Comets

Comets are small ice-rich bodies in the solar system, orbiting the Sun in highly eccentric orbits. They are composed of ices, mainly water ice, and refractory materials—silicates and organic compounds. Their structure is often bilobed, always porous and fragile. When comets approach the Sun and absorb solar energy, the water ice evaporates and entrains dust particles, giving rise to the typical coma and tail that distinguish comets from other small bodies. Interaction with solar radiation and the solar wind causes the tail to point in the anti-solar direction. The sudden and spectacular appearance of comets roused interest in these objects since ancient times, while their link to the formation of the solar system and possibly to the origin of life, prompted intense study and numerous space missions in recent years. Comets formed in the outer region of the solar nebula and were deflected and scattered by the giant planets into dynamically stable regions of the solar system: the Kuiper belt, extending beyond the orbit of Pluto, now considered member of the belt, and the distant Oort cloud. Occasional perturbations of these comet reservoirs send comets into the inner parts of the solar system, where they become active. Expulsion of ices and dust, sometimes in violent outbursts, erodes the comet nucleus, which eventually breaks or disintegrates. Cometary debris is often spotted as meteor showers, when the Earth crosses the comet's original orbit. Comets may also end their lives by falling into the Sun, crashing onto planets or being deflected into interstellar space.