"Deep Impact probe shows a fragile, empty comet"

I guess comets really are dirty snowballs. At least, according to this article from Reuters.

"The comet is mostly empty, mostly porous," said Michael A'Hearn, a comet specialist at the University of Maryland. "Probably all the way in, there is no bulk ice. The ice is all in the form of tiny grains."

The material on the comet's surface, down to a depth of several dozen yards (meters) is "unbelievably fragile, less strong than a snowbank," A'Hearn said in a telephone news briefing to release early findings from the mission.

I hope this means they should be pretty easy to destroy if they ever threaten Earth. And that maybe this book got it wrong.