BepiColombo sheds its cruise module and begins its Mercury arrival

How do you slow a spacecraft at a small planet close to the Sun, where gravity is weak? That is the operational problem the BepiColombo mission, a partnership between the European Space Agency (ESA) and Japan’s JAXA, has begun to confront in its final stage. Launched in October 2018, the mission traveled billions of kilometers and used nine planetary flybys to adjust its speed and trajectory gradually before reaching Mercury.

On September 3, the team began the arrival phase by separating the Mercury Transfer Module, which had supplied propulsion during the cruise, from the stack carrying the two science orbiters. The procedure had to unfold autonomously: a distance of more than 200 million kilometers ruled out human correction in real time. Four mechanical connections were cut, and spring mechanisms gently pushed the module away at about 40 centimeters per second.

ESA deep-space stations in Cebreros, Spain, and Malargüe, Argentina, first received the expected shift in radio frequency caused by the change in velocity. They then acquired telemetry from the Mercury Planetary Orbiter, or MPO. At 15:52 Central European Summer Time, the agency confirmed a successful separation, nominal systems and solar arrays charging the batteries.

The separation did not immediately put the probes into their science orbits. MPO will keep powering the stack until capture into Mercury orbit, scheduled for November 21. On December 9–10, it is due to release the Japanese Mio orbiter; each spacecraft will then undergo commissioning and orbital adjustments. The science phase is planned for April 2027, when 16 instruments will examine the planet’s surface, interior, magnetic field and space environment.

The AFP report published by Phys.org adds operational context: the confirmation signal took almost two hours to reach mission control, while commands and checks faced about a 30-minute delay each way. The report and ESA’s record describe the same event in different roles — one documents the telemetry officially, while the other explains why distance turns a slow separation into a high-risk operation.

The September milestone answered only the first question of arrival: the cruise module departed and the two orbiters continued operating as expected. Orbital capture, Mio’s release and commissioning still have to occur. If those stages succeed, BepiColombo will become the first mission to observe Mercury with two spacecraft in complementary orbits, separating close study of the planet from investigation of its magnetosphere.