The closet and most palatable first step would of course be the moon and perhaps until communications occur instantaneously we should spread ourselves like breadcrumbs across the Universe. This does mean making some choices that might seem counter-intuitive along the way with selections of some planets that don’t offer food or water, but perhaps are loaded with raw materials (for fusion, now that we are almost there, etc.) I am speaking of the gas giants.
So starting on the moon and making our way out here is a pioneers reference guide:
THE MOON: On the positive side, radiation, the astronauts greatest enemy thus far, is easily negated by forming lunar colonies from moon dust. Besides making an excellent shield from radiation, it also can give protection from the micro-meteors. Basic calculations suggest that the habitat would need to be underneath 11-14 feet of the dust, but the moon would also make a great place for industry. Consider this. Mining of Helium-3, a rare product on earth but it is not so rare on the moon. The Sun emits Helium-3 and the solar winds deliver it to the Moon where it is absorbed by the rock and soil. And manufacturing, while needing some modification to the methods, can benefit tremendously; especially with our level of skill with robotics these days. Robots could handle most of the work and could be managed for the most part from down here on Earth. AND the low gravity and no wind means it would be easier to manipulate the product but also on wear and tear for the machinery in some cases would be reduced as it is an excellent environment for hardware and microprocessors. Problematic would be those instances when perhaps molten metals were necessary but a centrifuge under part of the factory could take care of that. One drawback might be the meteors. A metric ton in meteors hit our lunar shield every day. Granted more actually hit the earth but our atmosphere keeps a large amount from making it to the surface. There are many way this could eventually be solved with technology but what could we do in the meantime? Maybe drag an asteroid to orbit the moon so the moon has its own shield? The affects would be far reaching and the protection limited. Still if the habitat or manufacturing plant were under 14 feet of lunar soil and outside work was limited, it could just be the cost of doing business.
Next out, though really out there with 56 million miles best case scenario, Mars. Martian is a veritable hotbed of scientific interest. Apparently once a water world of sorts, the poles are in need of great scientific review as some scientists hypothesize that bacteria could live in the ice just below the surface. Building a base would be both a challenge and necessary. Perhaps robotic assistance would be the best way. The technology is there, but controlling them with the time delay in communications. NASA is working on that with their LRCD due to fly in 2014. This would eliminate the 3-17 minutes of delay. So next concern – food. Growing plants in the higher radiation, the low air pressure, reduced gravity and Martian soil. While the ISS has allowed the research needed to grow food in micro-gravity, but sun-light is not as strong and if a greenhouse of sorts were used to eliminate the pressure difficulty then the rays would be further blocked. So what then. Apparently the lower pressure is okay. Plants can still grow when down at one tenth of the pressure. Of course gardening in full astronaut attire could prove difficult. The radiation part, can’t offer any assistance, but technology always comes through. The long travel that limits the sending of additional food could be solved with some space stations sporadically placed throughout space. This would allow the shipments of food until the radiation could be dealt with appropriately.
Space stations and habitats could offer many solutions as we colonize. In space there is an abundance of sun light. Solar cells provide light for the plants to grow and engines that use solar power allow the stations to operate from computers to heat.
Problems that are presented by IO’s volcanos might mean it is ruled out for colonization, but temperatures drop off quick from these volatile sites. Yes the volcanos have incredible force, but the colony would be located at a safe distance. The colony would need to have a quick evacuation method, maybe escape pods in case a new eruption were to suddenly appear.
Also there is EUROPA. That moon is at the opposite side of the spectrum. It has a crust of ice several miles thick and beneath that is an ocean. This could mean under-water cities and a different sort of life then man would have on most planets. There is reason to leave as much of the ice undisturbed as possible, though it would likely quickly form new ice. The radiation level on the surface is extremely high, high enough to cause either extreme illness or even death after only one day. But under the under the ice is another world, a world that is protected from the radiation by its icy barrier and claims an ocean that is believed to be made of salty water. Alien life may not require water, however, it is a safe bet that some form of life inhabits this moon. If this is the case then a food supply could be no problem, underwater cities could draw solar-power from above the ice. The atmosphere is composed of primarily oxygen. The only area of concern as long as one is protected from the radiation, is protection from the weight of the ice if it ever cracked and threatened the cities beneath. Perhaps an array of hydraulics could offer safety. The pressure that would be experienced at the depths of Europa’s oceans would not be as extreme as the oceans of Earth.
Yet another thought for colonization would be JUPITER. Yes it is a gas giant, but floating cities that claimed the upper atmosphere could be possible. Of course Jupiter’s escape velocity is a concern. Perhaps a small scientific habitat would be all that could be managed. With all the minable raw materials available, it is possible that one of the levels of atmosphere would be appropriate for a floating habitat.
The SATURNian system is definitely worth considering. A great source of Deuterium and Helium-3, Saturn is also low in radiation and has its own system of moons. Titan holds incredible possibilities. Mostly uncratered and believed to be quite young it is comprised mostly of water ice, lakes of hydrocarbons in the polar reasons, and a temperature of 95k. Granted 95k translate to -288 but it should be able to support a camp. However, the thick atmosphere and potential flash floods are worthy of consideration.
ENCELADUS and its suspected pockets of water within a 10 meter reach of the surface could offer a hospitable home. The higher density of Enceladus indicates a large silicate core which could provide materials for base operations.
The further out we go the less we know for sure, but we do know that URANUS with the lowest escape velocity of the four gas giants is another site that has potential for a mining operation of helium-3, there are many natural satellites where the likely robotic presence on Uranus could be remotely controlled from. That or try the same floating cities suggested in Jupiter’s atmosphere but as its gravity is about 90% of Earths, a human outpost is more of an option. Remote vehicles would handle the extraction of the raw materials. NEPTUNE would carry the same possibilities.