's surface? That is important factor about the ballon maneurability. On the other hand ballon needs control over the air direction, then how is the air flow pattern around Titan? The other problem is about the distance and time to send and receive commands from Earth to probe might be solved with an even more sophisticated artificial intelligence, doing more alike as a robot obeying any developed rules.

Rodolfo

Bring oxygen to Titan is prohibitive expensive due to its weight (liquid oxygen) and of short duration. I seems it as not a good idea.

Rodolfo

Certainly all the proposed Titan rovers or flyers use RTGs to power their motors -- but some of the balloons use the RTG's waste heat to provide a balloon's hot air, and there's no reason that it couldn't be directly utilized to operate mechanical motors as well. After all, precisely that has been proposed for a long-lived vednus lander: having the motor for its internal refrigerator run directly by waste heat from its external RTG, rather than by electricity from that RTG.

As for the problems of using wind to move a passive Titan balloon: that really is a problem for the current design, which would hover only about 3 km off the ground -- an altitude low enough that local surface features seem to radically affect its direction, according to Huygens. 7 or 8 km altitude, by contrast, might get it back into the main westerly superrotation wind. But as for the long time between its taking images of an approaching landscape feature, and its reception of a command from Earth to lower itself to the surface at that point: the science definition team has already said that such a mission could still probably land accurately enough to get acceptable surface samples, since Cassini seems to be indicating that Titan's surface composition varies much more over long distances than over short ones.

I guess the way to think about some sort of floating Titan lander is as a succession of individual landers as opposed to a mobile roving platform that can provide a unified geological data set over a long distances. Seen in this way, 10 to 20 (maybe more) discrete landing zone portraits would seem to me to be able to establish a whole lot of ground truth, perhaps even more than a long term roving mission that focused on a much smaller area.

CH4 + 2O2 => CO2 + 2H2O, so you need four pounds of oxygen to each pound of methane. Methinks a RTG would give better value for money.

tty

How about a slow lander or glider? With Titan's low gravity and thick atmo, you could do several circuits around the moon before landing... all of this for very little complexity in the vehicle design. Remember that Huygens had to use a third smaller chute to be able to land faster.

What about fluorine?

I don't see anything you get out of this that you couldn't get much better from a balloon -- even a nonpowered one.

Not much better, and much harder to store. Unfortunately, it appears that we have to stick with RTGs and Pu-238, much though I would prefer not to.

No balloon has ever been successfully deployed on another planet. The inflation and the proper envelope material sound like a very complicated problem to solve... and therefore expensive. A slow lander could use a design very similar to Huygens, and it would be much cheaper as a result... and I think the science would be comparable to a balloon.

A balloon would cover a small area in more detail, but a slow lander or glider could cover almost the entire surface. I think it would be possible to design something that could take days or weeks to descend with altitudes of 500 km or so above the surface for most of the time, and maybe a day below 500 km on final descent.

Also a balloon would require better model of the low altitude winds then I think Cassini or Huygens will provide. I think it would be really hard to design such a system without accepting a risk level comparable to Beagle 2.

I vote for a very large parachute, with lots of scheduled drift time - who cares where - we don't know enough about the surface to know what is important and what is not. And we better be able to analyse the surface for something other than water and hydrocarbons. This substance 'not found on any other surface in the solar system' has to have a name.

...Except for the two Vega balloons deployed on Venus in 1985 by the Soviet Union.

The Vega mission balloons were 100% successful. They had a mission goal (I'd have to check to be sure) of 1 days and lasted some 2 days till batteries depleted. Science capability was somewhat rudimentary because of the small payload, power available and direct-to-earth transmission, but it was a considerable technical and scientific success. And the small payload was well thought out and designed.

Er... ...Titanium? It's like Unobtanium. Oh, damn, someone's used that one. What about natiTium?

Bob Shaw

How about a rover that uses legs instead of wheels, like the Dante robots?

Well, first, the balloon has already been test-inflated repeatedly in Titan-temperature air -- including the cigar-shaped blimp. That's no problem, especially given the fact that Titan's low gravity means that the craft is falling quite slowly while it releases and inflates the balloon. (The large electric motors they'd need for a blimp have also been tested at Titanian temperatures.)

Second, a glider can only land once -- whereas the desired goal for a Titan Organics Explorer is to make landings at several different places (around five). The big problem with a nonpowered ballloon is that, of course, you only have very rough control over where you land -- but to a great extent that's also true with a glider. The Titan Organics Explorer appraisal group carefully considered the possibilities of a Titan airplane as well as blimps and balloons, and ended up saying firmly that the airplane was seriously inferior.

As for a rover with legs instead of wheels: first, they're complex and haven't really been proven yet in a real-world environment (consider how short a time it took for the "Dante" legged rover to foul up). Second, Titan's surface is so nasty -- riddled with very steep slopes and likely quagmires -- that a legged rover probably would find it as hard to get around on the surface as a rover with big inflatable tires, and maybe more so.

Finally, any short-range rover of any type has a scientific problem mentioned by the Organics Explorer appraisal group: the impression we're starting to get of Titan from Cassini is that long-distance compositional differences are much more important than short-distance ones.

This sounds like a job for Tumbleweed Rover!!!!!

Strong winds should allow it to travel all over, but calling back home seems to be a bit of a puzzler.

Why did you resign?

Bob Shaw

I just realized the problem with a slow parachute... a parachute kills all velocity vertical _and_ horizontal. A balloon would be much better because it would have longer to drift on the wind.

