system
Given the following post, respond as if you're commenting on Reddit:
human
Slow moving asteroids?

When we see or hear about asteroids they are always racing through space at crazy speeds. Is it possible that we would someday encounter a slow moving asteroid, that simply landed on earth quietly?
gpt
Suppose that there is some asteroid that's more or less still in relation to Earth. Then it's set on a collision course with Earth, but at a very slow speed, again in relation to Earth. This is more or less the same as dropping a big rock to Earth from a great height. Even though the rock initially is still, it'll gather a lot of speed falling down. How much speed exactly it gathers is the escape velocity from the surface of Earth, or about 11 km/s. So that's the minimum speed anything will hit Earth.

Why escape velocity then? Well, escape velocity is the speed you need to shoot something away from Earth so that it'll get far away. If you do it at exactly the escape velocity, then the speed of the object will approach zero far away from Earth. But this also works symmetrically in the other direction. If it starts far away at zero velocity, then it'll hit Earth at escape velocity. And far away here doesn't actually mean very far away. Even dropping something from the orbital distance of the Moon will cause it to hit Earth at 99% of the escape velocity.