Missions to Comets Comets are believed to be very old objects that formed far out in the solar system. They contain ices and dust PARTICLES which, at least in those comets that have never come near to the Sun, must have remained unchanged since the formation of the solar system. It is therefore of great interest to explore the comets, not only to learn more about the behaviour of the individual objects, but also to study the materials of which they consist in order to learn more about the processes that took place when our solar system was formed. Moreover, since the observations of Comet Halley in 1985-86, it is known that comets contain carbon-rich (organic) materials. Such studies have thus taken on a new dimension in connection with the search for the building blocks of life. It has even been suggested that comets may possibly carry (fossil) bacteria. Although some of the cosmic dust particles that are picked up in the stratosphere by high-flying aircraft undoubtedly come from comets, the best way to study cometary material is obviously to send dedicated spacecraft to them in order to perform in-situ (on-the-spot) analysis. Past missions The first visit by a spacecraft to a comet took place on September 11, 1985, as NASA's International Cometary Explorer flew through the tail of Comet Giacobini-Zinner to study its interaction with the solar wind. Soon after, in early March 1986, an armada of no less than five spacecraft visited Comet Halley, the most famous of all comets. It has been observed by astronomers during more than 2000 years and moves around the Sun in an elliptical orbit with a period of about 76 years. The most detailed investigation, in particular of its "dirty snowball" nucleus, was carried out by ESA's Giotto spacecraft, which flew by it at an altitude of about 600 km. Ambitious goals One of the most ambitious future cometary missions is ESA's Rosetta that will be launched towards Comet Wirtanen in 2003, for a rendezvous eight years later. American missions to comets are also underway. The NASA Stardust spacecraft is now en route to Comet Wild 2. While flying through its coma in January 2004, it will capture a few micrograms of cometary dust and bring them back to Earth for detailed laboratory analysis two years later. The NASA Deep Impact mission plans to create a gigantic fireworks display on the 4th of July 2005. It will impact about 350 kg of copper on the nucleus of comet Tempel 1 at high speed - this will make a big hole in the comet and will release a lot of freshly exposed cometary material. It is then the intention to perform a detailed analysis of the chemical composition of this material. All these missions will provide a fantastic wealth of new data within the next 15 years. They will give young astronomers, physicists and chemists a great chance to analyse very primitive cosmic material and, who knows, perhaps to find some of the missing links in our understanding of the formation of planet Earth. Most of all, however, it is hoped that they will also detect some of the building blocks of Life in the solar system.

An artist's impression of the Rosetta mission to comet Wirtanen