Figure 15-26: The Structure of a Comet The solid part of a comet is the nucleus. It is typically about 10 km in diameter. The coma typically measures 1 million km in diameter, and the hydrogen envelope usually spans 10 million km. A comet’s tail can be as long as 1 AU. Comet Wild 2 (inset) is examined further in Figure 15-28. (This drawing is not to scale.)

(Inset: NASA/JPL)