MERCURY ORBITER - AN INTERDISCIPLINARY MISSION

Mercury is the innermost and a less known terrestrial planet of the So lar System. It possesses a very high density (5.3 g/cm-3 at 10 kbar), a small but unexpected magnetic moment (6 x 10(-3) that of Earth), and a tenuous exosphere; ground-based radar observations indicate that wa ter ice may exist at the poles. There are still fundamental questions about its accretion and cratering history, and its thermal and chemica l evolution. The size of Mercury's magnetosphere is just 5% of that of Earth; substorms last 5 min, on average, and their generation process is influenced by the absence of an ionosphere. The model payload of M ercury Orbiter includes a multi-spectral imager, a gamma- and X-ray de tector, a magnetometer, charged-particle analysers, a wave receiver an d an ion emitter for spacecraft potential control. The spacecraft, the design of which is inherited from ESA's Cluster spacecraft, has a dry mass of 626 kg and is stabilised at 10 rpm, but the telemetry antenna is despun. The bit rate varies between 1.4 and 9 kb/s over the range 1.6-0.64 AU. The spacecraft, launched from Kourou with an Ariane-5, re aches its destination after two gravity assists at Venus and two at Me rcury. Its orbit is polar with periherm and apherm altitudes of 400 an d 16 800 km, respectively. The spacecraft's operating lifetime around Mercury is 3 Hermean years.