Halley's comet is on an eccentric and inclined orbit. Strip away its garish but insubstantial tail, and a comet is nothing more than a dirty snowball several kilometers across. During its last perihelion passage, in 1986, a small flotilla of spacecraft visited Halley's comet. One of the craft, named Giotto, approached within a few hundred kilometers of the central icy nucleus of the comet. The comet's nucleus itself is the dark blob at the lower right.
Craters are visible on the surface of the nucleus.
When a comet is close to the Sun, it grows an ion tail and a dust tail. A comet which approaches the Sun has the following structure. Up to 100,000,000 kilometers long, but even lower in density than the coma. Usually slightly shorter than the ion tail. Thus, a comet has TWO tails.