Most comets are so small and so far away that we cannot see them, even in the biggest telescopes. But we can see them when they head inwards toward the Sun and grow tails of gas and dust. (The word comet means 'hairy star'.)
The heat of the Sun makes the ices turn into gas that streams for millions of kilometres into space. Despite their length, the dust particles they contain are no bigger than grains of sand.
In 2014, Europe's Rosetta spacecraft will make history by going into orbit around a comet and dropping a lander on its surface.
Last update: 15 November 2010