BepiColombo: Two-Orbiter Design and Mercury Observations During Cruise

BepiColombo is a joint Mercury exploration mission of the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA). Named after the Italian scientist Giuseppe Colombo, it is intended to deliver two scientific orbiters to Mercury, combining studies of the surface and interior with measurements of the magnetosphere and its interactions with the solar wind.

Europe supplies the Mercury Planetary Orbiter (MPO), equipped with 11 experiments or instrument packages; Japan supplies the Mercury Magnetospheric Orbiter (MMO), named Mio before launch, carrying five instrument packages. The plan to place the two spacecraft in different orbits is intended to obtain simultaneous data from two locations, helping distinguish spatial differences from changes over time.

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After many years of study, the European Mercury orbiter mission was selected in 2000 as a major project in the Horizon 2000 Plus programme. Early plans also included a Mercury surface element, but its mass and funding requirements were too great, and it was cancelled in 2001. In 2003 Europe formally approved a mission combining its spacecraft with Japan's magnetospheric orbiter; the two agencies subsequently issued a joint call for instrument proposals.

Severe heat and ultraviolet radiation exposed problems in critical components such as solar cells during testing, while increased mass prompted a switch to an Ariane 5 launcher. The redesigned mission received final approval in 2009, but subsequent thermal-balance tests and problems with the power-processing unit caused further delays. On 20 October 2018 the spacecraft finally launched from the Kourou spaceport in French Guiana.

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Industrial teams undertook different parts of the work: Airbus Defence and Space in Germany led design and procurement for the European contribution, while Thales Alenia Space in Italy was responsible for several power, thermal-control and communications systems. In Japan, JAXA organized a team led by NEC to develop MMO. At launch, Johannes Benkhoff and Go Murakami were the European and Japanese project scientists, respectively.

In the cruise configuration, the two orbiters are stacked on the Mercury Transfer Module (MTM), with an additional sunshield protecting Mio. MPO's insulation uses high-temperature-resistant materials, and reflective fins on its radiator reduce infrared heat from Mercury. The solar arrays also have to change their orientation to balance power generation against overheating. The thermal-control design must cope with both direct sunlight and radiation from the planet's surface.

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Reaching Mercury requires continual adjustments to velocity relative to the Sun and the target planet. The mission combines solar electric propulsion with planetary gravity assists: the transfer module's solar arrays power ion engines that operate continuously at low thrust, while flybys of Earth, Venus and Mercury help reshape the trajectory. The two scientific orbiters remain mechanically and electrically connected during cruise.

The first Earth gravity assist took place on 10 April 2020, followed by two Venus flybys in October 2020 and August 2021. Such encounters serve navigation while also providing opportunities to test instruments and make observations. However, the stacked spacecraft and sunshield obstruct some instruments' fields of view, so the available data differ from the full orbital science observations possible after separation and independent operation.

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On 1 October 2021 the spacecraft stack made its first Mercury flyby, passing through the previously less-studied southern magnetosphere. Mio's two electron analyzers operated alongside its ion and energetic neutral atom analyzers. Researchers including Sae Aizawa, Yuki Harada and Nicolas André analyzed these data, identified magnetospheric boundaries along the flight path, and found that the magnetosphere was more compressed than average at the time.

In the post-midnight region, electron flux increased repeatedly. In some of these events, high-energy electrons arrived before low-energy electrons. Using a magnetic-field model, the team interpreted this as energy-dependent drift following repeated impulsive electron injections, supporting a process similar to substorms in Earth's magnetosphere. Their paper, published in 2023, also explained that the cruise configuration restricted the field of view and that telemetry gaps prevented direct measurements of some boundary crossings.

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Between 2021 and 2025, BepiColombo completed six Mercury flybys. Mio's Plasma Wave Investigation was developed jointly by Japan and Europe, with its search-coil magnetometer produced through Japanese–French cooperation. A study by Mitsunori Ozaki and colleagues compared these records, identifying clear whistler-mode wave activity during the first four flybys and discerning fine structures in frequency variation in the 2023 data.

This activity was more pronounced on the dawn side and weaker around midnight, but the six trajectories still did not cover all local-time regions. The paper discussed cruise-phase measurements separately from model-based interpretations, rather than equating a small number of flybys with continuous global monitoring. Long-term coordinated observations by the two orbiters remain a subsequent phase of the mission design; the published flyby findings provide specific questions and a basis for comparison.

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