Mercury probe BepiColombo thrusters are acting up, but science marches on

The BepiColombo spacecraft is to make a closer-than-planned flyby of Mercury this week, whizzing past the planet at approximately 165 km from the surface after the European Space Agency's (ESA) Flight Dynamics team tweaked the trajectory to compensate for malfunctioning thrusters.

The ESA and Japan Aerospace Exploration Agency (JAXA) mission was launched in 2018, and planetary flybys had always been part of the plan to steer the spacecraft into orbit around Mercury in 2025 without requiring prodigious amounts of thrust.

The project comprises ESA's Mercury Planetary Orbiter and JAXA's Mercury Magnetospheric Orbiter. The two are carried together by the Mercury Transfer Module (MTM), where the problem and reason for the trajectory adjustment lie.

In April, it was noted that the thrusters on the MTM were not operating as they should due to an issue with the distribution of electrical power. At the time, former ESA Senior Science Advisor Mark McCaughrean said the Solar Electric Propulsion (SEP) system had been "glitchy" for a while.

"Engineers identified unexpected electric currents between MTM's solar array and the unit responsible for extracting power and distributing it to the rest of the spacecraft," according to ESA. "Onboard data imply that this is resulting in less power available for electric propulsion."

This means that the original plan has had to be altered to compensate for the lower-than-expected thrust levels provided by the SEP. Engineers have spent months working on the problem and concluded that a trajectory change is needed if the spacecraft is to perform its baseline scientific mission.

A December 2025 arrival at Mercury is therefore off the table. Instead, BepiColombo will arrive in November 2026. The fourth, fifth (December 2024), and sixth (January 2025) Mercury flybys will go ahead as planned, but the fourth flyby will now be closer than planned, meaning less propulsion will be required to reach the fifth flyby. The sixth flyby will then send the spacecraft onto its new trajectory.

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"The rest of the BepiColombo mission is expected to go ahead as planned," ESA said, despite the delayed arrival. Of the 16 instruments on the spacecraft, ten will be active during the flyby.

The main science camera is shielded until the ESA and JAXA orbiters separate. However, ESA does plan to snap some bonus images from the three monitoring cameras on the MTM. Designed to monitor the deployment of the spacecraft's solar array and instruments, the cameras will be pressed into service to capture images of Mercury when BepiColombo will be approximately 200 km above the planet's surface.

Johannes Benkhoff, BepiColombo Project Scientist, said: "It's so exciting that BepiColombo can boost our understanding and knowledge of Mercury during these brief flybys, despite being in 'stacked' cruise configuration.

"We get to fly our world-class science laboratory through diverse and unexplored parts of Mercury's environment that we won't have access to once in orbit, while also getting a head start on preparations to make sure we will transition into the main science mission as quickly and smoothly as possible."

And ESA's Flight Dynamics team will once again get to demonstrate their prowess at trajectory calculation.

The spacecraft is scheduled to make its closest approach at 2148 UTC on September 4. The images captured during the flyby are expected to be downlinked and released on September 5. ®