BepiColombo Discards Transfer Module, Commences Final Mercury Approach

Eight years after launch, the Mercury probe separates from its transfer module as orbit approaches

After nearly eight years, the ESA and JAXA BepiColombo spacecraft is nearing Mercury, with engineers initiating the arrival sequence by jettisoning the transfer module that propelled it this far.

The spacecraft stack separated from the Mercury Transfer Module (MTM) on September 3 at approximately 1400 CEST. Following a tense interval, controllers confirmed a preliminary Doppler signal, indicating successful separation; by 1552 CEST, signal acquisition confirmed all procedures went to plan. The Mercury Planetary Orbiter’s (MPO) solar panels began charging the spacecraft’s batteries.

The next phase involves entering orbit around Mercury on November 21. The Mercury Magnetospheric Orbiter (MMO), also known as Mio, will be released between December 9 and 10. A week later, the MPO will maneuver into its final orbit of 480 by 1,500 km, with scientific operations beginning in April 2027.

The arrival is one year behind schedule. BepiColombo was originally expected to orbit Mercury by December 2025, but issues with the Solar Electric Propulsion (SEP) system required a trajectory reassessment, extending the cruise phase.

BepiColombo is no stranger to Mercury, having performed six flybys of the planet since 2021. It also conducted flybys of Venus and Earth after its 2018 launch.

The mission exemplifies international cooperation: the MPO and MTM are ESA contributions, while the MMO originates from JAXA. ESA also provided a sunshield to protect the MMO during transit, which will be ejected after the orbiter’s release.

The MPO is the larger component of the partnership, with a main body measuring 2.4 by 2.2 by 1.7 meters and a mass of 1,230 kilograms (including an 85-kilogram science payload). JAXA’s MMO measures 1.8 meters in diameter and 1.1 meters in height, weighing 255 kilograms (45 kilograms of which is the science payload). The MPO is 3-axis stabilized and nadir-pointing, whereas the MMO will be spin-stabilized at 15 rpm, with its spin axis perpendicular to the Sun.

The mission’s goal is to study Mercury, “from the structure and dynamics of its magnetosphere and its interaction with the solar wind, to the properties of its large iron core and the origin of its magnetic field.”

The initial mission at Mercury is planned for one Earth year (approximately four Mercury years), with a possible extension of another Earth year.

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