Comets are irregularly shaped bodies made of non-volitale grains and frozen gases. Comets have unusual orbits which bring them close to the Sun and then far out into space past the orbit of Pluto. As a comet approaches the Sun, the frozen gases warm and begin to evaporate. This creates a coma around the comet nucleus and a tail of particles that can be millions of kilometers long. Actually, a comet forms three different tails. One is a tail of dust and grains evaporated from the nucleus, one is a tail of ion particles, and last is the hydrogen envelope, which is a tail of hydrogen gas formed from chemical reactions. Most of the comets that enter the Solar System come from the Oort Cloud of comets, which surrounds the Solar System at a distance of about 1 light year from the Sun.
Asteroids are rocky material that orbit the Sun but are too small to be classified as planet, thus, they are called minor planets. Asteroids can range in size from pebbles to almost 1000 kilometers wide. Most of the asteroids in the Solar System lie in a main belt between the orbits of Mars and Jupiter. Asteroids are made of material left over from the formation of the Solar System. This makes them a topic of interest for scientist. The Galileo spacecraft took the first high resolution image of an asteroid, called Gaspra. Galileo later made images of another asteroid, 243 Ida.
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