Now, I'm leaning towards a plane powered by an RTG heat engine. The airspeed on Titan would be very slow, and the wings would be very small. An airplane would be almost as good as a helicopter or blimp, but much simpler to build. And there is less of a need to predict the atmosphere.

I like the tumbleweed approach, but there do not appear to be strong winds. How about an airbag balloon with spider accessories? Not a blimp, but a collection of hydrogen filled airbag-like cushons configured in a spherical shape with the landing probe in the middle, and an array of instruments dangling from tethers that analyses and broadcasts. When the probe is ready to land, the instrument clusters are reeled back in. Once on the ground, the probe can inertially drive the balloon coccoon across a verity of terrains, navigating with radar that can see through the bag fabric. Surface analysis is accomplished with drop down analytical tools.

How about a robot probe that moves along the ground like a snake or catepillar?

Why reinvent the wheel that Nature has already developed for the last few million years?

I would call it The Undulator - though they call it Hydra:

http://europa.eu.int/comm/research/headlin...5_12_26_en.html

http://www.robots.org/images/2001_Robot_Ga...e%20planets.htm

http://www.space.com/businesstechnology/te...s_010911-1.html

http://astrobiology.arc.nasa.gov/news/expandnews.cfm?id=367

Paper - Biomimicry as Applied to Space Robotics:

http://cswww.essex.ac.uk/technical-reports...cott_Ellery.pdf

Nix - a parachute will, after a short while, drift with the wind just like a balloon. If You could invent a parachute that doesn't You would rich, since landing with a parachute in a high wind is difficult and can be dangerous.

Also I can't understand where you got the idea that an airplane has less need to predict the atmosphere than a blimp - if anything it is the other way around. Also an aircraft has the same problem as a helicopter - it can never be realistically test flown on Earth.

In my opinion the only realistic first-generation air vehicle for Titan is a free balloon or possibly a blimp. A blimp can always revert to a free-flying mode if there are control problems or the computer has to safe itself. At such times a crude altitude hold function could be maintained by a simple pressure sensor directly linked to the RTG heater. Try that with an aircraft!

Incidentally a spherical balloon while the optimal form for a free-floater is not good if you want to moor it when "landing" it since a moored spherical balloon is violently unstable. In such situations a "kite baloon" that works as a weather foil is preferable.


tty

The Titan Organics Explorer design team is now trending toward two alternatives (I think I've mentioned this before):

(1) The more expensive one is a hydrogen-filled blimp with a complex celestial navigation system (apprently usable even on Titan if you use near-IR cameras to observe the Sun and Saturn itself), with two big motors that can be swivelled to drive it temporarily downward vertically for sampling.

(2) The less expensive is a big (and round) wind-driven balloon filled with hot air from the RTG, which would ordinarily hover at higher altitude (about 3 km, versus 1-2 km for the blimp) so that its ground controllers could see in advance if it was being blown toward a region worthy of sampling, after which they'd lower it in classic hot-air balloon fashion by opening a vent. Obviously far less control over landing spots; but the team concluded that it may be acceptable anyway, given Cassini's growing conclusion that Titan's surface composition varies more over long distances than over short ones. (The balloon would also have those navigation cameras, simply to tell the ground precisely where it was at any given time.)

Both these things would descend to within about 100 meters of the surface for 5-10 times over their lifetimes (that number has been downscaled from the previous 20 times), and drop a tiny tripod down to the surface on a line. The tripod would then instantly fire a core tube into the surface, which would then be detached from the tripod and reeled back up into the gondola on the cable. Thus ground contact is required for only a second or two, and there's considerable slack in the cable during the contact.

The point of this mission is that they want to make multiple sampling landings, primarily to look for local spots where liquid water/ammonia lava has been mixed with the deposited organic smog prticles to produce interesting prebiotic compounds that actually contain oxygen. Which mission will be chosen would depend both on cost and on the need for precision landings (the blimp would actually spend about a week quartering each of its 20-km wide landing regions in detail before deciding precisely which spot to land on in it). But if you want to make repeated landings, an airplane or glider is simply out of the question.

Alternatively, if you just want to do aerial reconnaissance (which is another real possibility for the first Titan mission, perhaps accompanied by a stationary lander at one good spot picked out by Cassini), an airplane would be usable, but wouldn't be any more scientifically productive than a balloon with a long lifetime. The trouble -- as with Europa missions -- is that the time gap between Titan missions is so long that you don't want to make any mistakes in designing each mission for maximum scientific output.

Thanks Bruce,

Any word on instrumentation? Are they planning on including a Mossbauer?

Well, the payload is strictly strawman at this point -- and it's also pretty barebones:

(1) Cameras and air temperature and pressure sensors (naturally).

(2) The main GCMS package to analyze the core samples taken, along with atmospheric samples (also naturally). They definitely want an ability to look for chirality.

(3) A microscope to inspect the retrieved cores of surface ice before they thaw (looking for physical structure, not microbes).

(4) A Raman spectrometer to analyze inorganic materials in the ice cores. This intrigued me; it turns out that they're looking for rocks from Titan's interior that might be ejected onto the surface along with the ammonia-water lava flows that they're looking for.

One can easily envision other things that could be added to this: other types of weather sensors; a near-IR spectrometer for remote surface composition mapping (even given the limited spectral winds through which near-IR sunlight hits Titan's surface); a subsurface radar sounder; a magnetometer to look for induced magnetic fields and thus a subsurface ocean.

That would be telling.

(since no one else replied)

Before anyone objects, this topic is entirely, er, on-topic, being on the subject of a Rover which travels across a beach-like landscape, and has some form of AI!

Be seeing you!

Bob Shaw