This idea dates back to the Russians in the 1970s. The surface of Venus is far too hot, and the atmosphere too dense, for Earth life. However, our air is a lifting gas on Venus, with about half the lifting power of helium on Earth.

A habitat filled with an Earth atmosphere would rise naturally and float high in the dense Venus atmosphere, at a height where the pressure is 1 bar, i.e. same as the Earth atmospheric pressure at sea level. The temperature is also around the range tolerable for Earth life, over 0C.

Also, just as weather balloons naturally rise to its operating level high in our atmosphere - and doesn't need to be engineered to hold in high pressures, so it works in the same way for our habitats. They float at a level where the pressure is equal inside and out, and can be of light construction.

In this way, humans perhaps could colonize floating colonies just at the tops of the clouds of Venus. The surface of Venus is harsh in the extreme, way outside the range of habitability for any known form of Earth life. However, the environment at the cloud tops is surprisingly habitable.

Russian idea for a cloud colony in the upper atmosphere of Venus, proposed in 1970s
 original article (in Russian) - and forum discussion of the article

I mentioned this idea in my Trouble with Terraforming Mars, but not in much detail. Since then I've found that, though there are only two recent peer reviewed articles on the subject, there is a very active discussion of these ideas currently in forum posts in various places on the web.

This includes discussions by the members of the Venus Society, by Jeff Goff and his commentators on the Selenium Boondocks, and by users on the New Mars forums. The ideas they discuss seem so interesting that it is surely worth further study, it seems to me, and yet has been almost totally neglected by academic researchers. That's the basis for this article.

WHAT ABOUT MARS, MOON AND THE ASTEROID BELT

Mars has often been suggested as a second home for humanity. But it might not work as well as you'd hope, see my Trouble with Terraforming Mars, and Ten Reasons NOT to Live on Mars, Great Place to Explore.

The asteroid belt seems a much better choice, also NEOs, if you use the materials mined from the asteroids to make new space habitats. See my Asteroid Resources Could Create Space Habs For Trillions; Land Area Of A Thousand Earths

The Moon is a good first choice because it is so close to Earth for resupply and emergency issues.See If Mars Is For Hardy Explorers Only, Where Is The Best Place In The Solar System For First Time Colonists?

Venus though has surprisingly many advantages, and is worth looking at seriously I think.

LIGHT CONSTRUCTION OF THE HABITATS

These habitats could be of really light construction, maybe tensegrity structures - or Buckminster fuller domes or some such. A slightly higher internal pressure would be useful to keep the fabric tight - but you don't have to do this either.

The Venusian cloud cities could use Tensegrity structures, or else Buckminster Fuller doomes - like this one - to achieve lightweight but strong construction. Earth normal air inside would act as a lifting gas to let the colonies float naturally in the habitable region of the atmosphere.

All you have to do is to make sure that the nitrogen and oxygen inside the habitat doesn't diffuse out into the Venus atmosphere - and then it will float at just the right level for an Earth normal atmospheric pressure inside and out.

For instance normal glass such as for house windows would be fine also for outside windows of the structure - or lighter plastics. The only thing is, they have to be acid resistant to resist the concentrated sulfuric acid in the Venusian cloud droplets.

This is a major advantage over space habs such as the ISS, or Moon colonies and so on. These have to be massively engineered to withstand ten tonnes per square meter of outwards pressure. That's why they are built of cylinders and spheres mainly, and is also why they have few windows, and their windows are made of such thick strong glass or multiple layers of glass.

This image shows the ESA Columba module docked to the ISS. You can see the heavy construction, and cylindrical style habitats with few windows. This is necessary because the structures have to withstand tons of pressure from inside, because of the vacuum of space outside the habitat.

Venus cloud colony habitats, by contrast, can be large, lightweight, and airy and with many windows.

Also, we have much less experience of space habitat type buildings, just the ISS so far, as well as construction of spaceships of course, such as Apollo etc.

Venus cloud colony habitats can draw on our experience of buildings for the Earth surface, especially Buckminster fuller type lightweight domes - and experience of building dirigibles.

For the first Venus habs, you could use a Bigelow type inflatable habitat - but far lighter than their space habs. Also, you get much more living space for your inhabitants for the same weight of habitat.

Bigelow's commercial space station plan - made of inflatable modules. The first Bigelow inflatable module is due to be docked with the ISS in 2015. At a later stage they plan to build an entire space station of inflatable habitats, as in this image. The habitat inflates and gets larger when it is deployed.

A similar inflatable habitat idea could work for a Venus cloud colony in early stages of colonization, but could be far lighter or provide a far larger living area for the same weight.

LIGHT CONSTRUCTION OF THE ACID RESISTANT SUITS FOR VENUS

There is no need to wear pressurized spacesuits either as the outside atmosphere is already at the right pressure. Spacesuits are amazingly clumsy in a vacuum because of the internal pressure. It's been described as like trying to work with your fingers inside a pressurized hose. They are also not as easily reusable as you might guess from science fiction stories. Modern spacesuits take ages to put on and take off, is a long process (on the ISS the process of donning a spacesuit starts the previous day with lowering of pressure of the entire ISS).

Space suits like this one are very clumsy in use, because the high pressure inside makes the gloves hard to work with. 

In a Venus cloud colony you would not need a pressurized space suit, just protection from acid. Your gloves could be lightweight and flexible, perhaps more like gardening gloves.

In a Venus cloud colony, if you go outside your habitat you need protection from the acid, and aqualung type breather apparatus. However, your clothes and gloves don't need to be pressurized and would be as easy to use as normal work gloves, e.g. gardening gloves.

So - that's one of the plus points, there are many more. There are many challenges and disadvantages also, though sometimes with surprising solutions to the problems.

Before I go into this in detail though, let's just say a bit more about the state of research into this topic.

PREVIOUS RESEARCH

As far as I know, there are only few serious studies of this idea. Few enough so that I can give just about the entire literature on the subject in a short list.

•  Geoffrey Landis's technical paper on his idea, see Colonization of Venus
• "Rehabilitating" VENUS as a Human Destination by Peter Kokh which was published in Moon Miner's Manifesto.
• Paul Birch's paper, see Terraforming Venus Quickly
• The earlier Russian work in 1971, see Dirigible Venus (in Russian, but you can use Google auto translate to get a rough idea of what ti says).
• The article I used as the source of the image above, with several artist's impressions of the colonies  original article (in Russian) - and forum discussion of the article. (Rough translation of the article in that forum discussion).

And as far as I know, that's it by way of published literature. Also a fair amount of forum discussion but I'll give that in the next section.

Venus cloud colonies have had nothing like the level of attention of Mars surface colonization. There is a small  Venus Society at Linkedin of just over a dozen people. But, there is nothing remotely resembling the Mars society for Mars (headed by Robert Zubrin).

CURRENT RESEARCH IN SPIRIT OF SCIENCE 2.0

So, Venus cloud colonies have had nothing like the attention given to colonizing Mars. But there are a fair number of people now discussing it in forums on-line.

It is a great example I think of Science 2.0 in action. In Science 1.0 then the science is done via peer reviewed publications - the researchers may discuss it before hand of course -- but the final result is a publication in a peer reviewed journal - and that is when others start to cite your results and refer to them and so on.

In Science 2.0 then much of the working out of ideas happen collaboratively over the web in forums and discussions and so on. You will get still peer reviewed papers, they have their place and are important, but are not the only way the research is done.

Image showing difference between Science 1.0 (left) and Science 2.0 (right) made by Tomwsulcer for Wikipedia

This also helps to make it a way of doing science that permits collaboration with amateurs as well, who often come up with innovative ideas that no-one else thought of - and sharing of ideas at an early stage- and making mistakes also. You no longer have the idea that everything has to be totally worked out and polished before you share your ideas with others.

