We are all familiar with impact. Lesser impacts such as hammering a nail, cracking an egg or stubbing a toe are part of everyday life. More violent impacts such as those caused by car crashes or bullets are fortunately less common but are still well enough known to be taken for granted. Very violent impacts such as meteorites striking the earth are outside our personal experience but we are aware of them. Despite this, impacts remain mysterious. They occur too quickly for us to follow what is happening and the evidence they leave behind is often ambiguous. Over the last thirty years...