Stages of a comet's orbit, illustration. Comets are icy bodies from the outer solar system that boil and form a bright tail of gas and dust as they approach the Sun. The trajectory (green arrow) of this comet shows the cometary nucleus passing the orbit of Jupiter (shown at top centre), and developing a coma as it starts to boil. An ion tail develops as it reaches the orbit of the Earth (shown at upper left). A dust tail (centre left) develops as it nears the Sun. Perihelion (lower left) is the closest approach to the Sun. As the comet moves away from the Sun, the boiling ceases and it returns to being a dark cometary nucleus (bottom right).