Venus I think is far more habitable than Mars. You wouldn’ t live on the surface of course, you’d live in airships in the clouds. Earth’s atmosphere is a lifting gas there. So, as long as you maintain the atmosphere of the hab, which you have to in any space habitat, you float and are safe. Gravity levels are as for Earth pretty much. You are shielded from cosmic radiation and also from solar storms, only vulnerabilities from them at ground level are to long distance conducting cables and you wouldn’t have those. UV is easy to protect against. Sulfuric acid droplets are far far easier to protect against than the vacuum of deep space or Mars - you can do it with a thin layer of teflon on the outside of your suit or habitat or other acid resistant plastics. And the Venus atmosphere is rich in all the main ingredients of life: C,N, O, H and also it has a fair bit of sulfur too. The launch mass for the habitat is far lighter than anywhere else for the same population and with a larger interior volume. The main drawback is getting back, which doesn’t however require a Saturn V - you just need a capsule to take the crew to a receiving ship in low orbit above Venus, so something like a Pegasus air launch would do the trick.

But Venus doesn’t have a business case. Nor does Mars, and it also has planetary protection concerns, introducing Earth life to the planet can potentially confuse the search for life and could rob us of the next big discovery in biology. The only place that could perhaps have a near future business case is the Moon. Whether that leads to large numbers of people there, or large numbers of robots and a few people, or just doesn’t work out at all, I don’t know. I think that the Moon can however be a tourist destination, a place for scientists and explorers and much easier of access from Earth and much more habitable than you might guess.

I don’t think either can be colonized right now. Mars shouldn’t be anyway for planetary protection reasons, because we could be robbing ourselves and our descendants of discoveries in biology as significant as the discovery of the double helix structure of DNA, or the discovery of evolution itself. It could be that significant, what we might find if we find either some early form of life on Mars, e.g. RNA world life with no DNA or proteins, or some other form of life made extinct by DNA life on Earth - or some exobiology based on a totally different biochemistry. Also, some of the things we could find there might be very vulnerable to introduced Earth microbes.

Humans aren’t the problem but the microbes that accompany us are. It’s especially easy to see this I think for Mars because the whole planet is interconnected through global dust storms which can shield microbial spores from UV light and it has potential surface habitats such as the RSLs that Earth life may be able to colonize, with many microbes and even lichens shown able to survive Mars conditions in Mars simulation experiments.

Robert Zubrin and a few astrobiologists also have put forward a bold hypothesis that what we will find on Mars is identical to Earth life in all respects, or sufficiently identical that there is no need for planetary protection of Mars. However, this is not at all proven and indeed very much a minority view.

It is indeed possible that some life has got to Mars from Earth. Microbes hardy enough to withstand the shock of ejection from Earth after a giant impact (such as the one 66 million years ago), re-entry and impact on Mars, a century of extreme cold, vacuum and cosmic radiation - and to survive the percholorates, and to find some habitat they can survive in when they get to Mars. There are some lifeforms that can do all that but we don’t know if they have made it ot Mars. Most Earth life certainly hasn’t and wouldn’t last even a fraction of the time needed for that journey. The easiest time for this to happen is in the very early solar system over three billion years ago.

As an example of what we might find on Mars and might be vulnerable to Earth life - the shadow biosphere hypothesis suggests that there may be RNA based life on Earth that uses RNA without DNA or proteins. We haven’t found it, but something simpler must have preceded DNA based life. The reason we can’t find it on Earth is probably because DNA based life made it extinct. So what if it still exists on Mars? Even if a few microbes from Earth got to Mars they might not have been enough to make all RNA based life there extinct. After all if the RNA shadow biosphere is a possible hypothesis on present day Earth - why not on Mars?

As a result, it is not at all proven that any life has transferred from Earth to Mars, it remains a hypothesis at present.

Also Elon Musk himself says the first landing on Mars would be very dangerous, and it’s the landing itself that is the most dangerous part. After a Challenger style accident with astronaut bodies, air, water, food and belongings scattered in small pieces over the surface of Mars, and spores blown in the global dust storms roughly every two years, that would be the end of any chance of planetary protection of Mars.

Venus might possibly have life in its clouds, because of the discovery of non spherical particles, also detection of Carbonyl Sulphide - a clear sign of life here on Earth (though it could be created inorganically on Venus). And Earth microbes probably couldn’t survive the sulfuric acid, but they are not far off being able to, so what if there are microclimates where the acid is not so strong in the clouds? Especially if there is life there already, maybe creating microclimates of some sort? If there is Life in Venus Cloud Tops - Do we Need to Protect Earth - or Venus - Could Returned XNA mean Goodbye DNA for Instance?

So I think we need to explore the Venus clouds robotically first just to make sure. COSPAR has passed it as safe for sample return, but there was a minority dissident view there that it is not safe, in comments by exobiologists after the decision, so I think it needs to be re-examined.

