Comet structure and 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. They have a dark crust and an icy interior (upper left). Heat from the Sun (yellow arrows, upper right) melts the ice. Cracks in the crust form as jets of vapour form (centre left). The resulting coma is impacted by the solar wind (blue arrows, centre right), with ionised gas (green arrows) forming a plasma pointing away from the Sun. The trajectory (red line, lower left) of this comet shows a comet developing an ion (plasma) tail as it reaches the orbit of the Earth. A dust tail (lower right) develops as it nears the Sun.