Comets have highly elliptical orbits that bring them very close to the Sun and swing them deeply into space, often beyond the orbit of Pluto. While most of the planets' orbits are near-circles, the orbits of most comets are extremely elongated ellipses. Because a comet experiences its strongest solar heating at its perihelion point, that is usually when it is brightest. This can be due to a fresh eruption of new material from a comet's surface, or sometimes this occurs when the nucleus splits into two or more pieces, exposing previously hidden sections of its material to the sun's heat for the first time. On the average, about a dozen of Jupiter's family of comets will pass perihelion during any given year.
Some comets travel is such long orbit that they are near the sun once in thousands of years.
All comets are part of the suns family, just as the Earth and the other planets. As comets approach the Sun they develop enormous tails of luminous material that extend for millions of kilometers from the head, away from the Sun. When far from the Sun, the nucleus is very cold and its material is frozen solid within the nucleus. When a comet approaches within close to the Sun, the surface of the nucleus begins to warm, and volatiles evaporate. The evaporated molecules boil off and carry small solid particles with them, forming the comet's coma of gas and dust.