orbital period of long period comets are over 200 years up to 30 million years: There is a cloud of rocks and dust that perhaps surrounds our solar system. This cloud is called Doort cloud,  which comes from the name Jun H.Doort who introduced its existence in 1950. It has bean assumed that Doort cloud is the reason for periodic mass extinction on the Earth. Short  period comets have less than 200 yearperiodic time. A region beside Neptune with at least 70,000 small orbits is called Kuiper belt and is located from 30 to 50 A.U and discovered in 1992.

There are only primary remaining from the ancient accretion disk of solar system with 4.5 billion years back. The comets like Halley, are short period and Kuiper can be imagine the source of them. Existence of Kuiper belt was predicted by Gerund P, Kuiper; a Dutch – American astronomer in 1951 and that is why it is named kipper belt.

                        COMET EXPLORATLON                            

NASA's stardust Mission will take sample of comet wild 2 and bring them to Earth in 2004. One small space craft about 770 pounds or 350 flew in feb 7, 1999 and  met comet wild 2 in jan 2004 and will return to Earth or jan 15, 2006 in western utah USA.