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What is the difference between comets and asteroids ?

Asteroids are rocks found orbiting the sun, generally found between the planets of Mars and Jupiter and also in the Kuiper belt. Those between Mars and Jupiter were never allowed to assemble into a planet because of the constant tug of Jupiter’s massive gravity. They have very little or no ices around them that can be released by the heat of the sun and glow . So they are detected only by the sun light that they reflect from their relatively dark surfaces.

Gaspera
Asteroid Gaspra

Comets on the other hand are in the coldest part of the solar system with their dust and ices well preserved. So their visibility is provided by the evaporating gas and the released dust as they near the sun. When the spacecraft Giotto tried to photograph the nucleus of Comet Halley in 1986 its instruments were soon saturated with dust blowing out from it, even when it was more than 500 kilometres from it.
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