The American astronomer Fred Whipple was born on November 5th, 1906. As a young graduate student he helped to plot the orbit of the newly discovered planet Pluto, and in the 1930’s he showed that meteor showers are the result of particles shed from passing comets. But he is best known for his work in comet theory: in 1950, he came up with the basic model for comet composition that is still in use today. It’s called the “dirty snowball” theory, and it proposes that comets are basically big chunks of frozen ice, mostly water ice, with lots of rocky pebbles and dust grains mixed in. When a comet approaches the sun, these ices melt or sublimate and form an atmosphere or coma, around the comet nucleus; the solar wind and the pressure of sunlight blow this atmosphere out into a long comet tail. When the Giotto spacecraft flew by Halley’s Comet and imaged its 20-mile-wide nucleus during the comet’s last appearance in 1986, it confirmed his theory.