comets

comets

Short-period comets are thought to originate in the Kuiper Belt Long-period comets are believed to originate in the Oort cloud, consisting of debris left over from the condensation of the solar nebula, located well-beyond the Kuiper Belt.

Comets are thrown from these outer reaches of the Solar System towards the Sun by gravitational perturbations from the outer planets (in the case of Kuiper Belt objects) or nearby stars (in the case of Oort Cloud objects), or as a result of collisions between objects within these regions

Comets are distinguished from asteroids by the presence of a coma or tail, though very old comets that have lost all their volatile materials may come to resemble asteroids Asteroids differ from comets because they come from the inner solar system.

As of May 2009 there are a reported 3,648 known comets

Comet nuclei are known to range from about 100 meters to more than 40 kilometers across. They are composed of rock, dust, water ice, and frozen gases such as carbon monoxide, carbon dioxide, methane and ammonia

cometary nuclei are among the darkest objects known to exist in the solar system. The Giotto probe found that Comet Halley's nucleus reflects approximately 4% of the light that falls on it. Asphalt reflets 7 percent

As a comet approaches the inner solar system, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them. The streams of dust and gas thus released form a huge, extremely tenuous atmosphere around the comet called the //coma//, and the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous //tail// to form, which points away from the sun.