# How feasible would be smallish (1km radius) space station / artifical planet with something dense in the middle for gravity and energy

I was wondering about feasibility of small 'shell' world build around something very dense, like black hole or some 'artificial mini star' that could also be source of energy, and of course gave us gravity of around 1 g at 1km radius.

If my math is right, we would basically need something of mass around 1.50E+17kg, collapse it into something less than 1km radius, make it stable, and build around it.

$a = 10\frac{m}{s^2},$ (just to simplify)

$r = 1000m,$

$M = \frac{a * r^2}{G} = 1.5 * 10^{17} kg$

With something like this, gravity at 1010m from the center would be: $a = \frac{M*G}{r^2} = 9.8\frac{m}{s^2}$ so tidal forces shouldn't be a problem

EDIT: removed some fluff, added some math

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Commented Aug 18, 2017 at 13:16
• What would prevent whatever your space station consisted in to fall into your black hole and just become part of it ? How would you enter orbit and land your black hole on things - I guess planets and/or asteroids? How would you not make these corpses to not fall into your black hole themselves ? I must say that I find the idea of orbiting something with a black hole fairly odd, et only landing it. Black holes usually don't land on things, things fall in them. Commented Aug 18, 2017 at 13:17
• Hello there. I cannot help you with your question because I'm not an astronomer, I'm not even a physicist. But I can say that if you did some math and if you even can provide some numbers (which is a big plus because most people don't), it would be nice if you could include how you ended up with that number so the smart people can check if you are correct and maybe point out mistakes. Commented Aug 18, 2017 at 13:17
• I suspect that the gravitational gradient would be quite uncomfortable, or at least mean that you were restricted to a shell.. Commented Aug 18, 2017 at 13:31
• @AndrewDodds This obligatory XKCD confirms your suspicion: what-if.xkcd.com/68 Commented Aug 18, 2017 at 13:32

I assume that your people found somewhere a black hole according to their needs. (That kind of black hole might exist.)

Whether it can have a have an atmosphere doesn't depends on the gravitational acceleration but on the escape velocity, which is small in your case. (About $0.14\frac{km}{s}$ compared to $11.2\frac{km}{s}$) The average speed of air is about $0.464\frac{km}{s}$. It would just fly away.

Otherwise I don't see any problems with your construction.

• Great, thanks, I suspected atmosphere wont work, just wasn't sure why, now I know :3 Commented Aug 18, 2017 at 20:54
• To add some numbers - Schwarzschild radius of such mass is 0.22 nm so such black hole would be quite possible and it would emit 0.015 W so it shouldn't fry anything. With lifetime of 8.99e27 years it shouldn't disappear immediately as well (universe is 'only' 13.80e9 years old). That said I don't think I would feel safe on it... Commented Aug 18, 2017 at 21:39
• I think I wouldn't feel to safe there, too. However it is certainly possible to make accidents unlikely by good construction. Also if you grew up on it, you'd probably feel different about it. Commented Aug 18, 2017 at 22:10

Such a contraption, even assuming you could make it stable, would have a very large gravity gradient and thus would feel very awkward with gravity noticeably decreasing while going up (going down would be very dangerous).

Actually you can land on a rotating space station, you just have to do it on the inner surface. The maneuver is actually quite simple: just approach diagonally (crablike, as a plane landing on a strong wind) the station aligned with internal surface, with the tangential component of your speed matching tangent velocity os space station. When over it kill lateral speed and s.s. surface will rise to keep shuttle in place.

• If my calculations are right, going up 10m from 1000m doesn't change gravity that much, so gradient shouldn't be that much of a deal?. And thanks for the idea of landing on a rotating space station, looks like fun maneuver to do, I'll add it to my list :3 Commented Aug 18, 2017 at 20:50