Some of the moons of the outer planets of the solar system are large enough to be suitable places for colonization. Even small moons could be dug out to create habitats. They are not big, but they are many. Many of the larger moons contain water ice, liquid water, and organic compounds that might be useful for sustaining human life. Colonies in the outer solar system could also serve as centers for long term investigation of the planet and the other moons. In particular, robotic devices could be controlled by humans without the very long time delays needed to communicate with Earth. There have also been proposals to place robotic aerostats in the upper atmospheres of the gas giant planets for exploration and possibly mining of helium-3, which could have a very high value per unit mass as a thermonuclear fuel.
The Jovian System
Currently, human colonization of the planet Jupiter is beyond the abilities of any type of envisioned technology. Due to its lack of surface, any type of colony on Jupiter would have to be airborne. Airborne cities for human colonization would prove inefficent and have little scientific value, and would also be dangerous. Jupiter's atmosphere is predominantly composed of hydrogen, which means that leaks of certain gases which could be used to propel the colonies would likely end in catastrophe. In addition, Jupiter has the highest gravity of all the planets at more than 2 times that of Earth, and thus would make leaving the planet very difficult.
The Jovian system in general poses particular disadvantages for colonizing because of its severe radiation environment and its particularly deep gravity well. Its radiation would deliver about 3,600 rems per day to unshielded colonists at Io and about 540 rems per day to unshielded colonists at Europa. Exposure of approximately 75 rems over a period of a few days is enough to cause radiation poisoning, and about 500 rems over a few days is fatal. Dug out colonies under meters of rock and ice would not have this problem. Artificial magnetic fields could be used for protection from radiation too.
A more promising alternative is to use the last Galilean moon, Callisto, as a site for a manned base. On average, an observer standing on Callisto's near side would receive about 0.01 rems per day, which is a level that can be tolerated by human explorers without the use of cumbersome shielding. The small outer moons of Jupiter also escape the radiation problem, and have low escape velocities, but would not pose any scientific value.
The Saturnian System
The Saturnian System is the most promising gas giant for colonization due to its relative proximity to Earth (compared to Uranus and Neptune) and its lower gravity (compared to Jupiter). A floating city in Saturn's atmosphere would have the same problems as one in Jupiter's atmosphere, but the lower escape velocity and the Earthlike radiation levels would make travel to and from the colony much easier. Saturn also has higher levels of Helium-3 than Jupiter (the levels of this isotope in the atmospheres of gas giants increase with the distance).
Robert Zubrin identified Titan as possessing an abundance of all the elements necessary to support life, making Titan perhaps the most advantageous locale in the outer Solar System for colonization, and saying "In certain ways, Titan is the most hospitable extraterrestrial world within our solar system for human colonization." The surface of Titan is mostly uncratered and thus inferred to be very young and active, and probably composed of mostly water ice, and lakes of liquid hydrocarbons (methane/ethane) in its polar regions. While the temperature is cryogenic (95 K) it should be able to support a base, but more information regarding Titan's surface and the activities on it is necessary. The thick atmosphere and the weather, such as potential flash floods, are also factors to consider.
On March 9th, 2006, NASA's Cassini space probe found possible evidence of liquid water on Enceladus. According to that article, "pockets of liquid water may be no more than tens of meters below the surface." If these findings are confirmed, it would mean liquid water could be collected much more easily on Enceladus than on, for instance, Jupiter's moon Europa. Discovery of water, especially liquid water, generally improves a celestial body's consideration for colonization dramatically. An alternative model of Enceladus' activity is the decomposition of methane/water clathrates - a process requiring lower temperatures than liquid water eruptions. The relatively higher density of Enceladus indicates a larger than Saturnian average silicate core that should provide materials for base operations.
Other Saturnian Moons
Colonizing one of the other larger Saturnian moons would be somewhat similar to colonizing our own moon, although the moons of Saturn have lower gravity.
Because Uranus has the lowest escape velocity of the four gas giants, it has been proposed as a mining site for helium-3. If human supervision of the robotic activity would prove necessary, one of Uranus' natural satellites might serve as a base. An alternative is to place floating cities in its atmosphere. By using balloons filled with hydrogen, large masses can be suspended underneath at roughly Earth gravity. Saturn and Neptune could be suitable as well, but Jupiter would likely not be, due to its high gravity, escape velocity, and radiation.
Neptune and its satellites could also be used for colonization, but are farther away, and Neptune has a higher surface gravity than Uranus. Its satellites, especially Triton, could also be colonized. Triton's surface shows signs of extensive geological activity implying a sub-surface ocean, perhaps of ammonia/water. Tapping such geothermal energy would make colonizing a cryogenic world like Triton feasible, supplemented by nuclear fusion power.
Kuiper Belt and Oort Clouds
The noted physicist Freeman Dyson identified comets, rather than planets, as the major potential habitat of life in space. Artificial gravity colonies are theoretically possible, but long travel times would make external resources quite difficult to obtain.
There are various difficulties in colonizing the outer solar system. They include:
- Distance from Earth: The outer planets are much further from Earth than inner planets, and would therefore be harder and take longer to reach. In addition, the communications delay between the outer planets and Earth would be very great.
- Supplies: Food supplies would have to regularly be brought to colonies on any planet. The greater distance from Earth makes this much more difficult.
- Planetary conditions: The outer planets have no surface to land on, so any habitation would have to use floating colonies, increasing complexity and decreasing reliability. The moons/comets do not have this problem, although some have specific problems (ie., Europa is in Jupiter's intense radiation bands). Digging out mini-moons would be one solution, shielding out radiation.
- Power: Solar power is generally considered unsuitable because of the enormous distance from the sun. Nuclear power is believed to be the only suitable power source for the colonies. Vacuum energy is another possibility.
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