So that's the spirit in which I present this article. Hardly anything I cover here has been worked out in detail.

For the forum discussions see

•  Venus society at Linkedin
• Jonathon Goff's posts, and their comment threads, on the Selenium Boondocks blog
• Venus discussion in the New Mars forums (and other discussions of Venus there)

NO VENUS COLONIZATION ADVOCACY

I don't think any of those involved in this discussion are Venus colonization advocates - not in the sense of some Mars colonization advocates who have the colonization of Mars as their only goal, and no lesser objective will satisfy them. 

We don't know if any of this is possible. It's not had any detailed engineering type review, nor has it had any detailed scientific research or experiments. But it has not had any refutations either, nobody has shown it can't be done. This is simply unexplored territory so far.

So that is the spirit in which to take this article and the research being done on this topic, science 2.0 fashion by many enthusiasts. They do it as much for the fun ideas to explore and the interesting physics involved, as for any serious hopes that it will happen some time soon. Yet there is a tantalizing possibility that these early discussions could lead eventually to these colonies getting built at some point in the future.

This is similar to the spirit with which amateur observational astronomers engage in their hobby. Astronomy is one of the few scientists where amateur observers make significant contributions to leading edge science.

Perhaps you also may enjoy joining in the discussion and theoretical explorations of the Venusian cloud colonies.

Note, I won't try to be objective, not sure one can be with so little peer reviewed research on the topic. So, I will sometimes put my own point of view on the subject in this article, but will try to make it clear when I do. This is just to stimulate debate and my own views on the matters discussed shouldn't be thought in any way special. For that matter, they are also liable to change in response to new studies or information.

So first let's look more closely at the many advantages, and some disadvantages of the cloud colonies

ADVANTAGES OF CLOUD COLONIES FOR VENUS

The main advantages are

1. no need to build structures able to withstand 10 tonnes per square meter of outward pressure (something we have no experience of doing except with spaceships and ISS). Can be amazingly light weight. (I've already gone into that above)

2. Stable temperatures. The main reason ISS deteriorates with design life of 30 years is because of cracks from the expansion and contraction due to huge temperature differences between sun and shade as it turns - same stresses would also apply of course to lunar habs until insulated by thick layer of regolith).

3. Nitrogen from the atmosphere - Venus has 3 bars of nitrogen compared with 0.78 bars of nitrogen for Earth. This means that even if entire atmosphere were filled with cloud cities, with populations greater than the Earth, they would never run out of nitrogen for plants and for buffer gas for breathing. This is a major plus point as nitrogen is generally in short supply in most locations in the solar system (some meteorites are nitrogen rich, but this is rare).

Nitrogen is rare in the inner solar system. This carbonaceous meterorite is rich in nitrogen. So it may be possible to find enough in space for habitats to use.

In the Venus atmosphere however Nitrogen is abundant. This is a major plus point for Venus for resource utilization

4. CO₂ also - and no need to extract the CO₂ from a near vacuum, as you have to do for Mars.

5. It also has a fair bit of water vapour - but unfortunately at the cloud top levels of Venus this is all bound up in extremely concentrated sulfuric acid, with pH less than 0, similar to battery acid.

This is perhaps the main challenge, need some way to get the water out of the concentrated sulfuric acid, by biological or chemical methods, or through use of solar energy. Do that and you have all the ingredients to grow plants and trees apart from some trace elements.

6. Ability to grow construction materials for new habitats. Once you have water and trace elements you can grow trees. Since trees are 90% either water or CO₂, then most of the mass of the trees comes directly from the atmosphere. As the colonies expand, much of the construction material would be wood and fabrics grown almost entirely from materials in the atmosphere of Venus. Wood and plastic is easily strong enough to construct the lightweight habitats needed for Venus via Buckminster Fuller domes, tensegrity structures, and so forth.

90% of the mass of a tree comes from water or CO₂, See this video by Richard Feynmann - the famous nobel laureate physicist.

See this article also, Trees Come 'From Out Of The Air,' Said Nobel Laureate Richard Feynman. Really?

On Venus, the water and CO₂ could come from the atmosphere

The missing ingredients such as trace elements for plants, soil for them to grow in (or machinery for hydroponics) etc can come from Earth initially, later by space mining.

90% of the mass of these trees comes from the CO₂ in the atmosphere or from water. Similarly we could get 90% of the mass of trees and cotton or other plants for materials in the Venus cloud colonies from the atmosphere. This could be used to build new habitats.

7. Easy to send materials to the Venus cloud tops similar to Earth re-entry, just aeroshells 

Artist impression of Apollo re-entry. The same method can be used in the Venus upper atmosphere, no need for retro-rockets.

No need for retro rockets. Also there is no hard surface to impact on, so no risk of collision with the surface. 

After atmospheric entry, you would use parachutes, as for Apollo and modern returns from orbit 

Finally you would also inflate balloons with lifting gas as it slows down, so that your supplies level out at the right level to be easy to retrieve from the habitats. 

This artist's impression shows the European Venus  Explorer - a proposed ESA mission to Venus (which was submitted but not been approved). The Russian Vega probes in 1985 would have looked similar. 

Larger scale missions supplying materials to a Venus Cloud Colony could use inflated balloons after atmospheric entry to float at the desired level for easy retrieval of the materials by the cloud colony.

A simpler approach is to simply use a light-weight sphere for atmospheric entry

This shows a titanium helium tank for from the Russian Salyut 7-Cosmos 1686 (Kosmos 1686) spacecraft assembly, which went out of control and fell to Earth as a fireball, 3 to 4 years earlier than expected, in 1991. Because it is spherical and made of titanium it landed more or less intact with some ablation features.


Supplies for Venus cloud colonies, if enclosed in titanium spheres with the interior adjusted to the right buoyancy for the habitats, would not need balloons or parachutes, but simply need to be fired on a trajectory that will take them into the Venus atmosphere at the right point, and then would survive atmospheric entry intact all the way to the habitat level.

See also, Low-altitude Exploration of the Venus Atmosphere by Balloon

One way or the other, it would be far easier to export materials to the cloud colonies than to the Moon or Mars.

If time is of no concern, if you take the long view, then using the Interplanetary Super Highway you can send material to Venus from anywhere in the solar system, so that also could include ice for water for the habitats - you would send it to the colonies in aeroshells and with parachutes so no need to worry about it getting ablated into the atmosphere.

8. Materials also available from the surface of Venus. Later on as the colonies become more independent, you can trawl the surface much like trawling an ocean bed, You wouldn't send complex sensitive machines down there most likely. Instead you would fly down as far as is safe in airships - remote controlled probably from the habs for safety and because electronics could survive harsher conditions than humans. Then these airships would deploy nets or grabs to the surface and collect boulders, or sand to bring up to the habitats.

Surface of Venus imaged by Venera 11. This extremely hostile surface would be about 50 km below the cloud habitats.

It would be possible to recover materials from the surface. One way to do it would be to fly aerobots remotely down to as close to the surface as is safe. They would then drop nets or grabs to the surface on long cables. The cables could have balloons with lifting gas attached at intervals along the cable to reduce the tension in the whole cable. 

The surface boulders and sands could be dredged and then recovered to the level of the floating habitats for use for soil, extraction of trace elements and metals, and so on.

Later on you could have mining operations on the surface but to start with you could get ballast and useful materials from the surface by trawling in this way much as for an ocean bed on Earth.

