Why??????
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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.
All that material coming out of the comet is lighter than the comet moving towards the sun.So, the tail points against the sun.
All that material coming out of the comet is lighter than the comet moving towards the sun.So, the tail points against the sun.
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Radiation pressure