Do asteroids have a maximum speed or do they keep accelerating?

Asteroids are tumbling in space and spinning around and an interesting thing is that they're all spinning rather slowly. That is consistent with them being loose piles of rubble. They're not solid objects. When meteorites come to Earth, they tend to travel pretty fast, around twenty kilometres per second, although the maximum is seventy. The net result of that is that they make this wonderful fireball effect. If a big one comes, the atmosphere doesn't slow it down, and that's when trouble happens. It will come all the way down to the surface and explode. On eof the nest studied meteor craters is in Arizona and it's just over a kilometre wide. The object there was about thirty metres wide. The estimate for the asteroid that wiped out the dinosaurs is ten kilometres.