Comets are releatively unaltered ‘leftovers’ from the formation of our solar system, and so hold the key to answering questions such as where did the volatile materials (including water) in the inner solar system come from? They are also relevant to life-on-earth in much more prosaic ways: comets generate streams of dust that may endanger satellites or spacecraft (we see some of these dust streams as meteor shows) and a cometary impact on Earth poses risks.