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Re:James Webb telescope's coldest instrument reaches operati... by dorgo on Monday April 18, 2022 @04:45PM (#31076277)
>to hurl, propel, or divert some rock at us at some non-negligible fraction of light speed, and we would be powerless to stop it.

Wouldn't you need to solve the n-body problem to hit a planet (earth) from many light years away with a rock? And to do this without exact knowledge of planetary bodies in our star system. And even if you could, a stray hydrogen atom hitting your rock early on would make it miss its target by some light hours. So you better account for every speck of dust on the way and everything with gravitational pull on your rock. An all the quantum fluctuations..

I would even wonder whether the space itself is fine grained enought to precisely target stuff light years away.

Re:James Webb telescope's coldest instrument reaches operati... by chmod775 on Tuesday April 19, 2022 @04:31AM (#31080820)
You need to stick the tiny maneuvering thrusters to it in any case. Or maybe just some light sails will do the trick at those distances, depending on how good your aim with the primary means of propulsion was.

If humanity was doing this, we'd likely attach propulsion and accelerate the entire way anyways. Unless there's some new drive technology discovered by then or we hijack something natural/environs to do it for us.