Scientists recently discovered a propellantless drive capable of completely negating gravity with low energy inputs! Huzzah! Sadly the Earth is going to be destroyed soon by a rogue handwave. Looking at their possible escape candidates the scientists settle on Venus. They build flying cities and sit them at 50km up.

They're having a great time but some of them want to open a window. What is the fastest way for them to alter the atmosphere to be breathable?

Edit: We can assume our scientists have access to the technology above our current level but within 50 years of advancement so nothing too amazing (think asteroid mining and relatively quick interplanetary travel) and the infrastructure to produce any raw materials or tools needed within a reasonable time frame.

  • $\begingroup$ We need to know more about the technology available to your people. Can you estimate what future year this would take place? Can you give us examples of what technology they have? $\endgroup$
    – JBH
    Commented Dec 16, 2018 at 1:36
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    $\begingroup$ Lol “a rogue handwave” I like it. $\endgroup$
    – Dubukay
    Commented Dec 16, 2018 at 2:33
  • $\begingroup$ 1) Can you make more clear capabilities of your propulsion, energy systems. As for now they seem to break physics and allow you to pump free energy to help us terraforming. 2) You do know at 50km up weather is like 75C and acid paradise? Not best place for a stop even with no windows if you do not wan't to corrode your cities. For now you should stay in orbit. $\endgroup$ Commented Dec 16, 2018 at 2:50
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    $\begingroup$ This depends on your notion of "fast". It took around four billion years for Earth to oxygenate its atmosphere to levels where we can safely open windows. Earth had an ocean to help too. Venus does not. It's not going to be easy. Plus it will happen, if it can, in the blink of a geological eye. Only millions of years. They may need to keep their windows closed for longer. $\endgroup$
    – a4android
    Commented Dec 16, 2018 at 6:43
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    $\begingroup$ @NofP they were left off because any answers given are going to need a lot of suspension of disbelief even if they are as science-based as possible. $\endgroup$ Commented Dec 16, 2018 at 20:19

5 Answers 5


"Oxigenated" and breathable aren't the same thing.

Venus' atmosphere is mostly carbon dioxide. You can fixate the carbon to graphite with relative ease using the Bosch reaction and electrolysis of the resulting water, wherein the hydrogen gets reused as long as needed venting the excess oxygen.

This has only the effect of reducing the habitats' buoyancy and starting lots of interesting oxidizing reactions down on the surface (which by the way will be 50 km lower, not 500).

The problem is that to be breathable the CO2 concentration must be less than 5,000 parts per million (down from the current 975,000), while oxygen concentration must not exceed 60%. We don't have anything available to make up the remaining 40%.

Magically converting 60% of the atmosphere to oxygen would have people die of CO2 poisoning, and converting 100% of the atmosphere would have them die of oxygen toxicity.

Freeing enough nitrogen from the surface of Venus that its concentration at 50 km is breathable also looks like a losing proposition - it's mostly aluminum silicates. Also, I'm not sure we could work a viable solution for O2 and N2 partial pressures at that height.

So, the simplest alternative would be to either build really large habitats, or equip them with very good projectors and external displays.

As @Rekesoft noted, Venus does have nitrogen in its atmosphere, it's just not concentrated enough. To use this, we need to get rid of about 15/16ths of the overall CO2 atmosphere (and then some more, because of the predictable outgassing), leaving in place all the nitrogen. This should give us a 80% N2, 20% CO2 atmosphere, about four times as dense as Earth's; at a sufficient height, that ceases to be a problem (but now buoyancy is). Proceed with the conversion of CO2 to O2 and graphite, and you're left with a breathable atmosphere, as long as you don't go too near the surface.

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    $\begingroup$ Actually, Venus atmosphere contains four times as much nitrogen as the Earth's. Nitrogen is a small amount of Venus atmosphere (either in percentage or mass) just because Venus atmosphere is incredibly dense. $\endgroup$
    – Rekesoft
    Commented Dec 17, 2018 at 9:17
  • $\begingroup$ @Rekesoft right, I wasn't clear - edited answer. $\endgroup$
    – LSerni
    Commented Dec 17, 2018 at 9:53

There isn't a really fast way to oxygenize a whole planet.

You could use plants but no plant would survive. So you could maybe place microorganisms. The organisms will take in the poisonous gases and release oxygen and a little nitrogen. The organisms could be genetically modified to reproduce in a crazy fast rate.

Researchers have discovered a possible new species of bacteria that survives by producing and 'breathing' its own oxygen. The finding suggests that some microbes could have thrived without oxygen-producing plants on the early Earth — and on other planets — by using their own oxygen to garner energy from methane (CH4).