9. Venus is easy to get to, the easiest place to reach outside of Earth, the Moon and some NEOs. Many people assume that it must be harder to get to Venus than Mars - but actually - you can go there more often than for Mars. It also has a shorter transit time, five months, and there is less delta v needed for the Holmann transfer also, so it requires less fuel, compared with Mars. You can travel to Venus on a minimum energy path somewhat more often also, every 1.6 years, instead of a wait of over 2 years for Mars. Later on you could have a series of five cyclers to ferry passengers and freight between Earth and Venus

This diagram shows the Holmann transfer orbit for Venus. You can get to Venus more quickly than Mars (5 months), you can do it more often (every 1.6 years) and the amount of fuel needed is less also.

Then when you get to Venus it is easier for your humans to get down to the cloud colonies. It needs no extra fuel, just as for supplies of materials you only need parachutes and deployable balloons.

So, it is really easy to get to the Venus cloud colonies for a one way trip, as easy as almost anywhere in the solar system (though Moon is best for our very first colonies because it is so close to Earth in an emergency or for tourists and resupply and for short term visits).

It is not so easy to get back to Earth from Venus. We will look into that under disadvantages.

10. Has a day of 4 Earth days in the upper atmosphere. That's because of super-rotation of the atmosphere which carries the entire atmosphere around Venus every 4 days. This is short enough to be reasonable for Earth life, if not quite the optimum of 24 hours.

Venusian clouds (in ultraviolet). The high cloud layers super-rotate - they have high wind velocities and circle the entire planet every 4 days. This would give the cloud colonies a "day" equal to 4 Earth days.

11. Same gravity as Earth more or less. Is the only place in the inner solar system with close to Earth normal gravity already. It seems unlikely that Venus colonists would have any problems at all including normal births and upbringing of children, and Venusian colonists would be able to travel to Earth without problems to study or for recreation or whatever.

This may not be such an enormous advantage - depending on future research. Humans may be able to withstand a fair amount of coriolis force with no ill effects in rotating habs, making smaller rotating habs for artificial g possible. It might be that we only need a few hours of Earth normal g a day (e.g. when sleeping) to keep healthy in space. This is all unknown territory at present, despite all our visits to the ISS and many trips to space, there has been almost no research into effects of low g on humans or practicality of rotating tethers or spaceships for generating gravity in space (the long term experiments here simply can't be done on Earth). But does simplify construction at least if you don't need to have internal spinning habs for artificial g.

12. Protection from meteorites. Venus cloud colonies would have no micro-meteorites, a continual minor hazard for space habs. On Venus, as for Earth, these all burn up in the atmosphere long before they reach the colonies.

 Light microscope images of stony cosmic spherules. These are remains of micro-meteorites. On Earth as for Venus they often burn up in the atmosphere, and those that survive fall harmlessly to the Earth


Micro-meteorite damage to the ISS. This one was of no significance, but it is a minor hazard. Habitats in the Venus cloud colonies would not be affected by micro-meteorites - as for Earth they would be removed by its atmosphere.

The atmosphere also gives protection from larger meteorites too. The Earth is hit by meteorites with energy about 3 kilotons every 1.3 years, but this is no problem because they burn up as air bursts in the upper atmosphere. The same would be true of Venus (but not Mars).

13. If the habitat is seriously compromised, it will not explosively decompress like a space habitat. Also, you won't get all the air rushing out of the habitat through even quite a small hole. Instead, as the pressure is the same inside the hab and outside, all that would happen after even quite a large hole is that the outside atmosphere of Venus would slowly diffuse into the habitat. This would give plenty of time to repair any damage.

This could not happen to a Venus cloud colony

A Venus cloud colony is in no risk of exploding like this as a result of impacts - first, the atmosphere protects it from any asteroid impacts and debris up to the multiple kiloton level. Then if it does get breached, by any means including accidental collisions or internal explosions, the result is just a slow exchange of atmosphere between the Venusian atmosphere and the air inside the habitat.

There would be no immediate risk of explosive decompression at all, as the atmosphere and the interior of the habitat are at the same pressure.

All space habitats including lunar colonies and Stanford Torus habitats do have this risk. They can deal with it with shielding, meters thick most likely for a permanent colony, and designed in layers to slow down the impact and dissipate its effect quickly.

They can also move out of the way of larger debris (or adjust its trajectory) - or treat it as an acceptable risk - but if these measures failed and they did get hit the consequences would be severe. Venus cloud colonies don't have this issue at all.

Indeed for small holes, you might wait until you have a reasonable number of them before doing anything, schedule running repairs of smaller holes throughout the habitat. The outer envelopes could be quilt type construction, and internally compartmentalized to reduce effects of even major breach, e.g. caused by collisions with air vehicles.

14. Protection from cosmic radiation and solar radiation - RP100 like Earth. Something close to RP 100 is probably essential if you stay in a colony long term. The levels of radiation you experience in a space walk on the Moon or in the ISS etc can have serious health implications if you do them frequently, so in those places you would probably need to live in a habitat shielded by meters of regolith, and do most of the work via telerobotics anyway. For interesting background on all this I recommend this spaceshow discussion: Classroom with Drs. John Jurist and Jim Logan, (on radiation issues).

Venus is one of the few places in the solar system outside of Earth that humans could live long term without any reason for concern about cosmic radiation or need to take special precautions or to limit the amount of EVA. Another such place would be in the oceans of one of the Moons with sub-surface oceans (which though has obvious planetary protection issues).

In space habitats, shielded by tons of material, then you would be fine inside the habitat, so a large habitat such as a Stanford Torus would be fine. But you would still have to limit the amount of EVA outside of the habitat, to avoid exceeding whatever is determined to be your safe lifetime dose of exposure to the radiation.

With Venus cloud colonies, this is simply not an issue at all.

DISADVANTAGES OF VENUS CLOUD COLONIES (WITH IDEAS FOR SOLUTIONS)

1. Not so easy to leave Venus once there for humans - a Venus One type project suggests itself similar to Mars One

SOLUTIONS:

The Venus cloud colonies do have advantage of no need to build a launch pad. Instead, you can suspend your rocket before take off using a suitably large, probably toroidal hydrogen balloon.

Calculation of the size of torus, filled with hydrogen, needed to suspend a Soyuz, fueled for lift off, in the Venusian atmosphere next to a cloud colony

Hydrogen is a slightly stronger lifting gas for Venus with denser CO₂ atmosphere - and of course no concerns at all of combustion. Hydrogen has a density of 0.0899 Kg/m3. and Carbon Dioxide, 1.977 kg/ m3, for air it is 1.205 kg / m3. So a hydrogen balloon on Venus has lifting power of 1.8871 kg per m3, compared with 1.1151 kg / m3 on EartH

To counteract the mass of a fully fueled Soyuz - lift off mass 308 metric tons (not sure of lift off mass for the latest TMA-M), then you would need 308,000/ 1.8871 = 163213.396216 cubic meters (though I know it is not accurate to this many places, will keep to highest precision until the end of the calc).

Suppose we suspend it from a hydrogen filled torus of radius 50 meters say, then using volume = Pi r^2 * 2*PI * R putting R = 50, then the cross section area of the torus is 163213.396216/(2*PI*50) =519.524375732 so radius is sqrt(519.524375732/pi) or about 12.9 meters

So a torus filled with  hydrogen with radius 50 meters and with the radius of the tube around 12.9 meters, it seems, would be enough to suspend a fully fueled Soyuz for launch from the Venus atmosphere. This does not take account of the extra upward lift due to the volume of the spaceship itself.

Do correct me if I've got any of those figures wrong.

Anyway in short, then it would be possible using hydrogen balloons to suspend the return vehicles in the atmosphere next to the habitat.

You still have the problem, how to get the fuel to Venus but perhaps you can make it in-situ using the hydrogen from sulfuric acid. Certainly we can send it as hydrogen feedstock just as for Robert Zubrin's idea for Mars with an 18 times multiplier, create 18 tonnes of fuel from the CO₂ atmosphere for every 1 tonne of hydrogen sent to Mars - or in this case to Venus.

