BepiColombo is a joint mission of the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA) to the planet Mercury. The mission comprises two orbiters: the Mercury Planetary Orbiter (MPO), built and operated by ESA, and Mio (Mercury Magnetospheric Orbiter, MMO), built and operated by JAXA. Together the satellites will perform a comprehensive study of Mercury, including characterization of its magnetic field, magnetosphere, and both interior and surface structure.
During the cruise phase to reach Mercury, the orbiters were in a stacked configuration with two additional components: the Mercury Transfer Module (MTM), which provided power and solar electric propulsion for the cruise, and the Magnetospheric Orbiter Sunshield and Interface (MOSIF), which protects Mio from the harsh solar radiation until it is deployed. MTM was jettisoned before orbit insertion at Mercury, whereas MOSIF will be detached from MPO after the Japanese orbiter is deployed.
In 2017 the spacecraft's cost was estimated at €1.65 billion (US$ billion). It was launched on an Ariane 5 rocket on 20 October 2018, with Mercury orbit insertion planned for December 2025, after a flyby of Earth, two flybys of Venus, and six flybys of Mercury. In 2024, due to a power-related anomaly limiting the thrust levels on its solar electric propulsion system, Mercury arrival was delayed to November 2026.
Overview
The mission involves four components, including two orbiters which will separate into independent spacecraft upon arrival at Mercury: Mercury Transfer Module (MTM) providing chemical and solar electric propulsion, by ESA Mercury Planetary Orbiter (MPO), built by ESA Mercury Magnetospheric Orbiter (MMO) or Mio, by JAXA Magnetospheric Orbiter Sunshield and Interface (MOSIF) to protect Mio during cruise, by ESA
During the launch and cruise phases, these four components were joined to form the Mercury Composite Spacecraft (MCS)
The stacked spacecraft took eight years to position itself to enter Mercury orbit. During this time it used solar electric propulsion and nine gravity assists, flying past the Earth and Moon in April 2020, Venus in 2020 and 2021, and six Mercury flybys between 2021 and 2025.
Expected to arrive in Mercury orbit in November 2026, the Mio and MPO satellites will separate and observe Mercury in collaboration for one year, with a possible one-year extension. Although originally expected to enter orbit in December 2025, thruster issues discovered in September 2024 before the fourth Mercury flyby resulted in a delayed arrival of November 2026.
ESA is responsible for the overall mission, the design, development assembly and test of the propulsion and MPO modules, and the launch. The two orbiters are operated by mission controllers based in Darmstadt, Germany. The spacecraft operations manager of BepiColombo was Elsa Montagnon until 2021, and is now Ignacio Clerigo. ESA's Cebreros, Spain ground station is the primary ground facility for communications during all mission phases.
Naming BepiColombo is named after Giuseppe "Bepi" Colombo (1920–1984), a scientist, mathematician, and engineer at the University of Padua, Italy, who first proposed the interplanetary gravity assist manoeuvre used by the 1974 Mariner 10 mission, a technique now used frequently by planetary probes.
Mio, the name of the Mercury Magnetospheric Orbiter, was selected from thousands of suggestions by the Japanese public. In Japanese, Mio means a waterway, and according to JAXA, it symbolizes the research and development milestones reached thus far, and wishes for safe travel ahead. JAXA said the spacecraft will travel through the solar wind just like a ship traveling through the ocean. In Chinese and Japanese, Mercury is known as the "water star" (水星) according to wǔxíng.
Scientific objectives The main objectives of the mission are:
Study the origin and evolution of a planet close to its parent star Study Mercury's form, interior, structure, geology, composition and craters Investigate Mercury's exosphere, composition and dynamics, including generation and escape Study Mercury's magnetised envelope (magnetosphere) – structure and dynamics Investigate the origin of Mercury's magnetic field Test Einstein's theory of general relativity by measuring the parameters gamma and beta of the parameterized post-Newtonian formalism with high accuracy. Mercury is too small and hot for its gravity to retain any significant atmosphere over long periods of time, but it has a tenuous surface-bounded exosphere containing hydrogen, helium, oxygen, sodium, calcium, potassium and other trace elements. Its exosphere is not stable as atoms are continuously lost and replenished from a variety of sources. The mission will study the exosphere composition and dynamics, including generation and escape.
The orbiters are equipped with scientific instruments provided by various European countries and Japan. The mission is designed to characterize the solid and liquid iron core ( of the planet's radius) and determine the size of each as well as to perform gravitational and magnetic field mappings. Russia provided the gamma ray and neutron spectrometer (MGNS) to test the presence of water ice in polar craters that are permanently in shadow from the Sun's rays.
Launch and operations
Before launch The BepiColombo mission proposal was selected by ESA in 2000. A request for proposals for the science payload was issued in 2004. In 2007, Astrium (now Airbus Defence and Space) was selected as the prime contractor, and Ariane 5 chosen as the launch vehicle. The mission was approved in November 2009, after years in proposal and planning as part of the European Space Agency's Horizon 2000+ programme. The initial target launch of July 2014 was postponed several times, mostly because of delays on the development of the solar electric propulsion system. BepiColombo is the last mission of the programme to be launched.
