Orbits of comets
Comets are believed to be left over particles from beginning of the Solar System,when most of these particles collided with each other to build up the planets. The comet nuclei, containing crumbly rock particles trapped in frozen ice, were left. If their orbits take them near the Sun, the heat turns the outer ice to vapour and solid particles are released as dust. In their frozen state far from the Sun, comets are invisible. Each time a comet passes near the Sun it pours some more of its nucleus into space. Eventually it 'dies' and becomes just an orbiting rock.