The Comet Nucleus

This is a drawing of what the surface of a comet might look like. Click on image for full size version (40K GIF) Image courtesy of JPL

Scientists do not know whether the nucleus is very hard (like solid ground) or very soft and breakable (like a snowball). The Rosetta mission hopes to land a probe on the surface of a comet to find out just how hard it is.

As a comet approaches the Sun, it begins to sublimate, forming the coma and a spectacular tail. This picture shows that sublimation may happen only in specific places on the nucleus. These spots of evaporation are called "jets". The jets can sometimes help to turn the comet and make it tumble in space. Halley's comet was photographed with three distinct jets on its surface as it approached the Sun in 1986.