Thinking about all that methane rain descending on Titan's south pole, draining out into Mezzoramia and then - who knows where? - set me wondering . .

IF it seeps through a porous material it could possibly warm up at depth and re-emerge gradually at the surface as methane gas. This could be a peaceful process, like the emission of methane from a terrestrial marsh (or rubbish tip) rather than anything sudden or localised. Now suppose a large sheet of impermeable material is simply laid over the ground in such a place it might begin to trap methane underneath and become buoyant. Something like this is maybe what starts mid-latitude convection cells. Could it be possible to harness the phenomenon to lift balloons? Relatively cool balloons, smaller heat loss problems, very low power requirements. With a nitrogen atmosphere and methane both readily available on Earth the technology could be tested here.

I think coal gas was used in early balloons on Earth, so there is some precedent. However, what you
are proposing is more like using water as a lifting gas (molecular weight 18 - not too bad..)

The issue is that it isnt a gas - at 94K the saturation vapor pressure is only 100 mbar or so.

Separating hydrogen (0.1% at Titan) might be do-able, but hardly a mature technology to propose for a
near-term mission.

Don't forget also the controllability advantages of hot air.

I agree absolutely this is not a serious suggestion for something to try soon! Purely a flight of fancy on my part. For a start you'd have to know when and where warm methane gas was seeping out of the ground. But unlike water vapour on Earth you'd only have to keep the methane a relatively small amount warmer than ambient temperature for it to remain gaseous.

Whilst trying to think of how to get a suitable fabric to stretch itself out neatly on the surface of Titan I realised that another of your specialities could come into play - the frisbee! Attach the payload in the form of a lot of small elements around the perimeter of a circular membrane and somehow arrange for it to spin as it descends. Speeding up the spinning just prior to landing would stretch it out nice and flat.

Anyhow, back to reality . . .

That was interesting about the use of coal gas in early balloons. I didn't know that.

And a question about the Titan hot air balloon (a thrilling proposal). How warm will the air inside be when it's flying? Will it be above the boiling point of methane?

About 10K above ambient - http://www.lpl.arizona.edu/~rlorenz/balloonjbis.pdf

Boiling temp depends on altitude; I dont think this is above boiling, but well above the dewpoint

Many thanks. I'll read that thoroughly when time permits.

That temperature difference is a lot less than I'd have guessed, but is makes sense with heat loss being a major factor in such a dense atmosphere. Another 10 degrees or so would see methane boil, I think. (111K at 1 bar, but I don't know the figure for 1.5bar.) The reason I asked was that it occurred to me you could then envisage a hybrid design with a methane balloon inside the hot air one. Volume for volume that would provide a lot more lift. Again though, probably too complicated for the first go.

Good luck with the Titan project. Everything we are learning about the place tells me that - barring a total failure of civilisation - we are in at the beginning of a very long story.

Keep in mind that 10K above normal means about 10% higher temperature in terms of absolute zero. That would provide the same lift as heating a balloon on Earth by 32K above normal, so if you picture a hot air balloon on Earth where the balloon is hot enough to sting your hand if you touched it... that's the performance we'd be talking about. Of course, touching the Titan balloon would have a different effect on your hand.

Yep, I see that. Ralph has done all the sums and I'm sure his optimum solution is just that. What a fascinating paper. (Thanks again rlorenz!) I was particularly pleased to see the NASA and ESA proposals set in a larger context, not only their relationship (complete independence at this stage) and relative maturity described, but also their place in the wider space of interesting Titan balloon possibilities.

Just one surprise in the NASA Flagship balloon design. I expected all the warm electronics to be inside the balloon with only sensors and antennae poking out.

But then my surprises - this one and the last - are simply down to my ignorance of the subject.

Ralph: Just read your paper on hot air balloons on Titan:

http://www.lpl.arizona.edu/~rlorenz/balloonjbis.pdf

It's great! I'm ready to write my congressman and demand TWO! :-)

--Greg

Balloons are fine but has there been any talk on the prospects of using plane probes. That in my opinion is the final frontier of planetary exploration. An ariel surveyor is a tantalizing approach.

The same technology used for probing hurricanes can be used to perform such a project.
Mars is suitable for the first approach. Then Venus, Titan and then who knows where.

The technology is available. It is only a matter of willingness.

http://www.noaanews.noaa.gov/stories2005/s2508.htm

What can a plane do, that an orbiter can't do over more ground, or a lander do for longer, or in many instances a Balloon do for longer, over more ground?

I don't doubt that the Ares mission for Mars is a very very cool project - but I have doubts about it's scientific benefits.

Doug

I know that for Microsoft Virtual Earth we were unable to make do with just satellite images; we had to use airplane photos. I believe the competition ended up doing the same.

So the advantage of planes over satellites is quality.

Obviously, planes can cover far, far more ground than rovers, and -- unlike balloons -- you can control where a plane goes.

--Greg (only expressing my own opinion here -- I'm obviously not speaking for Microsoft)

I can partially agree with that - the 25cm GetMapping project that covered all of the UK was an aircraft project.

That's because a nice twin engined aircraft is cheaper than a satellite, can be flown hundreds of times, and cover an entire country over a period of a year or so.

But this isn't a terrestrial issue.

An aeroplane on any other planet is a full spacecraft mission, will fly once, and cover only a small swathe of ground over a few hours or so.

