Halley's Comet
In 1986, Halley's Comet was the first to be observed in detail by spacecraft. It gave the first data on the structure of a comet nucleus and how the coma (nebulous envelope around the core or nucleus) and the tail formed. These observations supported Fred Whipple's "dirty snowball" model. This correctly predicted that Halley would be composed of a mixture of volatile ices – such as water, carbon dioxide, ammonia, – and dust. The missions also adjusted these ideas. For example, it is now known that the surface of Halley is mostly dusty, non-volatile materials, and that only a small portion of it is icy.