Image: D.Davis (NASA)

When do impacts happen?

An impact could be defined as the event that takes place when a NEO is captured by a planet and falls onto it. This happens much more often than one could believe: all bodies with solid surfaces in the solar system show evidence of a process of cratering. Daily on Earth, about 100 tons of interplanetary material drift down to the ground.
 

Besides this simple definition, what exactly is an impact? This question is not so easy to answer to, since impacts can be of very different kinds, ranging from explosions similar to atomic bombs to romantic phenomena such as meteor showers. In other words, many questions about impacts need to be answered: what happens during an impact, how many kinds of impacts exist? Can they really be so dangerous?
Another very interesting problem concerns the time-scales of the impact process : how often do impacts happen? And furthermore, when are impacts scheduled and is it possible to foresee them?