However the chance doesn’t seem that great - but like Mars, if there is life in Venus clouds it would be a major discovery. Not easy for Earth life to get there, even less so than Mars, and it might well be a relic of early life on Venus which as the surface got uninhabitable migrated into the clouds where it could survive due to the very long residence time of months for particles to fall to the surface rather than days - and chance of updrafts taking them up again.

If it is okay for planetary protection, then the Venus clouds have many advantages. Sulfuric acid is not hard to protect against, you can use Teflon for instance and other plastics. Compare the cost of an acid resistant suit covered in teflon, with the $2 million spacesuit you need to protect against a vacuum, and there is no comparison. Far far easier to live in a Venus cloud colony than in space. Though that mainly highlights the difficulty of living in space or on Mars than ease of living in the Venus clouds - of course it is far easier to live on Earth where the air is breathable and there is no sulfuric acid in the atmosphere. But that’s not the comparison here. Compared to Mars, Venus cloud colonies are like paradise in terms of ease of living there.

It’s the same for the colony - the exterior only needs to be protected from acid with a thin layer of plastic. No need to protect from micrometeorites. UV radiation is easy to filter out with a thin layer also. Pressure is the same inside and out so even if it gets torn it’s a minor issue that you can fix in hours or maybe even days, rather than an emergency you have to fix in seconds to minutes.

As for need to keep Earth atmosphere inside so that it continues to float - well you have the same problem anywhere in space that you have to maintain an atmosphere inside. So long as it is breathable you float. So you don’t need to worry about remaining floating. You can have lifeboats in the form of smaller emergency airships docked to it which you get into in case of an emergency.

The main problem with Venus is that it is so hard to get back to Earth. But you only need to survive of the order of an hour or two and rendezvous with a ship in orbit in just the right trajectory to dock with you - as for the ascent stage for the lunar module rendezvousing with the command module for Apollo 11 - rather than needing to hold out for up to several days as you phase into orbit with a fixed space station as they do with the Soyuz TMA for MIR, Skylab or the ISS. You certainly don’t need something as large as the Saturn V or even the Soyuz.

You need some supplies to keep the crew alive for a few hours. You also need to be able to contain the air against the vacuum of space.

Still, it should be possible. The HAVOC mission would use something like the Pegasus air launched rockets. The Pegasus (rocket) has a payload to orbit of 443 kg. After removing 120 kg for a crew of 2 (say), that’s 323 kg. Not very much, I think it might need a somewhat souped up Pegasus? They don’t go into details of the return journey - this was a NASA concept study and most of the focus was on getting to Venus.

However it seems that a return from Venus is something we can do with humans.

The main problem is an economic case for it. But that’s the same for Mars. We don’t have an economic reason for going to either place. Venus is much lower maintenance than Mars, and much larger living area for the same mass launched to Venus per colonist. It would also have far fewer imports because you don’t need to maintain a seal against a vacuum and don’t need expensive spacesuits. Also all the most important ingredients for life are available in the atmosphere - C, H, N, O. with S also quite abundant in the upper cloud layers. You can grow trees for instance, so native wood, to build new habitats and make plastic, just using elements from the atmosphere and trace elements. So they could even build their own new habitats and so long as they can make or import the teflon to shield them, and any electronics etc, pretty much all of the structure itself can be made in situ. Not the same for Mars as the massive metal habitats protecting against the vacuum can’t be made in situ, not without a major industrial base there.

But they still have to pay somehow for the many imports they need. And it won’t be an easy place to live, just easier than Mars. And that’s where you run into problems, what could be valuable exports for them?

I go into this in detail in my Case for Moon first, see the sections Commercial value for Mars and following.

The most likely place for colonization to succeed, I think, is the Moon, up to a colony of perhaps of the order of millions of people. It is resource rich, even has volatiles including water, and significant amounts of CO2, NH3 for nitrogen, in the polar regions, in the form of ice, though we don’t know yet quite how much there is, some of the readings are promising that there are vast amounts there, also we don’t know how easy it is to extract.

The surface is rich in many metals. It may also have the heavier metals like platinum and gold - that’s Dennis Wingo’s hypothesis with some data that would seem to back it up, also makes sense that it must have been hit by many iron rich asteroids as well as the iron cores of the larger asteroids early on such as the 110 km asteroid that created the south pole Aitken basin.

If it is possible to return metals to Earth at a profit, the Moon is the easiest place to do it, because of the possibility of Hoyt’s cislunar tether which would use the position of the Moon high in the Earth’s gravitation well to make the movement of material from the Moon to Earth like rolling a rock downhill, if you use carefully placed tethers you can actually generate a bit of power, by exporting material from the Moon to LEO. By balancing the flow of materials to and from the Moon then you conserve energy and essentially use no fuel at all.

The tether is momentarily stationary relative to the lunar surface and that’s when you load it with materials sent to it from the surface. The lunavator tether in luanr orbit around the Moon to throw the payloads back to Earth. The complete system to transfer materials from LEO to the Moon and back weighs only 27 times the payload it is able to transfer. That’s using existing materials. Also the lunar tether can use payloads picked up from the surface to increase the amount of ballast and so its payload capacity. So it is not mega technology but something we could do in the near future.

