Just out of curiosity, has anyone ever done a study on putting a balloon in Jupiter's atmosphere? I think it would be fascinating and potentially quite scientifically valuable to have such a vehicle drift around the planet at the 1-bar level or so, acquiring meteorological data and even imagery, perhaps also equipped with a radar sounder to glean some data about what's going on far deeper.

(And, yes, I'm channeling Clarke's Meeting with Medusa... )

IIRC, someone once brought up the fact Jupiter's atmosphere is mainly hydrogen, the lightest gas there is. It wouldn't be trivial to produce a balloon with enough buoyancy to sustain an instrument package. Probably heated "air" balloons would be the only way to go. The question is how hot does the hydrogen have to get for a modest size balloon volume?

Good point. Gonna take a WAG here, but if you're looking at an isothermal air column then the deeper you go, the denser it gets, and therefore the less heat is needed. Of course, Jupiter's atmosphere is NOT isothermal and generally the deeper you go the warmer it gets, which would increase the amount of heat needed for the balloon to maintain buoyancy. There might be some favorable transitional temperature areas near, say, the tropopause, but have no idea what the ambient pressure is there.

Really silly idea follows that may well get me laughed right off the forum: What about a "hard" balloon? What I'm thinking of here is something like a carbon-fiber composite 'buoy' pressurized appropriately with hydrogen during the descent (at something like 0.5 bars), completely enclosed, with an internal heater to vary the container's ambient internal density for altitude control. Maybe maritime-analog systems like this are worth considering.

Okay, you may laugh now...

I know what you are doing Nick. You are sitting around bored silly because of the pounding rain, and now that you've read all the recent posts you are coming up with wild ideas to pass the time.

Meanwhile the storm has given me enough work to do.

Busted... , and, holy crap, Dan!!! Hope this was the only damage you sustained; sure as hell is more then enough!

The rain's pretty much over down here. Actually, I'm sitting around waiting for the cable guy; my box decided that it was a good day to die back on Wednesday.

It's an interest exercise in material science to determine if a "vacuum balloon" could work for any material and any radius. My hunch says no.

About as silly: sustain a gas of monoatomic hydrogen (which is unstable) and fill the balloon with that, achieving half the density of diatomic molecular hydrogen.

Even with hot "air", you still have very little buoyancy, since the molecular weight is so low. In fact, even with a vacuum "in" the balloon, you would have a lot less buoyancy on Jupiter than you would with, say, a neon balloon on Earth.

I think a light glider might be a better way to go, and see if you can get a mission to last some tens of hours. My guess is that the turbulence would spoil any dreams of a nice long smooth scientific cruise.

Note that things are even worse on Saturn (with more hydrogen) but somewhat better for Uranus and Neptune, where the helium abundances are higher, and there's some methane to boot. The easiest place to float a balloon in a gas giant would actually be Uranus.

I think it would work, but it might not be at the desired atmospheric density depending on the container weight/materials used. Could be that the lightest, strongest vacuum balloon we could build would only achieve static buoyancy at, say, 200 bars ambient, which in turn still might crush the container to say nothing of completely screwing up instruments, bus hardware, etc.

Definitely agree that it's an interesting problem in material science. Hopefully someone with some knowledge in this field will chime in. (As a side note, also interesting that apparently many applications traditionally reserved for marine engineering seem to surface when talking about exploring the Jovian system...)

EDIT: Just as a visualization aid, a submarine or, better yet, a diving bell is a direct analog of a "vacuum balloon"...the external ambient pressure is many times the internal.

Oh, constructing a vacuum shell that doesn't crumple would be easy. Making it buoyant in hydrogen is the trick. Hydrogen gas is about 5600 times lighter than water. This sounds like making a submarine so light that the crew could carry it on a hiking trail.

