BepiColombo Sheds Power Module, Begins Final Approach to Mercury

The joint European Space Agency (ESA) and Japan Aerospace Exploration Agency (JAXA) mission reached a critical milestone on September 3 when engineers commanded the separation of the Mercury Transfer Module (MTM) from the main spacecraft stack. After a tense few minutes, a preliminary Doppler signal confirmed the separation, followed by confirmation at 1552 CEST that all had gone to plan.

With the MTM detached, the Mercury Planetary Orbiter (MPO) is now charging its batteries via its solar panels. The next major step is dropping into orbit around Mercury on November 21. Following that, the Mercury Magnetospheric Orbiter (MMO), named Mio, will be released between December 9 and 10. A week later, MPO will begin moving to its final science orbit of 480 by 1,500 kilometres, with scientific operations commencing in April 2027.

The arrival comes a year later than originally planned. BepiColombo was initially scheduled to begin orbiting Mercury in December 2025, but problems with its Solar Electric Power (SEP) system forced a trajectory redesign that extended the cruise phase.

The spacecraft is no stranger to Mercury, having performed six flybys of the planet since 2021, in addition to flybys of Venus and Earth following its 2018 launch. The mission is a notable example of international cooperation: ESA contributed the MPO and MTM, while JAXA provided the MMO. ESA also supplied a sunshield to protect the MMO during transit, which will be ejected after the MMO is released.

The MPO is the larger component, with a main body measuring 2.4 x 2.2 x 1.7 metres and a mass of 1,230 kilograms, including an 85 kilogram science payload. It is three-axis stabilised and nadir-pointing. JAXA's MMO is 1.8 metres in diameter and 1.1 metres high, weighing 255 kilograms (45 kilograms of which is science payload). MMO will be spin-stabilised at 15 rotations per minute, with its spin axis at 90 degrees to the Sun.

The overall goal of BepiColombo 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 one-year extension.

Why This Matters

The mission will provide unprecedented data about Mercury, the least explored terrestrial planet, helping scientists understand the formation and evolution of rocky planets in the inner solar system.
Successful orbit insertion ends an eight-year journey that was extended by technical problems, demonstrating resilience in deep-space operations.
The dual-orbiter configuration (ESA's MPO and JAXA's MMO) allows simultaneous observations from different orbits, offering a more complete picture of Mercury's magnetosphere and interior.

Background

BepiColombo launched in October 2018 on a long, gravity-assist trajectory that included flybys of Earth, Venus, and multiple passes of Mercury. The mission was named after Italian mathematician and engineer Giuseppe "Bepi" Colombo, who helped develop gravity-assist techniques used by earlier missions like Mariner 10. The spacecraft originally had three components: the Mercury Transfer Module (providing solar-electric propulsion and carrying the orbiters), the Mercury Planetary Orbiter (built by ESA), and the Mercury Magnetospheric Orbiter (built by JAXA). The SEP system problems that delayed arrival are not fully detailed in public documents, but the trajectory redesign successfully compensated.

Key Perspectives

[ESA and JAXA mission teams]: Focus on executing the remaining critical milestones — orbit insertion in November, orbiter separation in December, and initial science operations by April 2027 — to maximise the scientific return after years of transit. [Planetary science community]: Eager for the first close-up magnetospheric and interior measurements of Mercury, which remains enigmatic due to its large iron core relative to its size and its unusual magnetic field. The data will complement NASA's earlier MESSENGER mission. [Critics/Skeptics]: Some may question the cost and time of a mission that took nearly a decade to reach its destination, but proponents note that gravity-assist trajectories and international collaboration reduce costs and technical risk compared to direct insertion.

What to Watch

November 21, 2026: Orbit insertion burn — success or failure will determine the mission's immediate future.
December 9–10, 2026: Release of the MMO (Mio) orbiter from the stack.
April 2027: Scheduled start of full science operations; look for first data releases and images.
Potential one-year mission extension beyond the initial one Earth year (ending around April 2028).

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