ESA Science & Technology - Firing of Ion Engines

Two sets of two ion engines (one set for redundancy) are situated at the end of the Mercury Transfer Module (MTM), in a square configuration around the nozzle of the main chemical engine. This artist's impression shows one set firing.

During the interplanetary cruise to Mercury, when within the orbit of Venus, one set of two ion engines will be fired periodically to brake against the Sun's increasingly stronger gravitational pull. When still outside Venus's orbit, only one ion engine will be used, due to limited power available from the solar panels.