I remember reading some NASA studies in the early 80s on this topic. They concluded that hot air balloons were the only solution because lift is hard to achieve in an H2 atmosphere. Even then, I remember that the balloons had weight problems (large balloons needed) and couldn't reach too far above the lower cloud decks.

In theory, a paraglider would give a long descent life time, but managing the data relay would get really tricky

All this is moot now, though. NASA has lost the expertise and facilities for a Jovian entry probe. I believe they've dropped consideration of any more entry probes for Jupiter after they realized the cost of recreating the expertise and facilities.

I was thinking in terms of external pressure resistance vs. weight savings (the thinner the shell, the better), but I see your point exactly, JR; this may require an unreasonably strong material.

Why so pessimistic? I took Wikipedia's figures for a "normal" hot air balloon

http://en.wikipedia.org/wiki/Hot_air_ballo...ry_of_operation

Which is 113 kg for a balloon with 2800 cubic meters of volume, plus their recomended max operating temperature of 120 C (393 K).

Then I took their figures for Jovian temperature at 1 bar (100k Pa) of 165 K http://en.wikipedia.org/wiki/Jupiter

Finally, using the ideal gas law, I figure the envelope to hold 7,290,000 moles of H2 at 165K and 3,060,000 at 393 K, meaning it displaces about 240 kg of H2. Since the mass of the balloon is less than half that, it leaves over 100kg for payload.

Mass of the balloon vs. volume enclosed goes up as the 2/3 power, so a large balloon (100,000 m^3) could carry an amazing 8 tons!

As usual, I may have a math error somewhere, but my first cut at this suggests it doesn't look bad at all.

--Greg

As I recall, the payload was about 100kg for a balloon in the water clouds (where you calculated). It shrank rapidly as desired elevation increased.

My pessimism comes from the expertise to build and the facilities to test the heat shield. That is the technology that has been lost. I'm sure that if we paid to rebuild it, we could do the balloon.

I had no idea that the obstacles to a Jovian balloon were so high, though they make sense, in retrospect. I remember reading about the Soviets once having interest in a Jupiter balloon probe (only natural the originators of Vega), but of course that fell through.

Me, I'd just be happy with a kamikaze multiprobe mission that could sample various latitudes...

I was really intrigued by the idea of a balloon that would study the Jovian atmosphere for months. I'd be even more intrigued now that we know that Jovian and super Jovian planets are common. I still think that it scientifically would be a great mission. All the reasons for doing a balloon at Venus apply at Jupiter. It would be a difficult mission to fit within a budget, though.

I really liked the idea of the multiprobes -- a great compliment to Juno. I was bummed out to hear that we no longer had the capacity to recreate the entry technology. That is one of the reasons that the idea of a Saturn multiprobe has gained such favor. We do have the entry technology for every other planet in the solar system. The current idea with a Saturn probe is that it would carry two entry vehicles that have microwave radiometers bolted to the back of the entry capsules (the radiometers would detach before entry). The radiometers would get deep atmosphere soundings prior to entry, and the probes would get get in situ measurements down to a few bar.

Can you elaborate a bit on that, VJ? I'm assuming that one or more facilities or other critical infrastructure components were closed and/or reconfigured.

I am dealing with a memory of minutes or discussion of a meeting about New Frontiers options. It may very well have been one of Moomaw's stories about options for (then) future New Frontiers selections. (This would have been pre-Juno selection.) At the time, a Jupiter probe/orbiter was one of the four options for the second New Frontiers mission. A number of papers were published in that time frame about Jovian probes, usually multi-probe missions or a deept probe mission. There was discussion about using Galileo heat shield material or newer technology material (which would lighten the probes). A quiet statement was made that, as I recall, the expertise to manufacture the material and/or the facilities (at Ames, I believe) to test the material or the probe design (might be specialized wind tunnels, which would fit with Ames' expertise) were no longer available and the costs of recreating the expertise and/or facilities was simply too high. That raised the importance of flying a microwave spectrometer to get composition data on the lower atmosphere (the instrument would also get atmospheric structure information deep into the atmosphere). That instrument will fly on Juno. At about the same time, the Saturn multiprobe mission began to be talked about seriously as a way to get probes into a large hydrogen/helium gas giant.

