UPDATE 2019-03-09. I changed the number of zones from 7 to 8, separating the Sun from Mercury and Venus, as I felt that these are really distinct settlement zones since the Sun and the space near it probably can’t or won’t be settled.
In this post, I’ll explain the 8-zone model of the Solar System that I’ve developed, which I think is helpful for thinking about space settlement. These settlement zones are regions of the Solar System defined more-or-less by the types of objects in them, which have similar features and might be settled in a similar time frame or in a similar way. The inner Solar System and outer Solar System are divided into four zones each.
The image below (not to scale) shows these zones. I’ve assigned each a colour.
Zone 1 (Yellow) 0–0.007 AU
In this zone, we find the Sun. It’s a distinct “settlement zone” only in the sense that it’s a region of the Solar System that cannot be settled due to the intense radiation and gravitational forces. The outer limit of this zone could be determined by the radius of the Sun, which is approximately 696,000 km, or 698,000 km (about 0.005 AU) if several of the Sun’s outer layers (namely the photosphere, chromosphere, and transition layer) are included. The Sun’s outermost layer, the corona, begins at approximately this radius and extends millions of kilometres into space, but has no upper boundary. Another option is to consider that the closest that a known asteroid comes to the Sun is approximately 0.092 AU. Yet a third option is to consider that the Sun’s Roche limit for a terrestrial planet is about 1.1 Gm or 0.007 AU. This is the closest that a terrestrial planet could orbit the Sun without being torn apart by tidal forces. I’ve elected to use this value, as it would probably be quite difficult to build a space station closer to the Sun than this.
Zone 2 (Orange) 0.007–0.95 AU
In this zone, we find the hot terrestrial planets, Mercury and Venus. Although very challenging to settle due to the high temperatures and radiation, and lack of water, these two planets have the advantages of proximity to Earth and the Sun, and gravity levels very similar to those of Mars and Earth respectively. There are no moons in this zone, although there are numerous asteroids. The outer limit of this zone is defined by the inner limit of the habitable zone.
Zone 3 (Green) 0.95–1.78 AU
This is the habitable zone, where we find the cool terrestrial planets, Earth and Mars. It’s where at least 80–90% of human activity will probably always be, even once we’ve expanded to different areas of the System. The habitable zone of a star is the region where liquid water is stable on the surface of the planet, which is considered a primary requirement for life. The boundaries of the habitable zone are not well-defined, because a planet’s habitability depends on its size, atmosphere, composition, and temperature, not only the distance from its parent star. However, although calculations of the outer limit of the Solar System’s habitable zone vary significantly, the inner limit has been determined by numerous experts to be 0.95 AU. The outer limit of this zone is defined by the inner limit of the Asteroid Belt.
Zone 4 (Cyan) 1.78–4.2 AU
In this zone, we find the Asteroid Belt, where the vast majority of asteroids are found. This zone represents vast material resources for building and supplying spaceships, space cities, and infrastructure. The value of these resources to our burgeoning spacefaring civilization suggests that we can expect significant exploration and mining activity in this region of the System. Hundreds, perhaps even thousands, of settlements may eventually be established in this zone.
Zone 5 (Red) 4.2–15 AU
In this zone, we find the gas giants, Jupiter and Saturn. This will be the more popular region of the outer Solar System due to its relative proximity to Earth and the Sun, spectacular views, and vast resources, including numerous major moons that could be settled. The outer limit of 15 AU is somewhat arbitrary, being approximately midway between the orbits of Saturn and Uranus, or approximately half Neptune’s orbit.
Zone 6 (Blue) 15–30.33 AU
This is the zone of the ice giants, Uranus and Neptune. This is an enormous region of the Solar System, which may eventually be settled by some hardy souls; although, doing so will require more advanced technologies, especially in the areas of space transportation, communications, and energy. At this distance, the Sun is just another star in the sky (albeit the brightest) and there is nothing to distinguish day or night. We might consider this zone equivalent to the Arctic or Antarctic of the Solar System. It is very dark, cold, and far from home, and may only ever be inhabited by scientists and robots. Its outer limit is defined by the aphelion of Neptune’s orbit.
Zone 7 (Purple) 30.33–200 AU
This zone extends from Neptune to the heliopause, encompassing the Kuiper Belt and Scattered Disc. Only dwarf planets and small Solar System bodies have been found here so far.
Zone 8 (Black) 200–200 000 AU
This is the zone of interstellar space, from which most comets originate. It extends from the heliopause to the very outer limits of the Solar System, encompassing the hypothetical Hills and Oort Clouds. This is the largest zone and the one about which we know the least. Galactic cosmic radiation here is very high. The outer limit of this zone is defined by the largest possible orbit for an object orbiting the Sun.
Zone 3 is where we live now, and is by far the best suited to settlement. Earth and Mars are the most hospitable and habitable planets, and Luna (the Moon) will be settled purely due to its proximity to Earth. Space stations will be built in Earth orbit, and the moons of Mars will probably also be developed into space stations. The asteroids in this zone will be the first to be explored and mined.
Human activity will then spread into Zone 2, since Mercury and Venus are the next easiest locations to reach from Earth, and these worlds have many similarities with Earth, Luna, and Mars.
We will then progressively expand deeper into the System, establishing bases in the Asteroid Belt (Zone 4), followed by the moons of Jupiter and Saturn (Zone 5), especially Callisto and Titan. As technology advances, a few people may even wish to expand to the moons of Uranus and Neptune (Zone 6).
Our pathway into the Solar System will therefore probably look like something this:
Zone 3 (habitable/cool terrestrials) →
Zone 2 (hot terrestrials) → Zone 4 (asteroids) →
Zone 5 (gas giants) → Zone 6 (ice giants)
It seems unlikely that anyone will choose to live in Zones 7 or 8. Only dwarf planets and small Solar System bodies have been found beyond Neptune, and the distances between them are vast. However, these objects represent a vast store of valuable ices of water, ammonia, methane, and other volatiles. Perhaps mining robots will be put to work out here, creating propellant depots where spacecraft entering or exiting the Solar System can refuel.