Floating Comet

Comet Churyumov-Gerasimenko is shaped like a big rubber duckie. It has one small blob of material and one big blob, connected by a skinny neck. And it has something else in common with a rubber duckie: If you could find a big enough bathtub, it would float. The Rosetta spacecraft has been bobbing along with the comet since last summer. Its instruments have made precise measurements of the comet’s size, shape, and mass. From those readings, scientists determined that the comet is about as dense as cork — hence the idea that it would float in a big bathtub. The low density means the comet probably consists of loosely stuck together clumps of ice that are separated by small voids. Churyumov-Gerasimenko is moving closer to the Sun, so its surface is getting warmer. As it does so, ice at the surface vaporizes and escapes into space, carrying particles of dust along with it. Rosetta found that most of this activity comes from the comet’s neck, although some comes from deep pits in the two lo