It's certainly not a question of whether a Jovian probe mission can be done -- it was done once. It is, as I recall, a question of costs of recreating the capability.

Unfortunately, I don't tend to be a pack rat, so I don't keep all those old minutes and articles, so I can't look it up. In putting out this statement, I was hoping that someone with more recent knowledge would tell me that the situation has changed. Anyone out there with better knowledge of the situation than I?

As I understand it, the NF announcement of opportunity laid out science goals for a Jupiter-interior mission, and everyone assumed that entry probes would be required to meet the goals, but the Juno proposal convincingly addresses the science goals without an entry probe. And the AO is specific first and foremost about the science, not the architecture. So as I was following things, it wasn't that a probe was ruled out on its own [de]merits, but that Juno out-competed any possible probe mission.

I don't doubt that the jovian heat shield technology has been pretty well dismantled, though. I actually used it as the basis for interview questions in an HR process, because it seemed so staggeringly obscure that absolutely no one would have any advantageous experience with it, so it was a good way to test people on how they think outside their expertise.

The Galileo heat shield overperformed, and a future jovian entry probe could conceivably save a great deal of mass with a more conservative approach. Saturn would really be far more advantageous, with only about 40% the gravity of Jupiter. Since Saturn, Uranus, and Neptune have almost identical gravity at the cloudtops, a single design could be used for all three, with Jupiter being the hard one.

I'd love to see a mass-produced outer planet entry probe with Jupiter gravity assists flinging them each on their way to the outer giants, but this looks like it's about a mile behind the back burner.

It could well be that NASA looked at the costs of recreating the technology/facilities for a Jupiter probe and found that the dollars were significant. Since the microwave radiometer on Juno captures much of the high value science and Saturn probes could capture much of the detailed shallow composition data, NASA may have decided to forgo the bucks to recreate the capability to do Jovian probes. It's a shame -- I'd love to see the probes.

However, as Bruce Moomaw pointed out to me, there is a Jupiter-Saturn-Neptune launch opportunity in the 2016-19 timeframe that could potentially drop of probes at at least Saturn and Neptune. And then flyby a KBO. Would be a really sweet mission.

Not a math error; but we are in a very different physical parameter regime.

Terrestrial hot air balloons are peak_T limited (the sealant degrades above 120C). Propane burners
(operated at a low duty cycle) have plenty of power.

Unlike a buoyant gas balloon (where you can just make the envelope bigger to get the volume=r^3,
area=r^2 scaling) you can't just make a hot air balloon bigger, as this increases the area over which
it loses heat, so for a given power you can only sustain a lower temperature, so you need more volume, so....

Turns out nobody (as far as I know) has considered this analytically until this year, since it is such a different
situation from Earth. I have a paper in review at the Aeronautical Journal where I derive an approximate analytic
solution (where the heat transfer is linear with temperature, more true for Titan/Jupiter than Earth...)

for a 2kW power source (1 MMRTG) at 1 bar on Jupiter, assuming heat transfer coefficient of
1 Wm-2K-1 (I bet it is higher for Jupiter, actually, so a worse situation) and a 0.1 kgm-2 envelope
you are able to float 0.07 kg of payload. My wristwatch (which has a pressure sensor and compass!)
would pretty much be it.

On Earth you could float just under 2kg - my laptop.

On Titan (ok at 1.4 bar, so cheating a little bit) you can float 270kg - me and a couple of friends!

High molecular weight and low temperatures are what buy you hot air balloon performance.
(These are all absolute maximum values - practical designs would be a factor of two lower).

Terrestrial hot air balloons are basically sporty - high manoeuvrability (and small envelope size
to facilitate launch/packaging in trailer) at the expense of low mass/power performance.

