Oort Cloud - It is a huge spherical region surrounding the Solar System extending about 3 light years from the Sun. The size estimation of the cloud is based on the Sun's limit of gravitaional influence. It is believed that long-period comets are originated from the Cloud that may contain about 100 000 000 000 (a trillion) comets. These comets are typically tens of millions of kilometers apart. They are weakly bound to the Sun and their orbits are very unstable. Influences from one another or even from neighboring stars may easily change
their courses towards inner Solar System or out into intestellar space. Long-period comets may approach the Sun once every 200 years or even every million years or more. For this reason these comets are not known until they appraoch near the Sun when comet's icy materials begin to melt, giving out comet tails. Examples of Oort Cloud comets are Hale-Bopp and Hyakutake.
Some of these comets may have pulled into shorter period orbits by planets such as Halley and Swift-Tuttle. Comets' orbits are usually difficult to predict not only because of the gravitational influence but also due to changes in shape and mass of the comets when materials are being ejected.
The existence of Oort Cloud is still rather speculative. This is due to its vast size and comets that reside in the cloud cannot be seen as they are very far and their sizes are very small. We only speculate its existence because comets appear at anytime and come from all directions.
Kuiper-Belt Objects - This is a disk-shaped region of space lying around ecliptic extending past the orbit of Neptune to a distance of about 100 AU from the Sun. It consists of small icy bodies and is believed to be the source of short-period comets. In fact Pluto and its companion satellite Charon may be the largest example of these object.
It is estimated that there are at least 35,000 Kuiper Belt objects greater than 100 km in diameter, which is several hundred times the number of similar sized objects in the Main Asteroid Belt. Examples are such as 1992 QB1 and 1993 SC.
Occasionally the orbit of a Kuiper Belt object will be disturbed by the interactions of the gas giant planets so as to cause the object to cross the orbit of Neptune. It will either move out of the Solar System or into an orbit crossing those of the other giant planets or even into the inner solar system. These 'refugees' objects are known as Centaurs and the orbits are usually unstable. Quite a few such objects have been found which are situated between Jupiter and Neptune. Example of such an object is the 2060 Chiron with a diameter of 170 km. Despite they look insignifant, Kuiper-Belt objects may hold remnant clues to the early stages of the formation of the Solar System.
Images above show detection of comets in the Kuiper Belt, taken with the Wide Field Planetary Camera 2 from the Hubble Space Telescope (HST). The pair images show movement of the object. It is estimated to be several miles across. The object is so faint and is close to the detection limit of HST. The dotted line is the possible orbit that this Kuiper Blet comet is following. The images show movement of the object (circled) 105 minutes apart. A star (lower right corner) and a galaxy (upper right corner) provide a stationary background reference.
Credit: HST: A. Cochran (University of Taxes)/NASA.