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BlackNoise comments on Asteroids and spaceships are kinetic bombs and how to prevent catastrophe - Less Wrong
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I think the idea is that active defensive measures (as opposed to just watching with telescopes) are a lot more difficult to set up, and there's little motivation considering that the space activities aren't military-oriented. Although I suppose if we'd be far enough in space exploration to have asteroid mining there'd also be some contingency plans for extinction-grade bricks on a collision course; plans that can probably be adapted to include 'hostile local' handling.
Regarding the physics, do not underestimate heavy things flying very fast, especially if they're good at staying in one piece - a ship/asteroid may well destroy a city when dropped at 10~30km/sec, and attackers will aim.
Things flying very fast through the atmosphere of the earth have the habit of insinuating in it. Would the kind of technology that you would need to do astroid mining really give you the ability to blow up a city?
Yes. You just need to nudge an existing sufficiently large asteroid onto a collision course. (Targeting a specific city is harder but I don't know how much harder.) There are how many near-earth and earth-crossing asteroids over 100m diameter?
Almost certainly. Whether it is practical to get the asteroid involved rather than adapting the large scale propulsion technology is a whole different question.
I was mainly thinking about Project Thor, which roughly means that going at mach 10 (~3km/s) is like being made of TNT energy-wise. Now, current space-shuttles and the ISS weigh around 100 tons, and I'd imagine being able to get at least 10km/s, if not 30 with asteroid mining-level space tech, which should bring spaceships into the kiloton TNT range, that while far from a hydrogen bomb, packs the punch of a smallish fission nuke. So, while it probably won't be easy to wipe out big cities, immense damage is guaranteed.
What I can't estimate properly due to insufficient knowledge is the atmosphere's ability at stopping/limiting such threats, for all I know spaceships/rocks going at too steep an angle might blow up very high up, while stuff going at a gradual entry might be significantly slowed, Although as a rule, bigger things should care less about the atmosphere.
Edit: CellBioGuy's comment points out that spaceships aren't (and probably won't be) built to withstand reentry at dangerous velocities, making at least spaceship-jacks less of a threat.