"We have developed a preliminary design for a 80 km long Earth-orbit tether boost facility capable of picking payloads up from LEO and injecting them into a minimal-energy lunar transfer orbit. Using currently available tether materials, this facility would require a mass 10.5 times the mass of the payloads it can handle. After boosting a payload, the facility can use electrodynamic propulsion to reboost its orbit, enabling the system to repeatedly send payloads to the Moon without requiring propellant or return traffic. When the payload reaches the Moon, it will be caught and transferred to the surface by a 200 km long lunar tether. This tether facility will have the capability to reposition a significant portion of its “ballast” mass along the length of the tether, enabling it to catch the payload from a low-energy transfer trajectory and then “spin-up” so that it can deliver the payload to the Moon with zero velocity relative to the surface. This lunar tether facility would require a total mass of less than 17 times the payload mass. Both equatorial and polar lunar orbits are feasible for the Lunavator™"

See CISLUNAR TETHER TRANSPORT SYSTEM

We don’t have anything like this for Venus or Mars, so I think that makes the Moon a clear winner for commercial exports to LEO in the near future.

Initially I can see it exporting to spaceships, things like water also metal components that could be made with 3D printers on the Moon, solar panels which are particularly easy to make on the Moon because of the high grade vacuum. Eventually though also precious metals to Earth. The metal is easier to extract than you might think. If it is a pure deposit unoxidized, as for the nickel iron asteroids, you can use the Mond process which turns nickel into a gas at 40–60 C.

Spheres of nickel made by the Mond process. We don’t need the first high temperature phase using Syngas to convert nickel oxide to nickel as it is already in metalic form. The nickel reacts with carbon monoxide at 50–60 C to make nickel carbonyl gas, mixed with carbon monoxide. This is a temperature within the reach of solar heating in a big bag enclosing the meteorite or sample. To extract the iron requires higher temperatures.

You then heat the result to 220–250 °C to recover the pure nickel, in an attached smaller facility such as a 3D printer.

Platinum group metals are digesetd by using halides. Or it might be simpler to just send the residue back to Earth to extract the platinum group metals here

Near Earth Asteroid Utilization and Carbonyl Metallurgical Processes

Here, platinum is the most precious metal, so you’d extract the iron and nickel to get the platinum as residue, but you can also use the nickel and iron in situ as valuable metals.

Paul Spudis writes about it here: Moon First—Mine the Asteroids Later

“Once again, this procedure is simple in principle, but doing such processing in space, millions of kilometers from the Earth, raises many difficult questions, the answers to which are mostly unknown. How could we collect and store the gaseous iron and nickel carbonyls? With no gravity, magnetic field separation might be useful, but this again requires high power and complex machinery to separate the components. The containment vessel must be isolated from other components and unreacted feedstock must be cleared and recycled or discarded; can such delicate and complex operations be automated? Having humans in the control loop might answer a lot of these problems, but the most valuable asteroid might not be close to the Earth – out of reach for human missions, at least in the early stages of asteroid mining.

“I outline these difficulties not to cast doubt on the feasibility of mining in space, but rather to point out that in complex fields of endeavor, we should crawl before trying to walk and walk before attempting to run. Extracting and making useful materials from space resources is an engaging challenge, one whose mastery can change the paradigm of spaceflight. We are fortunate to have within our near grasp, a Moon that possesses abundant “dumb mass” – those resources needed to both create new space faring capability and to perfect the skills and techniques we will need to reach, secure and use the wealth of the Solar System.”

The Moon is the obvious place to go first, and as far as colonization - I think surely a fair way down the road. Just because space is so harsh. Would you choose to colonize the Sahara desert if somehow all the air was removed from it and you also had cosmic radiation requiring you to shield your homes with meters of radiation shielding and you had to wear spacesuits whenever you go outside your house? Nobody would be interested in that.

Well it would be as hard as that and more so to colonize space. So I think we may well do it, but not as soon as the enthusiasts hope, I think they are encouraged mainly by fantasy, vivid analogies with Earth colonization, and a diet of many science fiction stories they have read which make it seem so easy.

The lunar caves however, are amongst the most promising interior as large as an O’Neil habitat quite possibly if we go by the Grail radar data - they may be the easiest to convert to a colony in the near future - already shielded from micrometeorites and radiation and most of the work done to contain the atmosphere / biosphere inside. Still - that doesn’t mean easy as in “as easy as colonizing the Sahara desert or Antarctica”. It would be very tough, initially. If the moon has very valuable exports that could make it worthwhile. Meanwhile it is a place where we can have scientific outposts, and also it is well within reach of tourism from Earth, eventually you just go up there for a week or so holiday within 2 days travel there and travel back, possibly faster as our rockets get faster.

Venus atmosphere is least maintenance but rather far from Earth so hard to see tourism being a major industry there, though scientific outposts for sure, and for long term colonization, then the export needs to become much easier, and you need something that can be made there that is not made much more easily on Earth.

See also Shouldn't Musk try colonizing Venus instead of Mars? in my Case For Moon First