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I've read that the atmosphere about 60 km from the surface of Venus is very similar to the atmosphere on earth (the oxygen/nitrogen/CO2 levels and pressures are about the same). If there was a way, to say, create a floating city at that elevation, what would it be like?

I understand that the surface of Venus is extremely hot due to it's thick atmosphere and it's crazy greenhouse effects, but how would the temperature and effects of UV rays be at this altitude? I'm also interested in how the gravity at this level would compare to the gravity on Earth. Are there any other issues that might arise from living there?

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  • $\begingroup$ Welcome to Worldbuilding! Your question sounds very similar to the aerostats found in the sci-fi RPG Eclipse Phase. They work primarily by keeping large quantities of air in an enclosed space, which keeps them at a steady altitude since breathable air is lighter than CO2. $\endgroup$ – emo bob Dec 8 '16 at 14:51
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    $\begingroup$ Obligatory XKCD, note the Venus section $\endgroup$ – Separatrix Dec 8 '16 at 14:59
  • $\begingroup$ Related: worldbuilding.stackexchange.com/questions/41447/… $\endgroup$ – kingledion Dec 8 '16 at 15:18
  • $\begingroup$ Nice first question. Welcome to the site. When you have a chance take a look at the help center to get familiar with the way things work. $\endgroup$ – James Dec 8 '16 at 15:19
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    $\begingroup$ The big issue with building cities anywhere is figuring out where the food's going to come from. So you basically have to figure out how to build floating farms first. $\endgroup$ – jamesqf Dec 8 '16 at 19:02
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Feasible, but with many engineering challenges

First, the background reading. Here is Wikipedia's writeup on the subject, and some more from space.com and The Atlantic. Most importantly, here is a 2003 paper from NASA.

Advantages of a cloud colony:

  • A cloud colony could maintain breathable air at the same pressure as outside atmosphere. At a height of 50-55km, the air pressure ranges from 1 to 0.5 atm. Humans commonly live from 0-3,000m with air pressures from about 1 to 0.7 atm, so there is a several km window of acceptable pressures.

  • Breathable nitrogen-oxygen air would be a lifting gas in Venus's dense atmosphere. That means that a 0.5km radius sphere would be able to lift 700,000 tons; equivalent to two empire state buildings or 7 aircraft carriers. The empire state building has 20,000 people working in it every day, and an aircraft carrier has 5,000 living and working, so a colony of up to 40,000 could be feasible; though 10,000 might be more comfortable.

  • Due to the air pressure being the same both within and without, any leaks or ruptures in the containment material would not be catastrophic. Instead, the oxygen-nitrogen would slowly ooze out, while carbon dioxide (and sulfuric acid) slowly oozed in. While this would not be good, it would allow time for repair teams to fix the problem before everyone suffocate, was melted by acid, or plummeted out of the sky into literal hell.
  • Gravity is about 0.9g and people won't have to worry about microgravity debilitation.
  • The sulfuric acid haze below 50km in the atmosphere is sufficiently reflective that you can get almost as much energy from a solar panel pointing downwards as you can pointing it upwards.
  • The atmosphere is sufficiently thick that even at 50km a person gets equivalent UV protection to when they have on Earth.

Disadvantages of a cloud colony

  • Temperatures at the 50-55km level of the atmosphere range from 75C to 27C. This is a little too warm. If we pushed higher in the atmosphere to the edge of the lowest allowable pressure for comfortable breathing, we'd still have to spend a lot of energy on air conditioning. EDIT As MolbOrg points out, separating the breathable air lift gas into two sections will allow you to partially mitigate this problem by saving energy on cooling the lower section, while getting extra lift from the hotter upper section.
  • Sulfuric acid clouds above the cloud city would produce rain on the city. All exterior surfaces would have to make a material that does not react with sulfuric acid. In particular, whatever is used to contain the 'balloon' of breathable gasses would have to be acid-proof.

Conclusion

While there are engineering, cost, and return on investment problems to be sorted out, floating cloud cities on Venus are feasible.

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    $\begingroup$ from 75C - temperature is not a big issue there as long as you have the second section well above and cool your main section by exchanging heat with upper (and probably lighter section). This also can be used for energy extraction. $\endgroup$ – MolbOrg Dec 8 '16 at 18:38
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Looking at Atmosphere of Venus on Wikipedia we can see how the atmosphere of Venus looks at 60km altitude:

  • temp. of -10°C
  • pressure of 0.2357 atm.

This would still not be survivable for human beings. If at all, go to around 55km height: here the temperature and pressure won't kill you and you could be fine with an oxygen mask.

In any case, both heights are still beneath the cloud layer on venus - which consists of sulfur dioxide and droplets of sulfuric acid. Whithout being an expert, that doesn't sound particularly healthy.

Last but not least, this is not looking at how you'd actually build your city on Venus and make it float at such such a high altitude.

--> Bottom line: you have to create your own atmosphere anyway where you regulate composition, pressure and temperature. Might as well do it in a place where you can at least build on solid ground.

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