Comets

Comets are ice-covered rocks that orbit the Sun, typically in highly elliptical closed orbits.Some comets graze the Sun so closely that they are broken up and consumed by the Sun. Other comets survive passage around the Sun and so return periodically, if their orbits are closed.Some periodic comets come from the Kuiper Belt and have orbital periods less than 200 years. These comets are called short-period comets. Other periodic comets come from the Oort cloud and have periods far in excess of 200 years. These comets are called long-period comets.Comets are distinguished from asteriods both by the eccentricity of their closed orbits, and by their appearances when close to the Sun. When about 1 to 2 AU from the Sun, comets become heated enough by solar radiation to grow a tail. Some- times the tail is fairly faint, and sometimes it is spectacularly long, easy to see at dusk or at night.Comets often exhibit two visible tails. One is the illumination of dust debris left by the comet, and is called the dust tail. The dust tail shows the direction of the comet's trajectory. The other tail is the ion tail, or coma, and is composed of glowing ionized gas particles. The gas particles, and thus the ion tail, point directly away from the Sun.If a comet leaves dust particles in the Earth's orbit, then those particles will be part of a meteor shower.