You only need a rocket able to go from Venus cloud tops to orbit, rest can be done in a spaceship your crew transfer to in orbit.

As for how you send the empty rocket stages to Venus from Earth - obviously need a bit of heavy lift for that - or perhaps you can deliberately send spent rocket stages to Venus gradually with repeated passes of the Moon and Earth first, with gravity assist.

Anyway however you do it,once they arrive at Venus, leave a bit of fuel in the tank,and they should be light enough to float high enough in the atmosphere for retrieval and re-use, as with Jonathon Goff's Venusian Rocky Floaties

So you just need to send one such rocket to Venus and if it is re-usable, e.g. Space-X style, then it can shuttle up and down as often as needed by the colonists.

Longer term perhaps you can have orbital airships as for the JP Aerospace idea. This would be just as for Earth - but they would be somewhat more buoyant, so work better on Venus

JP Aerospace's Orbital Airship. An airship can't fly straight to orbit because if strong enough to survive the winds at low altitudes, it would be too heavy to fly to orbit. But if you put a floating staging post at about 30 to 43 kilometers above the ground, you can then have airships designed to fly in the upper atmosphere that could float at higher levels and simply accelerate slowly,  until they reach orbital velocity.

 Passengers fly up to the orbital platform in conventional airships, then transfer to the orbital ships.  These are in between a spaceship and an airship in design, and would be the largest vessels ever constructed, 2 km long (compared with 380 meters for the largest current supertankers). However, they can be amazingly light and don't need much strength, because they don't need to withstand strong winds or indeed much air at all. They would remain permanently at high altitudes.

Venus airships would need a similar two stage process with the more flimsy higher altitude airships permanently stationed at a higher altitude, with passengers shuttling to them from the cloud colonies using normal airships. If the idea worked, they would then slowly accelerate into orbit with ion thrusters as for JP Aerospaces ideas for Earth. With the denser atmosphere the hydrogen would be a somewhat stronger lifting gas than for Earth.

Nuclear rockets, including ones powered by nuclear reactors (maybe from materials mined in space colonies rather than on Earth), or future fusion or microfusion devices, could also use the atmosphere itself as propellant - not for combustion unfortunately - but you can heat it up in the intake, and fire it out the back of your rocket at greater speed than it came in.

With a good source of energy in your rocket, the atmosphere of Venus could be your reaction mass. The advantage of this is that you don't have to carry reaction mass with you on the rocket in early stages when it is in the thick atmosphere..

2. possible planetary protection issues - likely to be minor but might be major issue if it does have indigenous life from the early solar system that evolved independently from Earth. This needs detailed discussion so I'll do it as a separate section below.

3. sulfuric acid issues - need to protect the habitats and anyone who ventures outside them

4.. Not so easy access to water or hydrogen as for places with ice available.

SOLUTIONS: It should however be possible to get water and hydrogen from the sulfuric acid - and the habitats would be mainly self-contained cycling the water, assume by now we have mastered closed system type recycling.

Once you get to the point where you grow trees to build new habitats though, that needs a constant influx of water so need to work out how to extract it from the sulfuric acid - or else import it from elsewhere.

One idea for production of water in Venusian cloud cities. The Venus atmosphere has a sulfuric acid cycle which produces water vapour naturally. Sulfuric acid droplets fall towards the surface until eventually the water evaporates, and the sulfuric acid also decomposes, to sulfur dioxide (eventually) and water vapor. These two ingredients then rise in the atmosphere and recombine to make sulfuric acid and the cycle repeats.

Perhaps by heating the cloud droplets with concentrated solar energy (solar furnaces) in the cloud colonies, and increase of pressure also if needed, we can separate the water from the sulfur dioxide in the same way to produce water for the habitats.

5. high UV from the sun.

SOLUTIONS: it is not hard to shield a habitat from UV or to protect from UV when you go outside the habitat.

6. day even with super rotation of the atmosphere at 4 earth days is a bit on the long side. But seems reasonably survivable for Earth organisms.

It seems, that the advantages possibly might far outweigh the disadvantages making it in not so distant future maybe an easier place to colonize than a space colony.

CLOUD NINE

The idea of Venus cloud colonies is related to Buckminster Fuller's "Cloud 9" tensegrity spheres. Using his tensegrity construction methods, he worked out that you could build spheres a mile across, strong enough to withstand strong winds and storms (like his domes), yet with the total weight of the structure only a thousandth of that of the air enclosed. Heating the enclosed air just one degree above the surroundings would cause the whole structure float in the air, creating a floating city for thousands of people to live in. He didn't see this as a near future prospect, but thought it might happen in the far future.

He saw the possibility of future floating cities in our atmosphere, tethered to the surface with cables, and able to be moved in response to changes of climate etc.

Project for Floating Cloud Structures (Cloud Nine), by Fuller and Shoji Sadao 1962. In practise they would probably be tethered to the ground with cables.

Venusian cloud cities are similar - except that the Earth atmosphere is naturally buoyant on Venus with no need to heat it up even by one degree to stay buoyant, and no mountains you could collide with.

Venus cloud colonies are like this idea except that they are naturally buoyant anyway, and can be far heavier in construction than his cloud 9, and of any size.

CONCLUSIONS FOR VENUS CLOUD COLONIES

I doubt if Venus cloud colonies in the near future could be easier to colonize than an Earth desert or under the sea - but for those really keen to start up a second home for humanity, Venus seems one of the few places you could do it.

It is even a place where you could maintain a colony with fairly low technology, at least compared with Mars and space colonies. You don't need to hold in the ten tonnes per square meter pressure of an Earth atmosphere against the vacuum of space or near vacuum of the lunar or Martian "atmosphere". You are not critically dependent on machines for temperature regulation. You do need oxygen, but once we master closed habitats like Biosphere 2, then this can be supplied using plants and algae.

In that way the Venusian cloud colonists may well be able to survive without any higher technology at all, just life processes, for most of the time, even repair of their habitats, and no need for space-suits, just air breathers and protective clothing. 

It is hard to imagine it working totally without technology but the demands are less than for other space colonies. A well established self sustaining colony in the Venusian clouds could continue fine for quite a while even if all the machines it uses break down.

This McMurdo dry valley is far colder than it seems, one of the coldest most inhospitable places on Earth.

Yet, any place to colonize in the solar system, even the Venusian cloud colonies, would be harder to make habitable than this. Anywhere on Earth even the harshest deserts, you can at least breath and walk wherever you like hindered only by the need sometimes for thermal protective clothing.

So, especially if you can generate the oxygen for your habs with plants and get life based solutions for extracting the water from the sulfuric acid etc, it seems quite a hospitable place.

It is also stable long term, and resistant to changes introduced by humans as far as we know. Venus is not going to lose its atmosphere in a hurry or go snowball Venus or anything like that .

The Moon surely would be the location of our first colonies, or in orbit close to the Earth, for emergency support if nothing else.
 
I am personally totally skeptical about any plans using present technology to try to colonize anywhere in the solar system, anything like permanently, with travel times of months to get there and 30 minute plus light speed delays. Even the Venusian cloud colonies, I think would surely fail in some Apollo 13 type disaster, with death of the colonists, if attempted today.

That's because so far we haven't yet demonstrated the ability to create a closed habitat that can be sustained and repaired long term without fast and easy access from Earth. But longer term, Venus may seem quite promising.

The main issue to be tackled for early colonists would be, to develop the ability to live in an almost closed habitat (though with arguably better resource opportunities than most other locations) and then, you also need a way to return to Earth - unless you are keen on the Venus One idea.

