Comets The word "comet", which originates from a Greek word meaning "long-haired", indicates that these objects look like hairy stars with tails stretching across the sky. What we see in a bright comet is actually sunlight scattered by dust particles and the glow of fluorescent molecules or ions. Cometary nuclei - "dirty snowballs" - consist of ices (mostly water ice) and dust. They come in all sizes, from a few tens of metres to several tens of kilometres. Whenever a cometary nucleus is heated by sunlight, some of its ices are turned into gases (by sublimation), which escape with some dust particles. The gases and dust form a bright halo, a few tens of thousands of kilometres around the nucleus, whose size is just a few kilometres, and are progressively pushed away from the Sun by the solar wind and by solar radiation pressure. Cometary orbits The famous Comet Halley, whose nucleus is about 15 kilometres across, describes its elliptical orbit in about 75 years. The comets we can see nowadays are mostly on elongated orbits around the Sun, and are progressively loosing all their ices. They have been preserved for aeons in two huge reservoirs, the so-called Kuiper belt, which corresponds to small bodies that remained at solar distances between 30 and 100 times the Earth distance (i.e., about 5 to 15 billion km from the Sun), and the so-called Oort cloud, which is made up of small bodies formed at solar distances between 5 and 30 times the Earth distance (i.e., about 1 to 5 billion km from the Sun), and which have later been pushed to the outer edge of the Solar System. Did comets deposit the building blocks for Life? Gravitational perturbations by background stars or gravitational tides have changed the orbits of some of these nuclei and eventually injected them into the inner solar system. As they are still quite representative of the disk out of which the Solar System formed, such nuclei are thus very indicative of the matter that hit the terrestrial planets at the early epoch of heavy bombardment. Indeed, the discovery through past cometary missions of large amounts of water in comets and of complex organic molecules in the tiny aggregates of cometary dust, strongly suggest that cometary impacts could have provided the early Earth with all the building blocks for Life, i.e. water and prebiotic molecules, that is to say carbonaceous compounds. Such impacts might also have allowed an early development of Life, now most likely now extinct, on the planet Mars.

The comet Hale-Bopp, visible at the end of the year 1996 and in March/April 1997, was one of the brightest comets of the last century. The nucleus of comet Halley, which appears every 76 years and was last near the Earth in 1986. This image was taken by the Giotto satellite. century.