Unholey Comet

Comets are weird. They're similar to asteroids--chunks of very old material orbiting the Sun, rough-hewn and in many ways primordial--but while asteroids are usually mostly rock or metal, comets have minuscule amounts of metal, and are mostly rock and ice. This makes them much less dense than your typical asteroid. Hold a chunk of comet in one hand, and a chunk of asteroid in the other, and the asteroid will feel much heavier. Typically, comets are less dense than liquid water. When water freeze...