Rosetta Lander—Philae: Implications of an alternative mission

“Rosetta” is a Cornerstone Mission of the Horizon 2000 ESA Programme. Its goal is to rendezvous with a comet and to study its nucleus and coma using an orbiting spacecraft and a landed platform. The latter is called Philae; the Rosetta Lander has been designed to land softly on the comet nucleus and is equipped with 10 scientific instruments to perform in situ studies of cometary material.The Lander system is provided by international consortium with the participation of Germany, France, Italy, United Kingdom, Finland, Ireland, Hungary and Austria.The original Rosetta mission was planned to launch in January 2003 to reach comet 47P/Wirtanen in 2011 at a heliocentric distance of about 3 AU. Due to uncertainties regarding the reliability of the Ariane 5 launcher (after a catastrophic failure in December 2002) the Rosetta launch was postponed and a new mission studied.Changed mission characteristics (e.g. in launcher capacity; no swing-by at Venus) strongly limited the number of possible alternatives. After careful investigation, a decision was taken for a mission to comet 67/P Churyumov-Gerasimenko with a launch date in February 2004 and rendezvous in 2014.This new mission scenario has significant consequences for the Rosetta Lander, because the nucleus of P/Churyumov-Gerasimenko is expected to be considerably larger than that of P/Wirtanen. Current best estimates assume a radius of about 2.0 km and, thus, a mass which is about two orders of magnitude larger than that of the original target comet. This impacts strongly on the Lander separation, descent and landing scenario. Analysis of the increased landing risk on P/Churyumov-Gerasimenko has led to modifications of the landing gear to cope with the increased impact velocities expected.