Figure 8-29The Structure of a Comet The solid part of a typical comet (the nucleus) is roughly 10 km in diameter. The coma can be as large as 10⁵ to 10⁶ km across, and the hydrogen envelope is typically 10⁷ km (6 million mi) in diameter. A comet’s tail can be enormous—even longer than 1 AU. Comet Wild 2 (inset) is examined further in Figure 8-24. (This drawing is not to scale.)