Some ideas can be sketched out, as above, but really, this needs detailed study and assessment if the idea is going to happen in the near future.

FUTURE TERRAFORMING ISSUES

Venus is one of the planets that could be terraformed to resemble Earth. Indeed it has about the same quantity of CO₂ as the Earth. The difference is that most of our CO₂ is locked up in limestone and other carbonate rocks. It also has no continental drift, as the continents are all "stuck" - and instead has a global resurfacing upheaval every few hundred million years. But this may be due to lack of lubrication by water, so if we were to add enough water to Venus, it might be enough to restart continental drift. That's not certain as its lithosphere is also thicker than the Earth's but is a possibility.

This all requires mega-engineering and would be surely a multiple centuries and probably multiple millennia long project. But still, it seems worth thinking about, could our activities such as cloud colonies on Venus cause any problems for future terraforming, and if so, does it matter?

It's main disadvantage for terraforming is its long day. One idea is to have mirrors in space that cause a simulated sun - shade out the sunlight on the day side at "night" and shine sunlight towards the surface as required, simulating rotation of the sun around the planet every 24 hours. Or it could be solved at ground level by using shades and artificial light. Or life just adapts to the long nights, after all some life on Earth already copes with the month long nights of Antarctica and the polar regions.

Note that Paul Birch in his  Terraforming Venus Quickly suggests that Venus could be fully terraformed over a period of a couple of centuries, from 2040 to 2250, and has put forward a step by step plan with costing, which he believes would work. In his final vision for the planet then he has a simulated sun, with mirrors in space. Cooling of the planet is accomplished using a sun shade in space. and he has cloud colonies, which gradually float lower in the atmosphere as the atmosphere cools down.

POSSIBLE PROBLEMS WITH TERRAFORMING VENUS

Venus doesn't seem anything like as troublesome as Mars, in the Trouble with Terraforming Mars. For instance it's orbit is near circular and its axial tilt is minute so it doesn't need a stabilizing Moon.

But there are some issues.

1. If the Venus atmosphere has billions of people in cloud colonies, then this makes some solutions such as impact of comets to add water more troublesome. as a hazard to the cities.
   This could be fixed by breaking up the comets before impact - if you have the technology to redirect comets, you could also surely break them up into chunks small enough so that all that happens is the inhabitants of the cloud colonies get a display of shooting stars in the upper atmosphere
2. It has only a very weak magnetic field. It's thought that it continually loses lighter gases such as hydrogen from the upper atmosphere, and this could be a problem long term for any water we add to Venus. It also has somewhat less protection from cosmic radiation, though the thick atmosphere would provide adequate protection here on its own
3. The long day after terraforming - without the upper atmosphere super-rotation. This seems only solvable via mega-engineering - such as the Soletta mirrors in space - or via artifical lighting - or use of life forms that can tolerate half Earth year long "nights".
4. As for Mars you might want to introduce life in a slow succession such as cyanobacteria first with no aerobes. The life introduced in the cloud colonies could interfere with this.

As with plans to terraform Venus, I think that any detailed ideas about how to do it, so including Paul Birch's are likely to be oversimplifications. For instance, he has the CO₂ / N ₂ atmosphere transformed to an atmosphere with oxygen and naturally stabilizing over a period of 40 years with 24 hour sunlight from the orbital mirrors. This seems optimistic to me, and then, how do we know, if it works, that you end up with an Earth like climate at the end?

We have no experience of terraforming anything, or even of closed system habitats in space. Earth took billions of years to reach its present cycles and climate. It might be possible to speed that up so it only takes thousands of years., but we can't know that for sure with our present limited knowledge.

Also we see from Venus that there is not only one possible end state for a planet like Earth. There may be many other possible end states, only a few of which are hospitable for humans. 

It might be a tricky job to steer the process of terraforming to the desired end state.

Since we have only had reasonably advanced technology for a century or so, we have no previous experience of technological milllennia long commitments, or even two century long technological commitments as in Paul Birch's idea, if it was feasible.

We do not know for sure that technological civilizations themselves can be stable over such time scales. Hopefully they can be, but I think we need a few more centuries of stability before we can realistically take on a project that requires stability of our civilization for centuries or millennia to be carried out successfully.

Knowledge we gain from exoplanets could help here, also our experience of building larger and larger enclosed habitats in space. If we ever are able to contact other long lived extra-terrestrial civilizations then their experiences of terraforming may also help a lot.

We do however have some ideas for a gentle transition from floating cloud colonies on Venus to a terraformed Venus, and there seems on the face of it at least to be far less danger of messing up the process of terraforming than for Mars.

But there is enough of a danger of that to make it important, I think, that any current ideas should be open ended. I think it is best that we don't make any irreversible decisions for some time yet.

FAR FUTURE IDEA TO TERRAFORM MARS AND VENUS IN ONE GO

Venus has so much CO₂ that even if you could somehow project all of it into space, it remains within the gravitational influence of Venus, and the planet would just gather it all up again and be back again where you started.

Perhaps you could fire the atmosphere of Venus to Mars, using a railgun or similar, as a stream of dry ice pellets enclosed in reflective wrappers to protect them from the suns heat, thus making both planets habitable in a single project. But that is a far future idea, and verging on science fiction, at least at present.

Of course this is something that is way beyond our levels of understanding or wisdom to do right now. Maybe our descendants might think about it a few thousand or even millions of years into the future though....

PLANETARY PROTECTION ISSUES FOR VENUS

Some time back, a team of scientists for COSPAR examined the situation for Venus and came to the conclusion that even with the cloud tops, conditions are so different from Earth conditions that there is no need for planetary protection. They classified it as Category II meaning that you simply need to document whatever it is you do.

However, there was a dissenting voice at the time, by Dirk Schulze-Makuch who was not part of the team. I think it is possible that they might come to different conclusions now if it was re-evaluated.

See Planetary Protection Study Group Mulls Life On Venus

POSSIBILITY OF EARTH LIFE ABLE TO SURVIVE IN THE VENUSIAN CLOUDS

First, in 1991 researchers found some Earth microbes able to survive pH 0 or lower, so perhaps some Earth micro-organisms could live there after all. Only 1% of the bacteria on Earth can be readily cultivated in culture media. There are various reasons why this might be the case. See Strategies for culture of ‘unculturable’ bacteria for an overview.

Also, only a tiny percentage of all species have been studied in any detail. So it is hard to say for sure what the capabilities are of the micro-organisms we haven't yet studied, such as the majority of the archaea.

This is the issue of "Microbial dark matter". For instance a recent study found that - "Of the 100 major branches, or phyla, of microbes, less than one-third have any described species", see How Many Microbes Are Hiding Among Us?

The microbes carried by humans can have hidden extremophile capabilities - because microbes do not lose their capabilities, usually, when they move to a different environment. Some are polyextremophiles able to live in a variety of extreme environments as well as in much more ordinary ones (for humans).

A typical human has 100 trillion microbes in 10,000 species - and the species mix varies from one person to another. Most of these will be unknown to science, and some may well have extremophile capabilities.

For example a recent study of microbial populations of human belly buttons found a couple of species able to thrive in extreme cold and extreme heat. Another example is the discovery of a microbe on a human tongue able to thrive in conditions of very low pressure.

POSSIBILITIES OF INDIGENOUS LIFE IN THE VENUSIAN CLOUDS

The other way then there have been suggestions of possibilities for life in the Venus clouds, indigenous life. There are one or two interesting hints, observations that could possibly be interpreted as evidence of indigenous life.

The most intriguing of these are, the presence of OCS which on Earth would be strong evidence for life. On Venus however it is just suggestive, not conclusive. There are processes that could create the observed levels of OCS without life.

