BepiColombo power struggle could leave probe short of Mercury's orbit

ESA/JAXA mission running on reduced thrust as engineers work to resolve the issue

Updated Thruster problems with BepiColombo, the joint ESA and JAXA mission to Mercury, could cause headaches for managers plotting the spacecraft's trajectory and insertion into Mercury's orbit.

On April 26, as the spacecraft was scheduled to begin its next maneuver, the Mercury Transfer Module (MTM) "failed to deliver enough electrical power to the spacecraft's thrusters."

BepiColombo is supposed to enter Mercury orbit in December 2025.

According to former ESA Senior Science Advisor Mark McCaughrean, the Solar Electric Propulsion (SEP) system has been "glitchy" for a while.

BepiColombo has three components – a pair of scientific probes and the MTM. During the cruise to Mercury, the MTM provides power for the probes and also the SEP. While the SEP is not particularly high-thrust, the plan calls for it to run for extended periods on the way to Mercury orbit insertion.

The SEP is therefore a critical component, and while the mission team managed to restore thrust to 90 percent of its previous level by May 7, the reduction could require a rethink of the mission plan.

SEP is not a novel technology and has been used on many spacecraft over the years. The tech uses power, usually from photovoltaic panels, and electric thrusters. It is appealing to mission planners since the SEP can provide a significantly higher specific impulse than chemical engines while requiring far less propellant mass.

The root cause of the issue is still under investigation. The team's current priorities are to stabilize the power level and estimate the reduction's impact on upcoming maneuvers.

ESA said: "If the current power level is maintained, BepiColombo should arrive at Mercury in time for its fourth gravity assist at the planet in September this year.

"Final orbit insertion at Mercury is scheduled for December 2025 and the start of routine science operations for spring 2026."

Writing on Mastodon, McCaughrean noted the criticality of the system for the mission, but said: "There is redundancy and I'm hopeful that they will be able to recover full operational capability and/or work on new options for getting into Mercury orbit."

The BepiColombo mission is planned to be only the second to orbit Mercury. The MTM will be discarded just before orbit insertion, leaving the remaining components – the Mercury Planetary Orbiter and Mercury Magnetosphere Orbiter – to collect data for analysis by scientists.

The next gravity assist flyby is expected in September, assuming there are no more power problems. Engineers have yet to determine what impact running at 90 percent thrust will have on the scheduled December 2025 orbital insertion and spring 2026 start of science operations. ®

Updated at 15.29 UTC on May 16, 2024, to add:

Following publication of this article, ESA sent a statement from Elsa Montagnon, head of Mission Management and Science Operations division: "The current thrust level is compatible with the next critical mission milestone, which is the fourth Mercury swingby on 5th September 2024, and the first of a series of three swingbys until January 2025.

"This swing-by sequence provides a braking delta-V of 2.4 km/s and provides a change of velocity vector direction with respect to the Sun as required for the trajectory end game in 2025.

"The team is continuing to work on understanding the root cause of the problem and its impact on the remainder of the trajectory until the arrival on December 4, 2025."

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