BepiColombo, the joint European-Japanese mission bound for Mercury, has shed the propulsion module that carried it across the inner solar system for the past eight years. Mission controllers confirmed the separation on September 3, 2026, clearing the way for the remaining two science spacecraft to begin their final approach toward orbit around the solar system’s innermost planet in the coming months.
The European Space Agency’s Mission Control Team at the European Space Operations Centre in Darmstadt, Germany, received confirmation that the Mercury Transfer Module had detached from the spacecraft stack, marking what ESA calls the first step of a long-awaited arrival sequence that will not be complete until next year.
How the separation actually unfolded
According to ESA’s own account of the operation, the mission team issued a formal “go” for separation at noon Central European Summer Time on September 3, with the module expected to detach roughly two hours later. Because the spacecraft was already more than 200 million kilometres from Earth, confirmation could not arrive instantly — controllers had to wait for a preliminary Doppler signal at 14:20 CEST, followed by full acquisition of signal at 15:49 CEST, before they knew the separation had gone as planned.
Once free of the transfer module, the remaining stack automatically entered a safe mode, adjusted its orientation and reconfigured its systems before reporting its status back to Earth. ESA and the Japan Aerospace Exploration Agency, JAXA, jointly operate the mission, which has traveled roughly 9.9 billion kilometres and completed nine planetary flybys since launch to reach this point.
What the transfer module actually did
The Mercury Transfer Module, roughly the size of a small car, carried two 15-metre solar wings that powered the entire spacecraft stack and drove its solar electric propulsion system. That system converts solar energy into thrust by turning xenon gas into plasma and expelling it through four ion thrusters, delivering gentle, continuous acceleration over weeks or months rather than the short bursts produced by conventional chemical engines. That efficiency is what made the mission’s roundabout, multi-flyby route to Mercury possible in the first place.
The module’s job effectively ended earlier in the year: its solar electric propulsion system was switched off on June 15, 2026, well before the physical separation took place in September. With no antenna or onboard computer of its own, the now-inert module will remain in a stable orbit around the sun rather than returning to Earth or continuing on to Mercury.
Why the mission still has months of maneuvering ahead
Even with the transfer module gone, BepiColombo remained more than three million kilometres from Mercury at the moment of separation. ESA describes the overall arrival process as one of the most complex planetary approach sequences the agency has ever attempted, unfolding over roughly six months rather than as a single event.
The next milestone will be the separation of Mio, the Japanese-built magnetospheric orbiter, expected in early December 2026. Ahead of that, the composite spacecraft will rely on the chemical propulsion system aboard the Mercury Planetary Orbiter, the European-built craft, to fine-tune its trajectory. ESA has confirmed that orbit insertion around Mercury is targeted for November 21, 2026, with the Mercury Planetary Orbiter then guided into its final science orbit by March 2027.
Two orbiters, one long-delayed science campaign
BepiColombo is designed to place two separate spacecraft in orbit around Mercury simultaneously: the European-built Mercury Planetary Orbiter, which will study the planet’s surface and interior, and Mio, the Japanese-built Mercury Magnetospheric Orbiter, which will focus on Mercury’s magnetic field and the surrounding space environment. According to JAXA’s mission page for Mio, the roughly 255-kilogram spacecraft will settle first into an extended elliptical orbit once released, while the larger, 1,230-kilogram Mercury Planetary Orbiter will gradually lower its altitude into a separate, lower orbit around the planet. Flying at different heights on the same orbital plane, the two spacecraft are designed to jointly separate Mercury’s own weak magnetic field from disturbances caused by the solar wind, something neither orbiter could reliably do alone.
ESA project scientist Geraint Jones has noted that the mission has already produced useful science from its cruise phase and nine flybys, but that reaching Mercury will finally allow both spacecraft to deploy their full instrument suites. Under the current timeline, the two orbiters are expected to begin their formal science investigations by April 2027, roughly eight months from now. That gap between arrival and full science operations reflects the additional months required to separate the two spacecraft, settle each into its planned orbit and complete instrument checkouts before Mercury observations begin in earnest.
A journey shaped by an early setback
The mission’s route to Mercury was not entirely smooth. According to ESA, an issue that reduced the power available from the transfer module’s solar arrays forced mission teams to rework BepiColombo’s trajectory earlier in the journey, extending the path needed to reach Mercury safely. That the spacecraft still reached the planet’s vicinity on a workable schedule, engineers say, reflects the flexibility built into the mission’s original flight plan.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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