# Is it more challenging to put an airship in the Uranian than in the Venusian atmosphere?

The Soviet Union's (in cooperation with some European governments like France and West Germany) Vega missions put balloons in the atmosphere of Venus back in 1985. It was 30 years ago, the last successful interplanetary mission of Soviet/Russia. There are advanced paper plans for airships in the atmosphere of Venus. But I've never heard of anything similar for Uranus or Neptune. I actually hear very little about any mission plan for Uranus or Neptune.

Would it be more difficult to put an airship in the atmosphere of the gas giants, compared to Venus? It is colder and another composition and probably another density profile, and overall less known. What difficulties would arise, and how might they be overcome? Would radio communication get difficult? I note that all three mentioned planets have about the same surface gravity, very similar to that on Earth (although "surface" might have a fuzzy definition on a gas giant). Of course it is much more difficult to get to Uranus, though its deeper gravity well and aerobraking helps, but I ask about what can be done with an airship once a probe has entered orbit there.

I want to add the side question if it would be valuable for exoplanet research to examine especially the atmospheres of Venus, Uranus, Neptune since the most common exoplanet size seems to lie between them, and their atmospheres might be analyzed with telescopes planned for the next few years or a decade. Is there a sensible synergy between studying those three planets here, and exoplanets?

• Vaguely related reading: what-if.xkcd.com/30 Intended as humor of course, but nevertheless does go into detail about the atmospheric conditions of each world. – Flynn1179 Feb 13 '15 at 0:14
• How about ROI considerations? Uranus is ~20 AE away, a frigid hydrogen / helium / methane atmosphere over a water-ammonia ocean. Getting there would be very expensive, and there is very little you could learn on or bring back from Uranus. Venus, on the other hand, has a solid surface, a massive greenhouse atmosphere with many interesting chemical and meteorologic properties, and is routinely no more than ~0.3 AE away. Comparatively easy to get there, and interesting as well. So, without even going into the physics of airships, there's your answer why Venus was considered and Uranus not... – DevSolar Feb 13 '15 at 13:07
• @DevSolar Well, Uranus is tipped over, and Neptune is believed to have shaped the Solar system by migrating outwards, past Uranus. And it has Triton. It seems to me that the ice giants hold the key to understanding the evolution of our planetary system. And since they are most closely like the most common exoplanets, they surely have great ROI to offer for planetary science. Venus isn't easy either. Seismology is wanted from Venus, but how to get it in its hostile climate? – LocalFluff Feb 13 '15 at 13:22
• @LocalFluff: Some valid points re. the importance of the outer giants for our solar system... but what would you expect to gain from exploring their atmosphere with an airship...? – DevSolar Feb 13 '15 at 13:40
• @DevSolar Synergy with spectroscopically studying atmospheres of exoplanets similar to them. But an airship in a hydrogen atmosphere doesn't seem to be a good idea, I understand now. A Galileo Probe, maybe. But that one wasn't a success. Studying Neptune for traces of its migration in terms of its impact history and captured satellites should be high on the priority list. For the 2030's. – LocalFluff Feb 13 '15 at 14:19

Buoyancy is a big problem. To stay aloft, the average density of the balloon envelope, lifting gas and gondola must be <= the density of the surrounding atmosphere.

The pressure inside a balloon must be equal to or slightly greater than the surrounding atmosphere, otherwise the balloon will collapse. If you look at the ideal gas law you'll see that see that there are two ways of satisfying these two conditions: Either the molar mass of the lifting gas must be (substantially) less than the molar mass of the atmosphere, or the temperature must be (substantially) higher than that of the atmosphere, or both.

On Venus the atmosphere is primarily CO$_2$ with a molar mass of 44. The Vega balloons were filled with He, molar mass 4. The buoyancy is proportional to the difference (not the ratio) of the molar masses, thus 40 grams per mole - quite a lot of lift, in fact more than a helium ballon has on Earth for the same ambient pressure.

Uranus' upper troposphere is about 85% hydrogen, 15% helium for an average molar mass of approx 2.3. If you use the lightest lifting gas possible - pure hydrogen - the buoyancy is only 0.3 grams per mole, vastly less than the Vega balloons. It would be extremely challenging (basically impossible) to build a light enough balloon envelope, gondola and payload to be supported by that. Hydrogen is also rather more difficult to transport across the solar system than helium.

