It is approximately 100 years in the future. A base has been set up on the Moon and further settlement is ongoing (total population ~1000). The first interplanetary research base has recently been established on Mars, following the discovery of subsurface life by a previous expedition. However, after extensive studies, it has been determined that:

  1. This bacterial organism is adapted to several different underground environments and widespread across the planet's subsurface lakes.
  2. This organism is COMPLETELY biologically incompatible with Earth life.

Because of this, it has been decided to cancel further human colonization (at least until more data/a plan to breed this organism can be formulated), to avoid both forward and back contamination by microorganisms and preserve Mars's ecosystem for further study.

The question remains, though: if Mars is "off-limits", where else to colonize in the Solar System?

Any input would be much appreciated. Thank you for your time!


4 Answers 4


The asteroid belt

Followed by the moons and potentially rings of gas giants.

The asteroid belt is already planned in real life to be colonized alongside Mars. It’s reasonable to assume that companies and governments would start there. And it’s not like the asteroid belt is unprofitable compared to Mars, it’s probably the single most profitable place in the solar system that will be within reach very soon. Not only that but asteroids can contain ice, which is the one of the most important resources you will need if you want to colonize anywhere. Ice can become fuel, breathable air, and it is just plain water, a very crucial substance for human survival.

After some time missions will focus on the moons of Jupiter and Saturn, possibly also their rings. It’s really only the next reasonable step. Venus is far too difficult to even get unmanned probes on it for more than a few hours, much less humans and I don’t expect Mercury will be any better. Mercury is very close to the sun making it also not too good of a second choice. Jupiter and Saturn are not only potential resources themselves, but their moons are not only the subject of numerous research endeavors and also have large amounts of resources. The rings of these gas giants could also provide beneficial, they are sort of like mini asteroid belts.

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    $\begingroup$ "The asteroid belt is already planned in real life to be colonized alongside Mars." I would like to know who is planning this exactly. Certainly people have ideas there and such, but there is no plan for colonization from any country or company. Even Mars colonization is only in concept stages, except for maybe SpaceX, who still have outlined little. I know I'm being needlessly particular, just wanted to clarify this for people. The closest to firm colonization efforts is through the Artemis program on the Moon, as well as the International Lunar Research Station. $\endgroup$ Jan 9, 2022 at 6:52
  • $\begingroup$ Sure, nothing is a set in stone plan, but If I recall correctly blue origin has plans to mine asteroids. I could be misremembering though. $\endgroup$
    – Topcode
    Jan 9, 2022 at 18:17
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    $\begingroup$ again, nothing super official. They have concepts like the O'Neil cylinders, but I don't consider anything an official plan until they at least say what rocket their using to do something and what year. $\endgroup$ Jan 9, 2022 at 20:20
  • $\begingroup$ @WasatchWind yeah, SpaceX doesn't have "a plan", but they at least have a credible path toward developing one, starting with figuring out how to actually get stuff there. Blue Origin has vague notions of millions of people in space stations, doing...something. There's some awareness that they'll need a bigger rocket, New Armstrong, but only its name really exists. And I haven't seen anything to indicate they have any interest in asteroid mining, people seem to be assuming that's where they'll get the materials for their habitats, but they actually have only shown interest in the moon. $\endgroup$ Jan 10, 2022 at 6:25

Many would probably propose Venus - personally I am against that, but it does have some scientific value in exploration, and there are methods to have people use airships in the atmosphere.

I'd say you're best bet is to go onward to the asteroid belt, especially to places like Ceres. Ceres would probably be a good inbetween point between Earth and the outer planets.

In the outer solar system, Ganymede is one of the best colonization targets. It's a large Moon that's farther out from Jupiter, so the radiation is diminished. The other Galilean Moons are very valuable for scientific exploration.

Going to Saturn, Titan and Enceladus are of interest. Titan would be pretty good for settlement and science, I'm not sure how well settlement would go on Enceladus. Many of these icy moons that may have subsurface oceans might be difficult to build on, and they might harbor life that you want to protect.

Further out then that it's just kind of wherever there's a rock you can build on.

I'll also note that even if you aren't planning on colonizing Mars proper, Phobos and Deimos are still a really great place to settle as a stopping point on the way to the asteroid belt.

  • $\begingroup$ I'm also a strong proponent of colonizing Ceres. Very large asteroid with some evidence indicating that it may have more water than Earth. Lots of raw material to work with. $\endgroup$
    – FontFamily
    Jan 9, 2022 at 17:27
  • $\begingroup$ "stopping point on the way to the asteroid belt" - space isn't like a planetary surface, having a "stopping point" on any space journey massively increases trip time and total delta v required to get from A to B. The only time a waystation makes sense is in orbit around a destination planet, where typically very different craft are required for planetary shuttle runs compared to interplanetary trips. $\endgroup$ Jan 10, 2022 at 4:36
  • $\begingroup$ @KerrAvon2055 For the most part you're right, but Ceres is in a similar orbit to the other main belt asteroids, allowing low delta-v (but typically relatively long) trips between it and the rest of the belt. Stopping there means mostly matching orbit with the Belt asteroids, and it has plenty of volatiles to allow resupply of propellant. It has some potential to be an actual gateway to the Belt, unlike the "Lunar Gateway" station which would just be a detour for any spacecraft on their way to the moon or the rest of the system. $\endgroup$ Jan 10, 2022 at 18:16
  • $\begingroup$ As for Phobos and Deimos, atmospheric braking has a far higher effective specific impulse than chemical engines, and while it's easier to just land if you're using it, it's theoretically possible to use it for orbital capture as well. If Phobos and Deimos have useful propellant resources, you can then take on another full propellant load in Mars orbit, and continue on to the belt with a higher-energy trajectory than you could manage from Earth. It's not as "efficient" and has more complicated scheduling, but has potential to be faster. $\endgroup$ Jan 10, 2022 at 18:18

Titan is actually one of the more hospitable spots in the outer solar system. If you need methane to refuel a spacecraft, there's plenty of it. If you need nitrogen to help terraform Mars when they finally reapprove that project, Titan is the place to go. Colonist applications will be available soon.

Avoid Jupiter and its moons. The last transmissions from the 5 crewed missions sent there should be warning enough for anyone. 😱


Mars is currently fashionable, but I doubt that it's as habitable as its proponents think. We know that microgravity is unhealthy, but there's unfounded hope that 40% gravity won't be. But what if its simply 60% as unhealthy as microgravity? That's still pretty bad. Then, there are the galactic cosmic rays. Earth's magnetic field and atmosphere stop most of them: only relatively benign secondaries reach the surface. But the surface of Mars is irradiated by them. There is again a hope, poorly founded in my opinion, that these are no more dangerous to humans than alpha particles of similar energy. So, I think a healthy human colony on Mars will require a heavy centrifuge as habitat.

But then, why go to Mars? You can put a heavy centrifuge anywhere. I think the solution will be to find poorly differentiated asteroids: they have an excellent mixture of light elements (hydrogen, oxygen, nitrogen, carbon, phosphorus, ...) for sustaining life, as well as metals (especially iron and nickel) for technology. Build your habitat either on the surface or in easily accessible orbit.


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