BepiColombo

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 35-metre (115 ft) 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 (3⁄4 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 3.475 km/s (2.159 mi/s). 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 2020 GL2.

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 1.76 km/s (1.09 mi/s). 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.

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

On 15 June 2026 at 13:24 UTC, BepiColombo switched off its solar electric propulsion for the last time in preparation for the Mercury Arrival Phase starting in September. Also in June, BepiColombo teams started a series of arrival simulations at ESOC. On 3 September 2026, the Mercury Transfer Module successfully separated from the two orbiters, marking the beginning of the Mercury Arrival Phase.

The spacecraft will be "weakly captured" by Mercury's gravity in November 2026 into polar orbit. Only a small maneuver is needed to bring the craft into an orbit around Mercury with an apocentre of 178,000 kilometres (111,000 mi). The orbiters then separate and will adjust their orbits using chemical thrusters.

Trajectory and timeline

As of September 2026, the mission schedule is:

Spacecraft design and components

Mercury Transfer Module

The Mercury Transfer Module (MTM) has a mass of 2,615 kg (5,765 lb), including 1,400 kg (3,100 lb) of Xenon propellant, and was located at the base of the stack. Its role was to carry the two science orbiters to Mercury and to support them during the cruise.

The MTM is equipped with a solar electric propulsion system as the main spacecraft propulsion. Its four QinetiQ-T6 ion thrusters operated singly or in pairs for a maximum combined thrust of 290 mN, making it the most powerful ion engine array ever operated in space. The MTM supplied electrical power for the two stacked orbiters as well as for its solar electric propulsion system thanks to two 14-metre-long (46 ft) solar panels. Depending on the probe's distance to the Sun, the generated power was expected to range between 7 and 14 kW, each T6 requiring between 2.5 and 4.5 kW according to the desired thrust level.

The solar electric propulsion system has typically very high specific impulse and low thrust. This leads to a flight profile with months-long continuous low-thrust braking phases, interrupted by planetary gravity assists, to gradually reduce the velocity of the spacecraft. Prior to Mercury orbit insertion, the MTM was jettisoned from the spacecraft stack. After separation from the MTM, the MPO provides Mio all necessary power and data resources until Mio is delivered to its mission orbit. Separation of Mio from MPO will be accomplished by spin-ejection.

Mercury Planetary Orbiter

The Mercury Planetary Orbiter (MPO) has a mass of 1,150 kg (2,540 lb) and uses a single-sided solar array capable of providing up to 1000 watts and featuring optical solar reflectors to keep its temperature below 200 °C (392 °F). The solar array requires continuous rotation keeping the Sun at a low incidence angle in order to generate adequate power while at the same time limiting the temperature.

The MPO carries a payload of 11 instruments, comprising cameras, spectrometers (IR, UV, X-ray, γ-ray, neutron), a radiometer, a laser altimeter, a magnetometer, particle analysers, a Ka-band transponder, and an accelerometer. The payload components are mounted on the nadir side of the spacecraft to achieve low detector temperatures, apart from the MERTIS and PHEBUS spectrometers located directly at the main radiator to provide a better field of view.

A high-temperature-resistant 1.0 m (3 ft 3 in) diameter high-gain antenna is mounted on a short boom on the zenith side of the spacecraft. Communications will be on the X-band and Ka-band with an average bit rate of 50 kbit/s and a total data volume of 1550 Gbit/year. ESA's Cebreros, Spain 35-metre (115 ft) ground station is planned to be the primary ground facility for communications during all mission phases.

The science payload of the Mercury Planetary Orbiter consists of eleven instruments:

BepiColombo Laser Altimeter (BELA), developed by DLR in cooperation with the University of Bern, the Max Planck Institute for Solar System Research (MPS) and the Instituto de Astrofísica de Andalucía.

Italian Spring Accelerometer (ISA), developed by Italy

Mercury Magnetometer (MPO-MAG, MERMAG), developed by Germany and United Kingdom

Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS), developed by Germany

Mercury Gamma-ray and Neutron Spectrometer (MGNS), developed by Russia

Mio (Mercury Magnetospheric Orbiter)

Mio, or the Mercury Magnetospheric Orbiter (MMO), developed and built mostly by Japan, has the shape of a short octagonal prism, 180 cm (71 in) long from face to face and 90 cm (35 in) high. It has a mass of 285 kg (628 lb), including a 45 kg (99 lb) scientific payload consisting of 5 instrument groups, 4 for plasma and dust measuring run by investigators from Japan, and one magnetometer from Austria.

Mio will be spin stabilized at 15 rpm with the spin axis perpendicular to the equator of Mercury. It will enter a polar orbit at an altitude of 590 × 11,640 km (370 × 7,230 mi), outside of MPO's orbit. The top and bottom of the octagon act as radiators with louvers for active temperature control. The sides are covered with solar cells which provide 90 watts. Communications with Earth will be through a 0.8 m (2 ft 7 in) diameter X-band phased array high-gain antenna and two medium-gain antennas operating in the X-band. Telemetry will return 160 Gb/year, about 5 kbit/s over the lifetime of the spacecraft, which is expected to be greater than one year. The reaction and control system is based on cold gas thrusters. After its release in Mercury orbit, Mio will be operated by Sagamihara Space Operation Center using Usuda Deep Space Center's 64 m (210 ft) antenna located in Nagano, Japan.

Mio carries five groups of science instruments with a total mass of 45 kg (99 lb):

Mercury Plasma Particle Experiment (MPPE), studies the plasma and neutral particles from the planet, its magnetosphere, and the solar wind. It will employ these instruments:

Mercury Electron Analyzers (MEA1 and MEA2)

Mercury Ion Analyzer (MIA)

Mass Spectrum Analyzer (MSA), developed by Laboratory of Plasma Physics (LPP), Max Planck Institute for Solar System Research (MPS), IDA of Technical University of Braunschweig and Institute of Space and Astronautical Science (ISAS)

High-Energy Particle instrument for electrons (HEP-ele)

Magnetospheric Orbiter Sunshield and Interface

The Mio orbiter requires additional thermal control on the cruise to Mercury, in addition to umbilicals to the MPO. The European Space Agency thus provided the Magnetospheric Orbiter Sunshield and Interface (MOSIF), a white shroud that is shaped like a conical frustrum to provide clearance, as Mio is spun up during its separation in 2026, before being ejected from the MPO.

Mercury Surface Element (cancelled)

The Mercury Surface Element (MSE) was cancelled in 2003 due to budgetary constraints. At the time of cancellation, MSE was meant to be a small, 44 kg (97 lb), lander designed to operate for about one week on the surface of Mercury. Shaped as a 0.9 m (2 ft 11 in) diameter disc, it was designed to land at a latitude of 85° near the terminator region. Braking manoeuvres would bring the lander to zero velocity at an altitude of 120 m (390 ft) at which point the propulsion unit would be ejected, airbags inflated, and the module would fall to the surface with a maximum impact velocity of 30 m/s (98 ft/s). Scientific data would be stored onboard and relayed via a cross-dipole UHF antenna to either the MPO or Mio. The MSE would have carried a 7 kg (15 lb) payload consisting of an imaging system (a descent camera and a surface camera), a heat flow and physical properties package, an alpha particle X-ray spectrometer, a magnetometer, a seismometer, a soil penetrating device (mole), and a micro-rover.