Together with the asteroids, comets constitute the Small Solar System Bodies. This family is nowadays considered original matter remainders from which the Solar System was formed, 4.5 billion years ago.
Comets and the Solar System
Contrary to asteroids, which are constituted of rocky blocks of diverse composition and texture (up to 500 km diameter, beyond which they are considered dwarf planets), comets appear to be little aggregates (smaller than 10 km nuclei) of non-volatile grains and frozen gas.
Laboratory simulation of cometary grains (J.M. Greenberg)
The trajectories of known comets are generally highly elliptical, which makes them pass periodically near the Sun. The outgassing of light components (mainly water and carbon dioxides) then leads to the formation of a dust cloud (coma) which can stretch over great distances and creates a tail under the effect of radiation pressure and solar wind.
Besides a contribution to the Solar System development process, comets seemed to have had a prominent role in Earth's biological evolution. Some theories suggest that they could have brought a major part of the terrestrial organic matter from which life's elementary bricks have been constituted.
After a quick comet fly-by (75 km/s by GIOTTO for the Halley comet), a mission must be conducted–through long observations in orbit and on comet ground–to make new measurements to confirm the current theories or develop new ones.