Comets

Icy bodies that develop a visible coma and often a tail as they approach the Sun, split by orbital period into periodic comets, which return on a predictable cycle, and long-period comets, which take thousands of years or arrive from far beyond the planets. Belongs here: any body whose defining characteristic is this ice-driven outgassing, whatever its current distance from the Sun. Does not belong here: rocky bodies with no coma, grouped under Asteroids and Minor Planets, or distant icy bodies never observed to develop one, grouped under Trans-Neptunian and Outer Solar System Objects.