NASA are sending a probe into the comet 9P/ Tempel-1, early on the morning of 4 July 2005. This is a unique event, from which they hope to find out how comets are made. Are they balls of ice and mud, the harbourers of life's components, or just solid rocks of sterile cosmic debris? The Deep Impact programme hopes to provide the answers, but this destructive act is not without its critics. The Sky at Night explores the pros and cons of hitting a comet.