Using Black Holes for Terraforming?

[K^2]

but what was that thing about centrifuge cities? How would that work? the centrifugal force pushes you out, away from the circle's center. would it be oriented Vertically? that would mean the moon's gravity would compliment you as you went down but not as you went up, and everyone would be bouncing up and down like jackhammers. If it were horizontal, the moon's gravity would pull you down as the centrifuge pushed you sideways, and you'd end up at like a 45 degree angle to the horizon. that's better, but then there's not much point in putting them on the moon's surface as apposed to in orbit.

Actually, you'd build the city on the interior of a paraboloid. Think of the shape the water surface makes in a spinning glass. The local gravity will vary depending on distance from the center of rotation, but it will always point directly towards the floor. If you build the whole thing large enough, Coriolis forces are going to be low, so you'll feel like you're walking around on a level surface, even though you are inside a giant paraboloid. You'd probably build habitats near the 1G area, and keep various life support equipment closer to the center.

[Rickenbacker]

It is. A stable black hole has to have at least a few hundreds (or tens? I can never remember) the mass of the Sun. Which is more mass than exists in the entire solar system.

[TheDarkStar]

It is. A stable black hole has to have at least a few hundreds (or tens? I can never remember) the mass of the Sun. Which is more mass than exists in the entire solar system.

That's just the mass required for a black hole to form from a star. Black holes can exist at much lower masses, but afaik there isn't much of a natural reason for them to show up.

[K^2]

Neutron stars collapse somewhere between 2 and 3 Solar masses. So that is the minimum mass for a black hole formed during a supernova. A black hole would be stable with a much lower mass, but yes, there is no mechanism known that would produce them. Some have suggested that black holes of all manner of sizes could have been produced during Big Bang, but we have no evidence of any sub-stellar black holes being around.