Not wanting to disrupt the flow of this thread (too much), we may have already colonised space unknowingly !:
NASA study will help stop stowaways to Mars
http://www.physorg.com/news107608651.html
The article talks about NASA's clean-room procedures, used when preparing interplanetary probes..
"it is extremely difficult to eliminate all dust particles and microbes without damaging the electronic instruments the process is intended to protect."
....
"Clean rooms are considered extreme environments for microbes because water and nutrients are in extremely short supply. Nevertheless, some bacteria are able to survive on what little moisture the low-humidity air provides and on trace elements in the wall paint, residue of cleaning solvents, and in the spacecraft materials, themselves."
So far, we've deposited heaps of probes throughout our solar system. One day, we might be staring into a set of green eyes we accidentally plonked on another planet !!
Cheers
PS: Sorry for the change in tack .. I'm done .. cheers.