> Something coming at the earth from the Moon, for instance, is moving at least 2.3 kps, plus another kps if you fire it forward. What do you mean "fire it for

> Something coming at the earth from the Moon, for instance, is moving at least 2.3 kps, plus another kps if you fire it forward. What do you mean "fire it forward"? If you fire it forward, it will never fall on Earth. To get from Moon to Earth, you need to fire it backward in Moon orbit, to slow it down. Essentially, you need to slow it down a lot if you want it to get to Earth surface, as opposed to orbiting it alongside Moon.