How to Colonize an Asteroid
Selecting the proper asteroid within which to build our colony could be a difficult process. Several robotic probes will have to be sent out ahead of the colony ship to do detailed analysis of the internal structure of a few different asteroids. The asteroids have been pummeled for eons by each other and by sundry space debris - are they filled with fissures or cracks? Do they contain hazardous materials (radioactive elements or caustic materials)?
The most stable rocks are probably the nickle-iron variety - these are the most common survivors of entry into the earth's atmosphere. While we would want a carbonaceous asteroid to supply many of our chemicular needs, these are very crumbly objects and we need the stability that a more dense material would provide.
We must keep in mind that we will have to set our colony rotating, so the size and shape of our asteroid are very important. It need not be perfectly symmetrical, but it must have a stable rotational axis. We can do some exterior trimming, and some balance can be fine tuned by the size and shape of the colony sphere on the inside, but proper initial selection will greatly decrease the amount of work involved. See the section on Design Alternatives for more information.
Another important consideration is the general orbit of the asteroid. Is it one which is on an eventual collision course with earth? An eccentric orbit could bring the rock within a few million miles of the sun every few months - this would not be a good thing. Another possibility is one with an orbit which goes almost to Jupiter - it would get REALLY cold for years at a time. Some orbital eccentricity can be to our advantage though - coming a bit closer to the sun would increase the efficiency of the solar concentration system and speed the work of carving out the main shaft and inner sphere. In addition, the ejection of large amounts of material from the interior of the asteroid will cause a significant orbital shift in the long run. Much of this energy can be directed to the task of circularizing the orbit of the asteroid - or toward making it more eccentric.
A good orbit might be one which wobbled between the orbits of earth and Mars. The colony could act as a trasport for large materials from earth to Mars, and carry large numbers of colonists to Mars. We could use the proceeds from passenger fares to purchase high technology items, like computers, which we would have difficulty producing ourselves.