It would be a brilliant engineering exercise - if you had to design a planet for gliding and flying planes, it would look a lot like Titan. But scientifically, you could only really justify it as the post-script optional extra of an Orbiter, a Lander, or Balloons.

Doug

To me, an autonomous plane seems much more complicated and
prone to mission-ending mishaps than a balloon. I'd rather see the
lion's share of the engineering costs going into the instruments rather
than the platform.

edit: Remember Huygens? During it's descent it was flipped around
violently and somehow ended up rotating in the opposite direction
it was designed to. (Did they ever figure that one out?)
How would an autonomous plane handle those circumstances!

Another advantage of balloon over plane on Titan:

At the tail end of Huygens' descent, the probe reversed course on it's
ground course. As far as I know that was unexpected and unexplained.
There is a lot of mystery in the winds of titan. A balloon could help study
wind behavior at various elevations. To a balloon, Titan's winds are a
natural object of study. To a plane, they are a hazard.

Yes.
see my discussion of the topic in LPSC and (more detailed) in JBIS almost 10 years ago
http://www.lpi.usra.edu/meetings/LPSC99/pdf/1088.pdf
http://www.lpl.arizona.edu/~rlorenz/jbis.pdf

Indeed UAV technology has made remarkable strides in recent years, as the missile-toting
Reapers attest.

But there are two big classes of issue
1. reliability - this entails substantial guidance autonomy without the benefit of GPS (on which
almost all terrestrial UAVs rely) - if you have a cpu glitch, your plane might flip and its all over.
There is also the doa-ble, but nontrivial, issue of deployment from entry shell or whatever.
2. fundamentals of energy and power.
Heavier-than-air flight requires power (whereas LTA requires less - Montgolfiere - or none - buoyant
gas) There is an interesting airship/plane tradeoff depending on how fast you need to fly and how
much payload you need
see e.g. http://www.lpl.arizona.edu/~rlorenz/flightpower.pdf
In the thin Mars atmosphere HTA is particularly challenging - low density requires high flight
speed and thus high power, hence only chemical propulsion (and therefore very limited lifetime)
I think there was one solar mars airplane study, but you need very low wing loading.

A lot of this all comes down to whether you are doing it because it is cool, or to answer what have
been determined to be major scientific questions.
I agree with Doug, an airplane mission of a few hours is exciting, but is it $400M-worth of science?
As a stand-alone, I think it may not be. I think any of these platforms (plane/balloon or
whatever) has to be part of a larger architecture to be viable from a risk/cost/science perspective -
you will note even the VEGA balloons were part of a lander/flyby mission.

Titan is a bit easier, since low gravity and the thick atmosphere both help you, but you are still talking
about hours or a day or two chemical or battery. Current-generation radioisotope sources do not have
the power:weight to fly in a compact package - maybe Gossamer Albatross kind of wing-loading, but then
you have severe packaging/deployment issues. (Remember to sell a mission you are not about
demonstrating that it is possible, but demonstrating that there is no way, really, really no way, that
this can go wrong..)

Now, a small battery-powered UAV to fly for a few hours around a Titan lander would be a neat and
in fact inexpensive add-on to a lander mission (it doesnt answer major scientific questions, but would
give some very pretty imagery, boundary layer profiling etc. - a good 10^6 or 10^7 dollar kind of
project - basically a special class of instrument.

Some of the nuclear gang have pointed out (irrelevantly as far as the present Flagship discussion is
concerned) that presently-unavailable nuclear-thermal sources could provide non-stop flying
capability at Titan - see the Howe/Young presentation at OPAG. In principle that could give you
global access, although an airplane is tougher from a surface-sampling standpoint than a balloon
might be (yes, yes, you can have sampling penetrators, little helicopters etc..... meet my friends
on the TMC review panel..)
http://www.lpi.usra.edu/opag/nov_2007_meet...clear_plane.pdf
(actually Bob Zubrin proposed related ideas a decade ago..)

This last remark is incorrect. Balloons and planes are both affected by wind.
The effects are inversely proportional in a sense to flight speed - I flew hanggliders
as an undergrad, and it was a lot bumpier than a commercial jetliner.

A balloon's trajectory is more or less completely determined by the wind, a
plane much less so. That makes a plane slightly less effective (though a
good IMU can help) in measuring the winds than is a balloon, but on the
other hand, a plane can go where you want it to.

In either case, winds on Titan are basically quite gentle, though it is true that
the wind reversal in the lower part of the Huygens descent was not widely
anticipated.

My comment was too absolute, but in a relative sense I feel that a balloon
is cheaper to develop, longer lasting (more science), and less prone to mishap. (Just my 2 cents.)



Doesn't the long communication delay eliminate any chance of determining
where you want the plane to go after it exits it's aeroshell? Only a preplanned
route could be used or you need to have a virtually intelligent autonomous
system. A balloon is much less maneuverable but there is much more time
available to plan actions (mostly elevation changes) to take advantage
of the balloon's slowly evolving circumstances.

I suppose a plane that takes off from a lander could be programmed to
fly a specific course, but a balloon could also be programmed to release
from a lander when the prevailing wind moved to a certain desired direction.
And the release and flight of the balloon remains much less prone to mishap
in my opinion.

I'm sure someone must have mentioned the obvious compromise
between plane and balloon: A powered dirigible?

Yes. It's all there in the Lorenz paper linked in post #4 which goes over the excitingly wide field of possibilities.