The solution is probably to use a hot air balloon, with the air being the Uranian atmosphere. It will still be challenging - you still need a much greater balloon volume for the same payload weight than on Venus, and you also have to bring a compact, lightweight heat source to heat all that atmosphere. These technologies have yet to be developed.

• How about using vacuum instead of hydrogen? It is available in enormous quantities en route to Uranus. Some kind of structure would be needed to prevent the baloon from imploding. And the aerobraking could also be a challenge. – Peter Mortensen Feb 13 '15 at 20:15
• @PeterMortensen that has been theorized before, see en.wikipedia.org/wiki/Vacuum_airship. But even on earth, where the atmosphere has much higher molar mass than on a gas giant, there are no known materials that are light and strong enough to make it possible – David Feb 13 '15 at 21:31
• @David yes but it's also a question of external air pressure. If you want to go high in the Uranian atmosphere where the pressure is only a few kilopascals, a hollow metal sphere can certainly withstand this. If you want to go lower, you don't make it a pure vacuum. You make it maybe half the pressure of the outside (check valves can do this automatically for downwards motion). The hard part is folding up a large solid shell to launch on a rocket, and then unfolding later...and then making sure it's still airtight after reassembly. That part seems pretty impossible. – DrZ214 Jul 18 '15 at 21:00

Venus' atmosphere is mostly CO2 which has a higher molar weight than N2 or O2 so a balloon filled with our atmosphere would be buoyant. Balloons filled with H2 or He even more so.

But Uranus' atmosphere is mostly hydrogen and helium. Balloons that float would be harder. For a more thorough explanation of this problem, see pericynthion's excellent answer.

There are other problems than make Uranian balloon probes a lot harder than Venusian balloon probes.

Orbital velocity just above Venus' atmosphere is about 7 km/s vs about 15 km/s for Uranus. Energy scales with velocity squared. (15/7)^2 is about 4.6. So entry into Uranus' atmosphere would entail shedding more than 4 times as much energy as Venus atmospheric entry.

Also it takes a lot more delta V to reach Uranus than it does Venus. From Low Earth Orbit (LEO) it would take about 8 km/s to send a probe along a Hohmann transfer to Uranus. Tran Venus Insertion from LEO is about 3.5 km/s.

If coming from earth along a Hohmann orbit, the probe would be moving about 22 km/s when it reaches Uranus' atmosphere. And about 11 km/s when reaching Venus' atmosphere. Assuming a Hohmann path, Uranus atmospheric entry is a bigger problem.

Earth to Uranus Hohmann trip time is about 16 years. Earth to Venus Hohmann trip is about 5 months.

Solar power is an option for Venusian probes, Venus receives about twice earth's insolation. At ~20 AU, Uranus receives about 1/400 of the insolation that earth enjoys. The Uranian probe would need to be nuke powered.

My first concern was the magnetosphere, but Venus seems to have little or no intrinsic magnetic field, and Uranus's is somewhat stronger than Earth's, but not ridiculously so.

The temperature and pressure of Venus' atmosphere at 50-65km altitude are pretty manageable, not unlike conditions that can be found near Earth's surface.

Uranus's atmosphere seems to be much colder at comparable pressures. Keeping your airship heated (at high altitudes) and its envelope pressurized (at lower altitudes) is going to be the challenge there.

Radio communication is hard enough at Uranus' distance outside the atmosphere; I imagine you'd want the airship to have a low-power radio link to an orbiter (using frequency bands selected for minimal atmospheric absorption) and the orbiter would have a higher-powered comm link back to Earth. A similar strategy might be necessary in Venusian atmosphere too, if you're deep enough; sending an orbiter with a separate atmospheric probe probably has a lot of other advantages anyway.

Regarding exoplanets: big exoplanets currently appear to be the "most common" because massive planets in close orbits around their stars are the easiest to detect. While we might learn something about Uranian-type exoplanets from studying Uranus, they still aren't going to be as important to us in the long term as more terrestrial-type planets.

• Why were you considering the magnetosphere as a first concern? Radiation effects on electronics? – pericynthion Feb 12 '15 at 22:02
• Yeah, and interference with comms. – Russell Borogove Feb 12 '15 at 22:28