Why do comets have tails when they come near earth?

It is one of the wonders of astronomical research that we can tell what distant planets are made of without the need for space probes to visit them. As long ago as 1862, Swedish physicist Anders Angstrom noted that dark lines in the solar spectrum a breakdown of the Sun’s light were in exactly the same position as they would have been in light that was passing through hydrogen. He deduced correctly that hydrogen is a major constituent of the blazing Sun.

By a similar process we can tell what comets are made of, and we know that their chemical structure is different from that of other members of the solar system. In 1950, after studying their structure, American astronomer Fred Whipple described comets as ‘dirty snowballs’. He discovered that they are composed largely of water and gases, all of which when frozen at very low temperatures turn into various kinds of ice. Bound up in these ices are dust particles.

When a comet nears the Sun, some of the ice vaporises, surrounding the comet with a cloud of dust and gas. When sunlight passes through this cloud, it lights it up, enveloping the comet in a fuzzy glow. As well as heat, the Sun creates a breeze, the solar wind, caused by electrically charged particles heading in all directions. This breeze blows the fuzzy cloud in a kind of slipstream behind the comet. That is why all comets have a luminous tail, which never faces towards the Sun.