A meeting with a comet
On July 4, 2005, a comet and a NASA probe collided. Did astronomers find what they were looking for under the comet's surface?
In Evolution at the edge, S. Alan Stern describes many ways in which comets are not quite pristine relics of the early solar system. The first mechanism he describes that affects comet nuclei is the damage caused by solar and galactic radiation. This radiation creates a rind around a comet's nucleus anywhere from a few inches to a few yards thick. Exactly how thick was one of the questions NASA's Deep Impact probe addressed.