Oh, well the thing is that nowhere in space is anywhere near as hospitable as Earth. So it’s not so much which is the most habitable as which places are actually worth going to for other reasons. If they have useful resources, then you may have humans there even if they are very inhospitable and have to be supported from Earth. None of them are anything like as hospitable as Antarctica. So I think the most likely situation is settlement supported from Earth. Which could be scientific research, tourism, exploration, or space mining. And for mining, then if the same operation can be done for less cost with robots, you would have robots there instead of humans. I think that might well be the situation, so I expect that mining operations in space would have only a few humans and lots of robots.

With that background, the Moon is the most likely to have lots of people in the near future I think. First, it is very accessible from Earth. If we have tourism in space at all beyond LEO, then the Moon is the natural place. Easier to support large numbers of scientists too, and explorers. And there is much to interest both scientists and explorers. Its potentially vast caves. Meteorites from early Earth and the rest of the solar system. Great place to build radio telescopes on the far side. For some of the benefits see my MOON FIRST Why Humans on Mars Right Now Are Bad for Science

• The Moon is resource rich
• Moon science surprises

I am not sure though. It might be that even for the Moon a lot of the exploration is done from Earth via telerobotics. Perhaps there would be fewer tourists than expected actually travel there?

It may depend on the costs. If we can get it as easy to travel to the Moon as it is to travel across the Atlantic, not impossible with ideas for planes like Skylon able to fly to orbit or perhaps orbital airships as for JP Aerospace’s ideas, then I think we may get a lot of travel back and forth from Earth to the Moon. See my: Projects To Get To Space As Easily As We Cross Oceans - A Billion Flights A Year Perhaps - Will We Be Ready?

If not, if it is a destination for billionaires only, and bearing in mind that probably only a small proportion of billionaires will be space nuts dead keen to go to the Moon - well, maybe thousands living there, not so sure about hundreds of thousands or millions.

However of all the places we can go, the Moon seems the most promising. See also my

• An astronaut gardener on the Moon - summits of sunlight and vast lunar caves in low gravity

As for the easiest place to live, I think Venus wins hands down. Not the surface, but the upper atmosphere. It’s nowhere near as habitable as Earth but far better than anywhere else. The sulfuric acid in the atmosphere is far easier to deal with, far lower tech than a vacuum. You just need teflon covered materials basically, or other forms of very acid resistant plastic coatings.

Longer term, then the lunar caves could be very low maintenance once you have got over the initial cost of setting up a closed system ecosystem inside them.

• Maintenance costs
• Reducing maintenance costs for space habitats
• Maintenance for habitats in free space and city domes
• Maintenance for lunar caves
• Venus cloud colonies - a surprising low maintenance solution

Eventually perhaps these places could become so low maintenance they compete with habitats on Earth, but not any time soon I think,. By the time we can do this in space, we’ll be able to feed four times the Earth’s population from 0.5% of the Pacific ocean just from sea cities floating on the sea, low environmental impact, main imports just sea water and air. We may be able to do that, almost have the technology now in some ways, but probably a bit of a while before we actually have it and use it.

The big problem with Venus is, what is the commercial reason for being there? What are their exports to pay for their imports? Also travel to Venus is easy, comparatively, but travel back is harder.

Mercury could have benefits, abundant solar power and likely to be metals rich, and it is thought to have ice at its poles.

The asteroids long term may have a lot of potential. Enough material there for habitats for a total area a thousand times that of Earth, habitats for trillions.

Callisto in the Jupiter system I think has a lot of potential also. It’s outside Jupiter’s radiation belts. Also it’s got abundant ice. And there are no planetary protection issues - probably. Need to check that but it is probably okay with its subsurface ocean totally insulated from the surface.

The places we should not colonize at present are

• Mars
• Europa
• Enceladus
• Possibly Ceres

because those places may have habitats for life on them. Just microbes but to discover an extra terrestrial biology could easily be the next major discovery in biology. Until we know more, the safest assumption is that every one of those is potentially of astounding interest for biology and should be kept as is. The problem is that once you introduce Earth life to one of these places, it may be irreversible. This closes off future options. Right now I think we need to keep our future open.

Making habitats in lunar caves, at the poles of the Moon, in free space, that all leaves all futures open to us, and expands on our options for the future if anything. So I think that is the direction we need to go right now. We can certainly go to Mars orbit and probably also explore its two moons from orbit, but let’s hold off from the surface for now. I think Mars orbit and Phobos and Deimos need to be our end goal for humans to Mars, with the aim of exploring Mars by telepresence from orbit.

See also my MOON FIRST Why Humans on Mars Right Now Are Bad for Science