A View from the Outside: ESA’s Mars Express Survives Ultra-Low Power Eclipse Season

ASK OCE — November 3, 2006 — Vol. 1, Issue 15

On September 26, 2006, the European Space Agency’s (ESA) Mars Express spacecraft completed an unusually demanding eclipse season. The spacecraft functioned in an ultra-low power mode dubbed “Sumo” (short for “survival mode”) that allowed it to conserve the power necessary for its survival as it orbits Mars.

From late August to late September, Mars Express faced an unusually challenging solar eclipse season. During these eclipse periods, the Mars Explorer spent as long as 75 sunless minutes during each six-hour orbit of the planet. During these “blackouts,” the ship’s solar panels generated no power whatsoever, leaving it to rely solely on battery power.

During eclipse situations, the spacecraft was designed to run on three lithium-ion batteries previously charged by the solar panels. However, shortly after the Mars Express was launched on June 2, 2003, flight controllers discovered an anomaly that limits the amount of electricity that can be produced by the panels and delivered to the rest of the spacecraft, including the batteries.

ESA mission controllers quickly realized that the anomaly might mean that the batteries might not fully recharge after each lengthy blackout during the recent eclipse season. In a worse-case scenario, the Mars Express could lose all power and be unable to continue its mission.

The problem was solved by creating a configuration for the spacecraft in which all but the most essential onboard devices were turned off or powered down. To collect all possible solar energy, Mars Express was aimed away from Earth most of the time, resulting in very short communication intervals with ground controllers.

A series of tests and validations as well as a formal review by ESA determined that the Sumo power-saving mode would reduce on-board power consumption from 400 to roughly 300 watts, a figure later verified in actual use and low enough for the batteries to be adequately recharged after each eclipse.

Mars Express switched to Sumo mode on August 23. The height of eclipse season came on September 9, when the orbital blackouts lasted up to 75 minutes. The extra-long eclipse season ended on September 17. The spacecraft is now functioning with all systems nominal.