The Rosetta mission:: Rendezvous with and landing on a comet

The European Space Agency will launch the Rosetta spacecraft in January 2003dagger on a more than 10 years voyage to comet Wirtanen. The Rosetta orbiter will detach a scientific package which will land on the comet surface to perform in situ measurements of the comets composition. The orbiter will escort the comet for about one year and its extensive array of scientific instruments will study the comet in the using optical imaging, remote sensing, chemical composition measurements and magnetic field measurements. The goal of this paper is to provide a detailed account of both the Rosetta mission and the spacecraft designed to cope with the principal challenges of the mission. The long mission duration is driven by the need to perform three planetary flybys in order to increase the spacecraft velocity to synchronise its orbit with the Wirtanen orbit. Included is a flyby of Mars. Passing at not more than 200 km altitude above the planets surface, this is considered a very critical manoeuvre for the spacecraft. In addition to the planetary flybys, two close asteroid flybys are planned, further increasing the scientific return. Even exploiting these multiple flybys, the spacecraft still must carry about 55% of its launch mass in propellant in order to adjust its orbit and finally to rendezvous with the comet. As well as the long duration and complex flybys, another real challenge of the mission comes from the very large solar distances at which the spacecraft will have to travel and operate. Flying beyond the orbit of Jupiter, Rosetta will be the most distant spacecraft ever to be powered using only solar cells. Special solar cells have been developed to be optimised for the low temperatures and low intensity operation, even so, the solar arrays need to be huge. At nearly 33 meters long, they produce about 10KW at earth distance reducing to about 400 W past Jupiter. The thermal control over distances varying between less than 1 AU and more than 5 AU is also a tough challenge requiring both super-insulation and heating in deep space and considerable heat rejection close to the earth orbit.