The atmosphere is also not in equilibrium, as it has both H2S and SO2. This disequilibrium is something that life could exploit. The upper atmosphere of Venus also has been shown to contain particles that are microbe sized, and non spherical, which might be an indication of life in the clouds.

Particles in the Venus atmosphere stay suspended for months, rather than the days for Earth. Still, they will eventually fall down to the lower layers, so that makes it an issue, how do the microbes stay aloft? 

I haven't yet seen a worked out answer to this, so here are a couple of suggestions to explore.

First, perhaps microbes in one droplet, descending, could send out spores (explosively perhaps) that land in other droplets that ascend, and so continue the reproduction?

Another idea is based on the observation that some microbes form gas vacuoles on Earth more or less permanent, They are used by cyanobacteria to regulate buoyancy in water, not that far off the idea of using hydrogen vacuoles to regulate buoyancy in CO₂, evolved over billions of years. Is it possible I wonder? The main difference is, that the gas vacuoles in cyanobacteria take up only a small part of their bodies (and are made up of smaller, rigid, gas vesicles). Apparently Anabaema has gas spaces occupying up to 9.8% of their volume (see page 124 of the paper "Gas vesicles"). But this is far below the levels needed for the Venus atmosphere.

For microbes to float in the Venus upper atmosphere, if the vesicles were filled with hydrogen, then with the density of CO₂ of 0.001977 (and hydrogen, 0.000089) compared with water, at 0C, then they would need to have so much hydrogen in the vesicles that they occupy approximately 98% of the volume of the microbe.

I'm not sure if this is possible. However, life solutions are often surprising. If we do find life in the Venus upper atmosphere, then we would need to find out next, how it managed to stay there and reproduce. For instance, could the cells provide hydrogen filled bubbles, or external vesicles filled with gas, which remain attached to their bodies, somewhat like the bubble nests created by some insects, and use those to float in the Venus atmosphere? Or indeed might there even be higher plants or animals that do this?

Froth of Spittle Bug, or Frog Hopper - Larval form - could a similar technique be used in the Venus cloud tops, using bubbles filled with hydrogen, attached to the microbe or higher life form as a type of froth or foam, for buoyancy?

It wWould need to have less than 98% of the volume for the bubbles and the body of the creaure, with the rest all hydrogen, to float at the 1 atm level on Venus.

(This is my own suggestion, not seen it published anywhere)

COULD INDIGENOUS LIFE COLONIZE EARTH AFTER A VENUS SAMPLE RETURN

The study came to the conclusion that due to the high acidity then these life forms if they exist are unlikely to be able to colonize Earth. But  Dirk Schulze-Makuch  was not convinced by this conclusion - so that suggests there is room for discussion here. I am not either.

ACID ENVIRONMENTS ON EARTH

First, the lifeforms, if acidophiles, could colonize some of the most acidic environments on Earth.

RETAINED CAPABILITIES

Also - microbes often retain capabilities that they no longer need. Even though it would be billions of years since these microbes lived in Earth like environments, still there is a possibility that some might retain capabilities from those times to live in less acidic environments than for the Venus clouds.

MICRO HABITATS IN THE VENUS CLOUDS

Another thought, this is my own idea, is that we haven't studied the clouds in detail close up, only from orbit, or with instruments with limited capabilities. Especially if there is life there, given that life can transform habitats and form micro-habitats.

If that's so, there could be micro-habitats in the clouds caused by life processes that are not so acidic inhabited by symbionts which would perhaps need the capability to live in such habitats.

GENE TRANSFER AGENTS

Another potential hazard for contamination, both ways is through GTAs. This was found to be a potential hazard of a Mars sample return. The report by the European Sapce Foundation made a special mention of it.

This assumes that the DNA is similar to Earth DNA with the same cell mechanisms, and the microbes similar to archaea and there is a common origin.

All of that is possible if Earth microbes colonized Venus in our Hadean period.

Then you have the possiblity of Gene Transfer Agents.

Many archaea have an extraordinary ability to share tiny fragments of DNA with other totally unrelated species. In one experiment, researchers added GTAs able to confer antibiotic resistance into a sample of ordinary sea water and left it overnight. By the next day 47% of the culturable microbes in the sea water had taken up this antibiotic resistance capabilities from the GTAs

The GTAs are also small, of order of size of a few tens of nanometers so hard to contain during a sample return.

In this way, Venusian cells even unable to survive on Earth, even if they are dead, might be able to transfer some of their genetic material to Earth archaea so transforming them and adding new capabilities.

This is also a potential issue for forward contamination of Venus clouds. If Earth and Venus life did have this genetic similarity and distant evolutionary connection, there is a chance that Earth archaea, unable to reproduce in the clouds themselves, yet could transfer genetic material to microbes in the clouds.

XNA BASED LIFE IN THE VENUS CLOUDS

Finally, there is the possibility that Venusian life is not based on DNA but some other basis such as XNA (change of backbone) or something more radical than that. If so then we can't really generalize from DNA to capabilities of XNA.

Rotating DNA animation. Could life on Venus have a different backbone from DNA , using PNA, HNA, TNA, GNA or other XNA?

Here XNA is a general term for nucleic acid analogues - with the same bases as DNA but a different "backbone", in place of the Deoxyribose of DNA. These include HNA, PNA, TNA or GNA (Hextose, Peptide, Therose or Glycol NA). The PNA world hypothesis for instance suggests that life on Earth went through an earlier stage where it used PNA (peptide nucleic "acid") before it started to use RNA or DNA. That's because DNA and RNA are so complex it is a little hard to see how they arose from non living chemicals alone.

Life on Venus could have done the same, but maybe didn't end up as DNA, may still uses PNA or evolved to some different form of XNA.

That raises the possiblity that XNA based life could be better at coping with Earth conditions than DNA itself. This could be possible, if it is really a completely different form of life with different metabolism, cell machinery etc. and has never had any previous contact with the Earth environment.

If there does turn out to be life in the Venus clouds, then, the situation is not that different from the situation for Mars.

The microbiologist Joshua Lederberg said

Whether a microorganism from Mars exists and could attack us is more conjectural. If so, it might be a zoonosis to beat all others.

On the one hand, how could microbes from Mars be pathogenic for hosts on Earth when so many subtle adaptations are needed for any new organisms to come into a host and cause disease? On the other hand, microorganisms make little besides proteins and carbohydrates, and the human or other mammalian immune systems typically respond to peptides or carbohydrates produced by invading pathogens. Thus, although the hypothetical parasite from Mars is not adapted to live in a host from Earth, our immune systems are not equipped to cope with totally alien parasites: a conceptual impasse.

Some of these possibilities may seem unlikely, but how do you assess the probabilities of them? Can we design a mission return to Earth able to cope with these possibilities?

Carl Sagan wrote in Cosmos:

“ If we wish on Earth to examine samples of Martian soil for microbes, we must, of course, not sterilize the samples beforehand. The point of the expedition is to bring them back alive. But what then? Might Martian microorganisms returned to Earth pose a public health hazard? The Martians of H. G. Wells and Orson Welles, preoccupied with the suppression of Bournemouth and Jersey City, never noticed until too late that their immunological defenses were unavailing against the microbes of Earth. Is the converse possible? This is a serious and difficult issue. There may be no micromartians. If they exist, perhaps we can eat a kilogram of them with no ill effects. But we are not sure, and the stakes are high. If we wish to return unsterilized Martian samples to Earth, we must have a containment procedure that is stupefyingly reliable. There are nations that develop and stockpile bacteriological weapons. They seem to have an occasional accident, but they have not yet, so far as I know, produced global pandemics. Perhaps Martian samples can be safely returned to Earth. But I would want to be very sure before considering a returned-sample mission.”