Launch The two orbiters were successfully launched together on 20 October 2018. The launch took place on Ariane flight VA245 from Europe's Spaceport in Kourou, French Guiana.
Gravity assist maneuvers
The stacked spacecraft left Earth with a hyperbolic excess velocity of . Initially, the craft was placed in a heliocentric orbit similar to that of Earth. After both the spacecraft and Earth completed one and a half orbits, it returned to Earth to perform a gravity-assist maneuver and was deflected towards Venus.
Following its Earth flyby in April 2020, BepiColombo was briefly mistaken for a near-Earth asteroid, receiving the provisional designation .
Two consecutive Venus flybys reduced the perihelion near to the Sun–Mercury distance with almost no need for thrust. A sequence of six Mercury flybys lowered the relative velocity to . After the fourth Mercury flyby in 2024, the spacecraft is in an orbit similar to that of Mercury and remains in the general vicinity of the planet.
Science during Venus flybys After the potential biomarker phosphine was tentatively discovered in the Venusian atmosphere in September 2020, ESA scientists suggested that BepiColombo might be able to detect the compound during its two Venus flybys in 2020 and 2021. However, it was not clear if the spacecraft's instruments were sufficiently sensitive and there has been no announcement of results. Since then, the claimed September 2020 detection has been disputed by follow-up studies.
During the first Venus flyby in October 2020, seven science instruments and a radiation monitor onboard the Mercury Planetary Orbiter, and three instruments onboard Mio, were active and gathering data. The observations were coordinated with JAXA's Akatsuki, the only active spacecraft orbiting Venus at that time, as well as Earth-based observatories.
The second Venus flyby in August 2021 happened only 33 hours after another interplanetary spacecraft by ESA, Solar Orbiter, completed its gravity assist at the same planet. Both spacecraft used their science instruments to study the magnetic, plasma, and particle environment around Venus during their flybys, offering unique multipoint datasets. The MPO's MERTIS instrument captured high resolution spectra of the Venus atmosphere and the Mercury Transfer Module's three monitoring cameras (M-CAM) captured a series of black-and-white images of the planet, documenting the various phases of the flyby.
Science during Mercury flybys During the first Mercury flyby in October 2021, the spacecraft captured its first images of the target planet using the M-CAM monitoring cameras on the Mercury Transfer Module. Some of the scientific instruments on both orbiters were also active during the flyby, exploring the magnetic and particle environment around Mercury and measuring the planet's gravity.
During the second flyby in June 2022, the M-CAM cameras imaged, among other targets, the crater Heaney with a candidate volcano, an important target for the spacecraft's primary mission. This crater has been recently named after Seamus Heaney following a request from the M-CAM team. Some of the scientific instruments have been again active, measuring the magnetic, plasma, and particle environment around the spacecraft.
During the third flyby in June 2023, the MPPE suite of instruments on Mio was used to map the magnetosphere of Mercury. Based on these data, scientists described various expected features of the magnetosphere, but also made new discoveries: 1) a low latitude layer containing particles with much broader energy range than ever observed on Mercury, 2) energetic hydrogen ions trapped at low latitude and near the equator, and 3) cold plasma ions of oxygen and sodium, as well as signatures of potassium, which were probably ejected from the planet's surface by micrometeorites or the solar wind. Mio's observations during this flyby also identified the chirping-like discrete whistler-mode emission waves previously observed in Earth's magnetosphere but so far unknown from Mercury.
In May 2024, computers on BepiColombo (as well as on another ESA mission, Mars Express) reported a sharp increase in the number of memory errors, coinciding with a massive solar flare from the active region AR3664, at that time facing away from Earth. The event was also observed in detail by ESA's Solar Orbiter.
During the fourth flyby in September 2024, the spacecraft had, for the first time, a clear view of Mercury's south pole. The M-CAM 2 and 3 cameras provided images of the polar region, as well as the Vivaldi crater and a crater newly named Stoddart after Margaret Olrog Stoddart following a request from the M-CAM team. The altitude of the flyby (165 km) was even lower than the expected final science orbit of MPO and it coincided with elevated fluxes of high-energy charged solar particles. Scientists used this opportunity to study how these particles interacted with molecules on the surface, using the SIXS instrument.
During the fifth flyby in December 2024, using the MERTIS instrument, BepiColombo became the first spacecraft ever to observe Mercury in mid-infrared light. During the sixth and final Mercury flyby in January 2025, the M-CAM 1 camera imaged the permanently shadowed craters Prokofiev, Kandinsky, Tolkien, and Gordimer near the planet's north pole.
Thruster issues On 15 May 2024, ESA reported an issue preventing the spacecraft's thrusters from operating at full power during a scheduled manoeuvre on 26 April 2024. On 2 September 2024, ESA reported that to compensate for the reduced available thrust, a revised trajectory had been developed that would add 11 months to the cruise, delaying the expected arrival date from 5 December 2025 to November 2026.
Arrival at Mercury