Objects move pretty fast in space, usually tens of kilometers per second.
A 20 meter of diameter object with velocity of 10000 mps is pretty deadly impact from Earth's point of view.
400-500 kilotons of TNT
I don’t know about you but I’d consider a Hiroshima-scale explosion pretty deadly. We just got lucky it didn’t hit the middle of a major population centre at a more direct angle. That could have been catastrophic.