Was skimming the System Information Window when I noticed something curious.
Asteroid #72, colony cost: N/A, surface temp -100.6, Gravity 0.14, distance 400m km, diameter 2000 km
...that is, incidently, well above the lower bound of my gravity tollerance - geosurvey even gave it about 2 million tons of minerals. I have colonies on smaller moons that work normally.
I guess bodies flagged as asteroids can't have a colony cost?
And yes, I know underground infrastructure means I can colonise it now. It's just curious - and would have been a terraforming candidate if not for this.. odd quirk? Oversight in programming? I'm assuming it's not a bug.