VENUSIAN CLOUDS MIGHT BE BEST CHANCE OF XNA IN OUR SOLAR SYSTEM

The Venusian clouds indeed might be one of our best chances of finding XNA in our solar system - in the remote case where there is life there. That's because for billions of years it has been almost impossible for Earth life to be transferred to Venus, the surface of Venus has been so hot it would be destroyed before it could reproduce even if it made it all the way to the surface of Venus.

The other way around also, then it has been almost impossible for the cloud top life of Venus, if it exists, to be ejected through the thick atmosphere as the result of meteorite impacts on the surface of Venus, with enough velocity to leave the strong Venus gravity and get transferred to Earth or Mars. A huge asteroid impact on Venus would disturb the cloud deck for sure, but could even a giant impact send significant amounts of the high Venusian atmosphere into space?

So, if life evolved independently on Venus, and has been there ever since, it would probably be a form of XNA. In that case all bets are off as far as planetary protection of the Earth. We can't say much by analogy with DNA life even about its size, or its properties or its adaptability to different environments.

There are other places that could have XNA, including Mars, and the Europan oceans, or comets. But Venus has been more isolated from Earth than any of those. Even the Europan oceans could potentially share DNA with Earth through impacts on Earth sending debris all the way to Europa.

This probably could only be possible for Venus in the very early solar system and the Venusian surface might well have been too hostile for Earth life already by the time Earth was habitable.

HAZARDS OF XNA FROM VENUS

That it is XNA does not make it safe for Earth life. Yes, as some say, the XNA would not be adapted to Earth life. But the other way around, Earth life would not recognize XNA as potentially harmful, and it might well not trigger the immune system either. And adaptations of microbes are usually in the direction of keeping the host alive for longer, it is of no benefit to a mcrobe that infects a human for the human to die.

It might also be able to out compete Earth microbes in their own habitats, and yet behave differently from them, transforming ecosystems. It could damage crops or animals, or change the balance in the seas. In the worst case, XNA based life might prove to be better than DNA based life all round. For instance it might be more efficient at metabolizing and reproducing. The very worst case is goodbye DNA.

For these and other reasons, then researchers in the field of synthetic biology, who are actually contemplating the possibility of creating new life based on XNA instead of DNA (by substituting XNA for DNA in a cell, complex process but most of it is now worked out) - they are exceedingly cautious about the research.

In the XNA specifications section of this paper: Xenobiology: A new form of life as the ultimate biosafety tool The authors talk about biosafety requirements for this procedure

"The ultimate goal would be a safety device with a probability to fail below 10^-40, which equals approximately the number of cells that ever lived on earth (and never produced a non-DNA non-RNA life formc). Of course, 10^-40 sounds utterly dystopic (and we could never test it in a life time), maybe 10^-20 is more than enough. The probability also needs to reflect the potential impact, in our case the establishment of an XNA ecosystem in the environment, and how threatening we believe this is."

So, the idea is that the experiments need to be designed so that there is less than a 1 in 10²⁰ chance of the XNA reproducing in the wild outside the laboratory (most likely by making it dependent on some substance not available "in the wild" outside of the laboratory).

IMPOSSIBILITY OF CONTAINING XNA AT SUFFICIENT PROBABILITY LEVELS

XNA returned from Venus could not be contained at those sort of probability levels. It would more likely be a one in a million type containment such as is suggested for the Mars sample return proposals, and is already a major engineering challenge if the particles to be contained are small, say 40 nm across or smaller.

(Incidentally, my own view, I also think the plans for one in a million level containment of a Mars sample are totally inadequate levels of probability for Mars, if the sample happens to contain XNA or substantially different life forms).

For the issues for a Mars sample return see Could Microbes Transferred On Spacecraft Harm Mars Or Earth - Zubrin's Argument Revisted and  Need For Caution For An Early Mars Sample Return - Opinion Piece

NEED TO STUDY IN SITU FIRST

For all these reasons, putting my own personal view here for discussion, I think we need to study the Venusian clouds "in situ" to start with, and send some of the miniaturized biological instruments to study the droplets and particles there first, not bring them back to Earth.

I think the chance that it is hazardous is tiny - but, much as Carl Sagan said for Mars sample return, we shouldn't take even a tiny possibility of existential risk, such as extinction of humans, or long term diminishment of our life prospects, with a billion lives.

Carl Woese, who first classified the Archaea, the third domain of life said in an interview:

When the entire biosphere hangs in the balance, it is adventuristic to the extreme to bring Martian life here. Sure, there is a chance it would do no harm; but that is not the point. Unless you can rule out the chance that it might do harm, you should not embark on such a course.

Carl Sagan wrote in his book Cosmic Connection:

…Precisely because Mars is an environment of great potential biological interest, it is possible that on Mars there are pathogens, organisms which, if transported to the terrestrial environment, might do enormous biological damage - a Martian plague, the twist in the plot of H. G. Wells' War of the Worlds, but in reverse. This is an extremely grave point. On the one hand, we can argue that Martian organisms cannot cause any serious problems to terrestrial organisms, because there has been no biological contact for 4.5 billion years between Martian and terrestrial organisms. On the other hand, we can argue equally well that terrestrial organisms have evolved no defenses against potential Martian pathogens, precisely because there has been no such contact for 4.5 billion years. The chance of such an infection may be very small, but the hazards, if it occurs, are certainly very high.

If there is clear evidence of life there, then we should proceed with extreme caution, and treat the cloud tops as category III, until we know what it is that we are dealing with in some detail.

NEED TO TREAT VENUS ATMOSPHERE AS A CATEGORY III DESTINATION FOR COSPAR

I am not sure whether or not the upper Venus atmosphere is correctly categorized, presently, as Category II (no restrictions only required to provide documentation describing any experiments), because of the, probably remote but not impossible, chance of contamination both ways. I think, personally, that the case deserves a review in light of more recent research and ideas.

Also, apart from classification issues, I feel personally that we should sterilize spacecraft and instruments designed to study the cloud tops of Venus, until we know a bit more about it, as the classification is not certain enough that it might not change in light of future discoveries.

Okay this may add 10% to the cost of the mission (sterilizing Viking added an estimated 10% to the mission cost). That is a big increase when margins are tight, I understand.

But that is well worth it to be totally sure that e.g. if you do detect apparent signs of life in the clouds, such as DNA or amino acids, that it comes from Venus and not your spaceship. Also to make sure you do not contaminate Venus samples or the clouds themselves with reproducing life, including the probably remote chance of some archaea with pH 0 acidophile capabilities getting transferred to Venus on our spacecraft, or some of the archaea able to share their DNA with Venusian organisms via GTAs  (this is a method that the archaea use for transfer of genetic material between unrelated species of archaea, more about this below).

This should be regarded as an exciting possibility. In my own view again, then if there is life in the Venus clouds, especially interestingly different, or XNA based life, this is such a wonderful and interesting result for biology and science and evolution - and in the long run for humanity generally - that it far outweighs the disappointment that we need to postpone colonization of the cloud colonies for a later date.

We should celebrate the discovery of other forms of life anywhere in the solar system. Also, if discovered, proper study of exoplanet life should take priority over colonization, in my view, if there are any conflicts of interest. What do you think?

COLONIZATION IS A LONG TERM PROJECT IF POSSIBLE AT ALL

Colonization of the atmosphere of Venus, if it happens, is a long term process. It might lead to terraforming Venus, if so that is likely to be a multiple millenia process - and even in Paul Birch's massively speeded up mega-engineering version of terraforming Venus, which may be over optimistic, it takes at least two centuries to complete.

