Europe is not sure if its ambitious Mercury probe can reach the planet

Artist’s rendering of the BepiColombo mission, a joint ESA and JAXA project, which will take two spacecraft to the harsh environment of Mercury.


This week, the European Space Agency published a slightly ominous note about its BepiColombo spacecraft, which consists of two orbiters bound for Mercury.

The online press release cited a “glitch” with the spacecraft that is affecting its ability to generate thrust. The problem was first detected on April 26, when the spacecraft’s main propulsion system was scheduled to perform an orbital maneuver. At that time there was not enough electrical energy supplied to the solar propulsion system.

According to the space agency, a team made up of its own engineers and those of its industrial partners began working on the issue. By May 7 they had made some progress, restoring the spacecraft’s thrust to about 90 percent of its original level. But this is not complete and the fundamental cause of the problem is still not well understood.

It is an ambitious mission, with an estimated cost of 2 billion dollars. Carried out jointly with the Japanese space agency JAXA, BepiColombo was launched on an Ariane 5 rocket in October 2018. So the stakes are high with these boosters. The critical question is, at this power level, can BepiColombo still perform its primary task of reaching orbit around Mercury?

The answer to this question is not so clear.

A three-part spaceship

The spacecraft consists of three components. The “transfer module” is where the current problems occur. It was built by the European Space Agency and is intended to power the other two components of the spacecraft until October 2025. It is essential for positioning the spacecraft for entry into orbit around Mercury. The other two elements of the mission are a European orbiter, MPO, and a Japanese orbiter, Mio. After their planned arrival in orbit around Mercury in December 2025, the two orbiters will separate and conduct at least a year of observations, including characterizing the small planet’s magnetic field.

The press release is ambiguous about the fate of BepiColombo if full power cannot be restored to its propulsion system.

Ars contacted the European Space Agency and asked if BepiColombo can still reach orbit around Mercury in this state. The answer, according to statements by Elsa Montagnon, head of mission operations at the space agency, is not entirely clear.

“Thank you for your legitimate questions about the current uncertainty,” Montagnon said. “We are working hard to resolve these uncertainties.”

Gotta have that delta-V

What is clear, he said, is that the current thrust level can support the next critical milestone, BepiColombo’s fourth Mercury pass, which will occur on September 5 of this year. This is the first of three scheduled approaches that will take place in quick succession from September to January and will slow the spacecraft relative to Mercury.

“This step sequence provides a braking delta-V of 2.4 km/s and provides a change of direction of the velocity vector with respect to the Sun as required for the end of the trajectory in 2025,” Montagnon said.

Currently, a team of experts is working on the implications of the reduced thrusters for the other two parts of this step sequence and other propulsion needs in 2025.

This transfer module is scheduled to be discarded from the rest of the stack in October 2025, and subsequently the remaining approach and insertion maneuvers into Mercury orbit will be carried out with the chemical propulsion subsystem of the European MPO spacecraft.

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