Using Electric Propulsion to get to Mercury - the BepiColombo Mission

BepiColombo is an ESA funded mission, in co-operation with JAXA, which will deliver 2 separate spacecraft into orbit around Mercury in 2019, following launch from Kourou on Soyuz-Fregat in 2013. The transfer to Mercury involves a large velocity increment for the spacecraft. Although this can be achieved by direct transfer or by using conventional chemical propulsion along with gravity assists, this requires the use of a large launcher. The use of electric propulsion combined with gravity assists can deliver the high total impulse required with reduced propellant mass compared to chemical propulsion systems, enabling the use of a smaller launcher. The transfer to Mercury has been optimized on this basis, with a Mercury gravitational capture scheme to improve the mission robustness. Ion thrusters with specific impulse levels of approximately 4600s have been selected for the electric propulsion system. The total thrust level is matched to the available power, varying between 90 and 290mN; a single thruster is used in the early near-Earth phases of the mission, with 2 thrusters being used simultaneously as the power increases at closer spacecraft solar distances. 4 thrusters in total are required to achieve the mission total impulse requirement with adequate redundancy. The high total impulse leads to over 410kg of Xenon being required, which drives the design of the Xenon storage and feed system. Specific requirements relating to electric propulsion system operations for this mission include interactions between simultaneously operating thrusters, and the high solar thermal load; these have been addressed and verified by technology demonstration tests.