There is no hurry about all this, we can afford to spend a few decades studying Venus's atmosphere carefully, and deciding what we want to do. We are not ready to colonize anywhere outside the Earth Moon system anyway right now, in my personal point of view.

This is my own view on the matter for discussion.

You may have your own ideas on all this. Do discuss this in the comments section of this article.

So anyway, as a suggestion, we could explore Venus from Earth using autonomous robots, sterilized to start with until we understand what is there better.

The cloud decks are probably less complicated than Martian terrain, so they may not involve much decision making. Certainly they are easier to navigate, as there is no risk of collisions. So, we might not need to send humans there, for scientific study, or to navigate the probes, at the cloud top levels anyway. 

At least the need for humans for exploration is far less than for Mars.

Idea for a Venusian solar powered airplane. (See also, Atmospheric flight on Venus). This could cruise permanently on the sunny side of Venus, and would be more manouverable than a balloon or dirigible.

One way or another we may be able to explore the Venus atmosphere in an adequate  preliminary way using autonomous robots. There are no obstacles needing real time action by a human pilot (e.g. via telerobotics) so they may be able to fly quite fast under auto pilot. Certainly it is easier than Mars which needs highly advanced autonomous robots, or else telerobotic control from orbit, for adequate exploration in a reasonable period of time.

But there may be some advantages to humans close up - e.g. to sample niches in the clouds that are different in composition - or for decision making during the experimental procedures. So, depending on what we find, perhaps we might send humans to Venus in the early stages. If so, if we still want to keep the clouds pristine, they can go to orbital habitats and study the clouds via telepresence.

Finally if it becomes reasonably conclusive that there is no life there, can start to think about possibilities for the cloud colonies. 

These colonies are not needed for scientific exploration, at least in the early stages. Whatever we find there, we can explore Venus adequately from orbit via telepresence. Orbital satellites can easily get close enough to Venus for telepresence operations of probes in the clouds or rovers on the surface. 

LET'S CELEBRATE VENUSIAN LIFE, IF FOUND

If there is interestingly different life there, let's celebrate it, and put off plans for colonization for a few decades or centuries, until we find out what it is, or are able to simulate / study it with reasonable confidence.

We could still colonize a Venusian cloud layer with life, depending on what we find, for instance, if confident that Earth and Venus life can co-exist without harm to either (this is possible if they occupy different regimes and neither can occupy the habitats occupied by the other).

LET'S MAKE THIS A TIME OF OPEN ENDED EXPLORATION AND DISCOVERY OF THE SOLAR SYSTEM

In this Venus is no different from Mars, Jupiter, Mercury or even the Moon. I think at present we simply don't know enough about our solar system to start making long term plans for colonization or terraforming. Instead we need small research stations like for instance the onees in Antarctica, also can send human explorers, like the early Antarctic explorers to the solar system so long as they are careful to keep it clean and explore in a biologically reversible fashion.

I think the exploration of Antarctica is a far better analogy for present day solar system exploration than the colonization of e.g. N. America or the Polynesian islands. Simply because space locations we can visit are so hostile, more hostile than Antarctica.

The Belgian Princess Elisabeth Antarctic Station - this is the first Zero emission Antarctic station - it generates all its own power through green energy sources such as solar and wind power.

At the present state of knowledge of the solar system, and the present state of our capabilities for closed system habitats, and the harsh hostile conditions in all proposed locations for space colonization compared with Earth, then Antarctic stations are a better analog than any historical colonizations such as of the Polynesian Islands or the Americas.

Early settlers to these places did not have to mine for ice to get water, or ship it in as ice, or extract it from sulfuric acid in the atmosphere, or extract oxygen for breathing from water or other sources. They didn't need to worry about whether the air was breathable and indeed most of them took this totally for granted.

Venus cloud colonies are perhaps the most hospitable places in the solar system according to some measures. Far more hospitable than Mars, I'd say, and more hospitable than the Moon also though not so close to Earth or so easy to return to Earth from.

Still, none of these places are anything like as hospitable as Antarctica where you can at least breath. Venus cloud colonies might perhaps be comparable to ideas for sea bed habitats on Earth, however - if the water supply and easy return of colonists to Earth issues can be solved.

If so they are far more hospitable than Mars or the Moon, and may be our best bet for a second home for humanity in our solar system.

We should however bear in mind that there have been almost no peer reviewed studies of this, just two papers in recent years. So it is necessary to keep an open mind about it. It is surely too soon to come to a final conclusion one way or the other.

What do you think?

SEE ALSO

Most of my other articles are about Mars. However some of the issues raised in them are also relevant for Venus.

Also you might like to read these if you think that Mars is the best choice for a suitable second home for humanity. It is natural to think that, since Mars is often suggested as a good place to colonize in news stories. However, if you read the articles, you will discover that there are many good reasons to consider, which suggest that Mars is not really such an ideal place to colonize in the near term. It is far more hostile to most Earth life than the harshest deserts on Earth, and is likely to stay that way for centuries. Also there are many issues that could arise in any centuries and millennia long project to terraform it.

On problems that could arise while terraforming Mars

Some of these issues could also arise while terraforming Venus.

Trouble With Terraforming Mars - has short section also on the Venus Cloud Colonies

Would Microbes From This Astronaut Make It Impossible For Anyone To Terraform Mars - Ever? - problems that may follow from introducing life to a planet in an unplanned way - this could also be an issue for terraforming Venus using ecopoesis - if you expect the life you introduce to create an oxygen atmosphere for instance.

On reasons for exploring Mars rather than colonizing it, in the near future

This is relevant because Mars is often cited as a possible second home for humanity.

Also, as I said above, though perhaps we might not mess up Venus as badly as Mars by colonizing it without much forward thought about the possible consequences, yet, what is the hurry to colonize Venus either? So,some of the same considerations apply to Venus as for Mars.

Ten Reasons NOT To Live On Mars - Great Place To Explore

"Ten Reasons Not To Live On Mars, Great Place To Explore" - On The Space Show

Mars, Planet Of Surprises, Great To Explore Not So Great To Colonize - 1. Is It As Good A Place To Live As A Desert? (first of five articles that go through it more slowly in detail)

Might there be Microbes on the Surface of Mars?

Space Habitats For Colonists - And Contamination Free Boots On Mars - With Telerobotic Avatars - we could use the same methods for Venus in the very early stages, if it was necessary to keep the upper atmosphere free of Earth life - and also because of practical issues of return of humans from Venus cloud top levels. So again there are close similarities between Venus and Mars exploration in the early stages - though with Venus we can probably explore the cloud tops at least more easily with semi-autonomous robots, since there are no hazards by way of terrain to navigate in real time.

On other places in our solar system which could be a second home

If Mars Is For Hardy Explorers Only, Where Is The Best Place In The Solar System For First Time Colonists? - this suggests the lunar peaks of eternal sunlight as a good nearby place for first colonies

Asteroid Resources Could Create Space Habs For Trillions; Land Area Of A Thousand Earths - suggests that long term, resources from the asteroids used to build habitats are more likley to be humanity's second home than any planetary or moon surface colony.

On forward and back contamination issues, for Mars

Could Microbes Transferred On Spacecraft Harm Mars Or Earth - Zubrin's Argument Revisted - though it is unlikely that Venus and Earth shared microbes for the last few billion years, still, they might have shared life in the early solar system - and indeed, Mars and Earth also are most likely to share microbes in the early solar system during the late heavy bombardment.

There are many practical, ethical and legal considerations for a Mars sample return. The same considerations would apply for a Venus sample return, if there is any chance that it contains indigenous extra-terrestrial life. See:

Need For Caution For An Early Mars Sample Return - Opinion Piece

Mars Sample Return - Legal Issues and Need for International Public Debate

Mars Sample Receiving Facility and sample containment