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One of the intriguing facts about Uranus is that somewhere within the gravity is about 0.9g. Now, if one wants to daydream about living on Uranus in slightly chilly balloons, it would be interesting to have a side by side view of local gravity and atmospheric density/pressure at different altitudes above Uranus' center.

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Balloons can be quite useful for carrying scientific payloads, but only on Earth, Mars or something else "benign". Not Uranus though. In that context, it's more science fiction (of the remote future, that is) than a serious design consideration.

They are a poor design choice because:

  • They are completely useless for the duration of the rest of the mission
  • They involve tremendous risk if the mission's success is indeed to depend on them
  • Using balloons would require very low injection velocities
  • The must deploy at a certain altitude, because otherwise the pressure would be too low and the balloons would likely pop

That means you need a controlled and very slow decent into a volatile atmosphere, which costs tremendous amounts of propellant, reducing the possible payload mass. Any real space mission to Uranus would therefore likely choose something like controlled high-speed descent of the main probe, until total destruction (which will unfortunately not be very deep).

But, more to the point: Some work has been done no this subject, but not in much greater detail than a basic exponential atmosphere. Existing Uranian atmospheric models differ quite significantly from one another. As you may know, they are all based on Voyager 2 (1989) spectral data for the composition (aided by telescopic data). A lot of the current knowledge is inferred from analyzing the formation mechanisms and dynamics of the clouds in the Uranian atmosphere. This, together with pretty basic physics (like the exponential atmosphere and insolation models to get the temperature profiles) composes all of mankind's knowledge about the Uranian atmosphere.

Analyzing balloon flights on Uranus, given its inherent uncertainties, certainly does not require anything better than an exponential atmosphere. It also does not require you to use a full-fledged temperature profile for the integration of the barometric equation; just use the planet's average temperature (see the bottom of this wiki), and keep it constant; that significantly simplifies the integration. Then it's just a matter of plotting $P(r)$ vs. $g(r) = μ/r^2$ for a range of $r$ that describes the ~20% of the Uranian radius.

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    $\begingroup$ Some more reasons why ballons are a badidea: Uranus Atmosphere is mostly hydrogen, any balloon at pressure and temp. equilibrium would be heavier. Even at the lower limit of the atmosphere, at 100bar the density is about 9kg/m³ - 1% of water - in case someone qants to build a "submarine." $\endgroup$
    – mart
    Feb 19, 2014 at 7:55
  • $\begingroup$ @mart: It's a mixture of H₂ and He, so it's possible, but you'd need some pretty funky type of balloon. But indeed, the specific volume of the mixture is pretty low, meaning that funky balloon would also need to be big :) $\endgroup$
    – Rody Oldenhuis
    Feb 19, 2014 at 9:32
  • $\begingroup$ Re balloons, I find this NASA paper on Atmospheric Mining in the Outer Solar System (PDF) quite interesting. It presents an aerostat conceptual design that indeed uses balloon canopy on "scoopers". They are supposed to be short duration fuel capturing and processing devices. Anyway, I just thought to mention that they are considered. I stumbled upon it when mentioned in Harvesting isotopes of Helium or Hydrogen from outer space. And that's about the extent of my knowledge on the matter. ;) $\endgroup$
    – TildalWave
    Feb 24, 2014 at 11:17

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