Q: Since comets are made mostly of...

Q: Since comets are made mostly of ice, what keeps them from evaporating in the vacuum of space? Ice sublimates in a vacuum.

A: They do sublimate, but only to a very limited extent because it is so cold in space, Caltech astronomer Harold Zirin said. As the comet’s orbit takes it closer to the sun, however, it begins to warm up and the ice sublimates at a much greater rate. That, in fact, is what forms the comet’s tail. After many passes around the sun, all of the comet’s ice will have evaporated, leaving only rocky remnants that appear as periodic meteor swarms.