One last aside - the reviewers of my Titan balloon paper (just out in the January 2008 Journal of
the British Interplanetary Society - www.bis-spaceflight.com) noted that the Daedalus interstellar
probe study long ago proposed reactor-powered Jovian hot-air balloons to do the propellant mining
for the probe....

Ralph

I believe Arthur C. Clarke's Hugo award winning novella "A Meeting with Medusa" <originally serialized in PLAYBOY!> involved a reactor powered hot air balloon.

Ralph: Thanks for such a scholarly and *polite* answer! I hadn't been able to figure how much power you needed for one of these things, but I had guessed you'd need a reactor, not just an RTG. (Propane didnt seem to be the answer.) :-) Does it help much to use a different material for the balloon -- something with better insulating properties?

Ed: In the Clarke story, the balloon was fusion powered. A big (but not heavy) laser fused lumps of frozen deuterium. Pity inertial confinement fusion didn't work out. :-(

--Greg

Indeed yes - double-walled balloon (shortens the length scale over which convection can work)
is the practicable solution [already used on IIRC the Breitling orbiter round-the-world balloon,
which was a Rozier rather than a pure hot air]

This decreases the heat transfer considerably (but is rather harder to model/predict)

Maybe bubble-wrap would work. (Or think of it this way - you could half the convective heat
transfer with a 1cm layer of something with a conductivity of 0.01 Wm-1K-1, say aerogel. But that
would likely multiply the weight per unit area by a factor of several-to-ten) And remember you
have to package this and inflate it somehow - so simple double wall is about as good as you can
realistically propose.

As I said, figuring this out is sort-of a new problem : 2 centuries of ballooning means the
whole field is rather empirical in nature, nobody needed to work with the fundamentals -
you could just evolve from the ample experience base.

It seems that pure H2 would be about 76% as dense as the jovian mix of gases. So thermals aside, some of the work could be done if a filter could ensure that the methane and helium could be removed. I can think of a few ways to do that, but it would be harder to do it rapidly unless you bring the hydrogen along. (Something funny about taking hydrogen to Jupiter...) Maybe bring a tank of compressed H2 that is jettisoned once the balloon is filled. And even if He+CH4 gradually leaked in, at least you'd start the journey with the pure H2.

Then you could use a pure-H2 balloon and you'd still have to heat it, but the margins would all get a bit easier to deal with.

As I recall, keeping H2 contained is a problem -- those small molecules creep out.

Another way to skin the cat would be to use the balloon technology -- the name escapes me -- where IR radiation can enter, but not escape. The air volume is heated from the surrounding environment through a green house effect. Lots of heat at Jupiter.

Note that the lower the molecular weight of a gas, the MORE thermally conductive it is. A H2 hot air balloon will cool faster, for example, than a He hot air balloon on Earth (if you were heating for extra lift)

I think He would sneak out even more readily: helium atoms and hydrogen atoms have the same outer shell of electrons, but helium is monoatomic.

The result is the same whether it's helium sneaking in or hydrogen sneaking out, however. Methane would permeate more slowly. And the same double-barrier (or bubblewrap or whatever) that tempered thermal loss would temper impurities seeping into an initially all-hydrogen balloon.

All told, I'm not sure that a long-lived balloon would really want a heavy payload. The most interesting results would come from the radio science. Another approach would be to have some instruments (like a GCMS) that perform their duty early, then drop off like ballast.

Clever idea.

Hmmm. Looks like it might not be all that hard to separate Hydrogen from Helium -- and NASA is already working on it (for different reasons).

http://isjaee.hydrogen.ru/pdf/AEE04-07_Hampton2.pdf

This is the conclusion "The alloy, LaNi5, was shown to have the kinetics and capacity needed to remove hydrogen from a flowing stream of hydrogen and helium, thus allowing purification of both gases."

--Greg