M. oxyfera can survive in mud that is inhospitable to other forms of methane-consuming bacteria such as Methylobacter mesophilic .
Digital rendering of magnified bacteria

Here are some other examples of microorganisms you can use:

Magnified bacteria

Benthic Bacteria
Magnified bacteria

Genetic modification will allow the organisms to survive

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    $\begingroup$ No, bacteria won't survive, with no changes to the atmosphere. Most they can survive are 80-120C. Temperature at the surface is 460C. Not counting living in corrosive pool of CO2 with no water and other problems. You do not want methane as it reacts with oxygen. You may try to make pool 50km up or more, with oxygen seeding and with millions of years you will have oxygen levels. But that is not enough to make it breathable. $\endgroup$ Commented Dec 16, 2018 at 20:57

Cheat and open windows in a few weeks

The stated objective is to be able to open a window in the hull of a floating city at 50km altitude.

Part of the appeal of opening a window on earth is that outside often smells better, or at least different than what it smells like inside. Also, cross breezes churn up the air inside to cool things off. We assume that humans in these Venesian habitats will want some or all of these benefits.

Cheating Approach

Changing the entire atmosphere of a planet is really expensive and really time consuming. Ultimately, humanity may want to do that but it won't be done any time soon.

So, if someone wants to open a window, wrap a big bubble around the living quarters. Fill the bubble with breathable atmosphere. Build in large fans to create winds and breezes. Add small chemical factories that generate aromatic compounds to seed the breezes with. Make these match Earth as much as possible.

Or, seed the outside of the habitat with trees, bushes and flowers. The load on life support goes down and the plants will make their own scents to make the breezes smell different.

Link the air circulators for the outside bubble to the regular habitat air system. This way, if someone opens their window they can get cross breezes through their apartment.

Expensive Approach

Do the terraforming thing. Seed the clouds with the right bacteria to turn the atmosphere into something breathable. Finish the process in your grandchildren's lifetime.


This will the large-scale application of completely gravity negating propellantless (CGNP) drives which, fortunately, only require low-energy inputs.

First, build all the floating cities you need to house the population of planet Earth. Second, build a two gigantic fleets of CGNP drive tanker spaceships. One fleet will scoop up the atmospher of the planet Venus, while the other scoops planet Earth's atmosphere. The two fleets transfer the atmosphere from one planet to the other.

Since Earth is about to be destroyed by a rogue handwave nobody will miss it if it now has Venus' atmosphere instead of its atmosphere.

Venus will have an oxygenated atmosphere. But before you start opening windows, disassemble the two gigantic fleets tanker spaceships and reconfigure them into a gigantic sunshade to allow Venus to cool down and remain cool enough for human habitation.

The floating cities can descend to the surface of Venus, once it's cooled down enough, since it's pretty barren it might be a good idea to bring some of Earth's oceans too, before that rogue handwave strikes. Nothing like prettifying your new planet when you move in.

When all that's done, you can open your windows.


There is no fast way to make it breathable. For that you would need to make at the least a partial terraforming to happen.

At 50 km up in the Venus atmosphere your problems are:

  • Acid clouds, people do not like to breath mix of acids and acidic oxides. So you would need to convert them in to salts, minerals or collect and send them somewhere.

  • Atmosphere is 96.5% carbon dioxide so even if you convert some of it to oxygen to have 21% oxygen at your altitude and deal with acidic compounds. What would take you to move or convert around 30% of the atmosphere of the Venus.

    That still would not do a thing as it still leaves you with a 65% carbon dioxide atmosphere, anything above 1% is not where you want to have a walk or 0.5% for a living.

    But converting more just to oxygen is not an option too as high oxygen partial pressure is toxic too.

    So for extra we can: convert it to water, move to Mars or any other places, convert to minerals, biosphere. And that leaves you at converting 95-96% of the atmosphere. You as well can finish the change and live at the surface and not float ten-something kilometers above ground.

  • Temperature outside is 75C. I would not find it a nice walk. As you make atmosphere smaller and deal with greenhouse compounds, temperature drops. But you will find hard to balance optimal temperature and breathable composition of the atmosphere

  • Around 4 times as much of nitrogen. Not a problem at altitude with a 1 bar pressure, but a problem at the surface. Deal with extra to live at the surface: removal, conversion to mineral, biosphere.

So you have options:

  1. I want to float in my cool cities with breathable outside. That leaves out the need to deal with mostly only nitrogen. Altitude of your cities will drop to ten-something kilometers.

  2. Complete terraforming. With extra 5% work deal with excess nitrogen.

    May as well move Earths oceans or move hydrogen from the gas giants to produce new ones if the Earth ones are already gone in a boom. That will give you a lot of extra work, but people like water and seasides.

Good decision is move some of unneeded atmosphere to the Mars or just plainly terraform it too.


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