As soon as a comet nucleus moves close enough to us for detailed observation, it is obscured from view by a shroud of gas and dust. Whipple believed they were like dirty snowballs - large chunks of water ice and dust mixed with ammonia, methane and carbon dioxide. As the snowball approached the Sun, its outer ices began to vapourise, releasing large amounts of dust and gas which formed the characteristic tails. Today, largely thanks to data from Giotto and the Russian Vega spacecraft, we now know that Whipple's model was fairly accurate. Despite these advances, scientists want to learn more about these occasional visitors to the inner Solar System.
Further studies are important for a number of reasons.
Comets probably acted as building blocks for the planets. They are thought to have formed about 5 billion years ago from the vast, rotating cloud of material which surrounded the young Sun. In those violent times, many comets crashed into the young planets. Craters caused by ancient comet and asteroid impacts can still be seen on the surfaces of the Moon, Mercury and many planetary satellites. Comets may also have provided some of the water which now forms Earth's oceans, and may even have delivered the complex organic chemicals which led to the first primitive life forms.