The European Space Agency's Rosetta mission to a Comet almost 900 million km from Earth is one of the most ambitious scientific missions to be launched during the next decade.
Rosetta was be launched in 2004 by an Ariane-5 from Kourou, French Guiana into the required escape hyperbola trajectory towards Mars.
A rendezvous with the Comet is the destination of ESA's Rosetta mission. On its journey to the comet, the spacecraft will pass close to asteroids. Comets are believed to be the most primitive objects in the Solar System, and are particularly interesting to scientists because they hold many clues about our Sun and its planets. To gain enough orbital energy to reach its target, one Mars and two Earth gravity assists will be required.

See the Rosetta web site for more information