Comet and meteor. Artwork showing a meteor (left) and a comet silhouetted against a starry sky and a city landscape. Comets are formed by dust and ice with diameters in the range of 200-1200m. The inset at top right shows the highly eccentric orbit of a comet in comparison with the almost circular orbit of the Earth. As the comet nears the Sun it forms a coma, a spherical halo of gas around the comet body, and a tail which is swept back by the solar wind and always points away from the Sun. Meteors are streaks of light caused by meteoroids, fragments of comet bodies or dust particles of the interplanetary medium, which enter and burn out in the Earth's atmosphere.