Some factors I can think of:
- Gravity of the planet or moon, as higher gravity requires more energy to take off from the planet, thus making industry/trade with Earth more expensive and making the planet less desirable for colonization.
- The presence of substances useful for life, like water.
- The ease of transporting people and goods to and from Earth (spacecraft delta-v and launch time window)
- Size of the object, some are just too small. (would asteroids of a few km in radius be suitable for this?)
- Stable, non-extreme climate (low eccentricity and temperature around 0C, the colder, the more energy is needed to maintain warmth)
Aside from being practical, would it be likely that the first significant humn settlement would be on that object, given the choice?
The Earth’s moon is an obvious choice but lets say that they want to stay out of the Earth countries’ military and political reach as much as possible? Anyhow, they won’t want to be on the moon for the purposes of this question.
One of the easiest to colonize might be Mars because of its proximity and relatively nice temperature, but it requires a lot of energy to launch objects from Mars, making trade with Earth significantly more difficult. Maybe one of it’s (tiny) moons Phobos or Deimos, but they are small, yet far larger than most Earth cities and comparable to it’s largest ones.
Next are the moons of the gas giants, some of which have more useful resources than any of the inner planets, having large amounts of water ice, and in the case of Titan, large amounts of a substance that could be used as fuel in a similar way that oil is used on Earth. But these moons are also of the larger variety, with a good amount of gravity. So perhaps a tiny moon is more practical, with the larger moons only used for resource extraction, where it is practical. Launching cargo from resource extraction into space using a mass driver might not be difficult, but mass drivers seem to create too many g’s for humans to handle without getting crushed. Thus, a tiny moon would be more practical.
Another option is Ceres, a small object but not tiny, maybe just the right size, it has plenty of space for human societies (2 million square km). Also it is in the asteroid belt, possibly the cheapest place to mine for valuable resources, and it could attract an “asteroid rush”, like a “gold rush”. The asteroids have many extremely valuable resources that are worth many trillions, the rare-earth materials and metals like gold. There are also comets in that belt, giving the colonists of Ceres water, if it isn’t on the dwarf planet itself, and it is heavily speculated that there is. If it isn’t subject to an asteroid rush, then it is a good strategic object for an Earth government to claim and establish a sizable presence, because of the surrounding material wealth. Ceres also does not have the radiation issues of the gas giant moons and it is closer to Earth.
Another possibility is a large space station, but that would probably be a lot more expensive to construct.
Looking forward to reading everyone’s thoughts.