BepiColombo is a joint mission of the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA) to the planet Mercury.[4] The mission comprises two satellites launched together: the Mercury Planetary Orbiter (MPO) and Mio (Mercury Magnetospheric Orbiter, MMO).[5] The mission will perform a comprehensive study of Mercury, including characterization of its magnetic field, magnetosphere, and both interior and surface structure. It was launched on an Ariane 5[2] rocket on 20 October 2018 at 01:45 UTC, with an arrival at Mercury planned for November 2026, after a flyby of Earth, two flybys of Venus, and six flybys of Mercury.[1][6] The mission was approved in November 2009, after years in proposal and planning as part of the European Space Agency's Horizon 2000+ programme;[7] it is the last mission of the programme to be launched.[8]
On 15 May 2024, ESA reported that a "glitch" prevented the spacecraft's thrusters from operating at full power during a scheduled manoeuvre on 26 April.[9] On 2 September, 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.[10]
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.[5] In Chinese and Japanese, Mercury is known as the "water star" (水星) according to wǔxíng.
The mission involves three components, which will separate into independent spacecraft upon arrival at Mercury.[15]
Mercury Transfer Module (MTM) for propulsion, built by ESA.
Mercury Planetary Orbiter (MPO) built by ESA.
Mercury Magnetospheric Orbiter (MMO) or Mio built by JAXA.
During the launch and cruise phases, these three components are joined together (with the Magnetospheric Orbiter Sunshield and Interface or MOSIF between Mio and MPO)[16] to form the Mercury Cruise System (MCS).[17][18]
The prime contractor for ESA is Airbus Defence and Space.[19] 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, which are operated by mission controllers based in Darmstadt, Germany, were successfully launched together on 20 October 2018.[20] The launch took place on Ariane flight VA245 from Europe’s Spaceport in Kourou, French Guiana.[21] The spacecraft will have an eight-year interplanetary cruise to Mercury using solar-electric propulsion (ion thrusters) and gravity assists from Earth, Venus and eventual gravity capture at Mercury.[1] ESA's Cebreros, Spain 35-metre (115 ft) ground station is planned to be the primary ground facility for communications during all mission phases.
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.[1] Although originally expected to enter orbit in December 2025, thruster issues discovered in September 2024 before its 4th flyby resulted in a delayed arrival of November 2026.[22] The orbiters are equipped with scientific instruments provided by various European countries and Japan. The mission will characterize the solid and liquid iron core (3⁄4 of the planet's radius) and determine the size of each.[23] The mission will also complete gravitational and magnetic field mappings. Russia provided gamma ray and neutron spectrometers to verify the existence of water ice in polar craters that are permanently in shadow from the Sun's rays.
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"[24] 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.
Planned orbits for Mio and MPO satellites, the two probes of the BepiColombo mission
The stacked spacecraft will take eight years to position itself to enter Mercury orbit. During this time it will use 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.[1]
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 is deflected towards Venus. Two consecutive Venus flybys reduce the perihelion near to the Sun–Mercury distance with almost no need for thrust. A sequence of six Mercury flybys will lower the relative velocity to 1.76 km/s (1.09 mi/s). After the fourth Mercury flyby, the craft will be in an orbit similar to that of Mercury and will remain in the general vicinity of Mercury (see [1]). Four final thrust arcs reduce the relative velocity to the point where Mercury will "weakly" capture the spacecraft 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.[28][29]
History
The BepiColombo mission proposal was selected by ESA in 2000. A request for proposals for the science payload was issued in 2004.[30] In 2007, Astrium was selected as the prime contractor, and Ariane 5 chosen as the launch vehicle.[30] The initial target launch of July 2014 was postponed several times, mostly because of delays on the development of the solar electric propulsion system.[30] The total cost of the mission was estimated in 2017 as US$2 billion.[31]
Schedule
Animation of BepiColombo's trajectory from 20 October 2018 to 2 November 2025 BepiColombo·Earth·Venus·Mercury·Sun For more detailed animation, see this videoSequence of images taken during the second Mercury flybyAnimation of BepiColombo's trajectory around Mercury
As of January 2025[update], the mission schedule is:[32]
According to Johannes Benkhoff of ESA, the probe may possibly be capable of detecting phosphine – the chemical allegedly discovered in the Venusian atmosphere in September 2020 – during this and the following flyby. He stated that "we do not know if our instrument is sensitive enough".[33] On 15 October 2020, the ESA reported the flyby was a success.[34]
10 August 2021, 13:51 UTC
Second Venus flyby
1.35 Venus years after first Venus flyby. Flyby was a success, and saw BepiColombo come within 552 kilometres (343 mi) of Venus' surface.[35][36]
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 is located at the base of the stack. Its role is 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 operate singly or in pairs for a maximum combined thrust of 290 mN,[49] making it the most powerful ion engine array ever operated in space. The MTM supplies electrical power for the two hibernating orbiters as well as for its solar electric propulsion system thanks to two 14-metre-long (46 ft) solar panels.[50] Depending on the probe's distance to the Sun, the generated power will 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. Moments before Mercury orbit insertion, the MTM will be jettisoned from the spacecraft stack.[50] After separation from the MTM, the MPO will provide 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
Mercury Planetary Orbiter in ESTEC before stackingRadio testing of BepiColombo 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.[50]
The MPO will carry 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.[50]
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.[50]
Science payload
MPO’s science instruments
The science payload of the Mercury Planetary Orbiter consists of eleven instruments:[51][52]
Spectrometers and Imagers for MPO BepiColombo Integrated Observatory System (SIMBIO-SYS), high resolution stereo cameras and a visual and near infrared spectrometer, developed by Italy, France and Switzerland
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.[3][58] 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.[3][59][60]
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.[59] 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.[51]
Science payload
Photo captured on 23 June 2022 as the spacecraft flew past the planet for its second of six gravity assist manoeuvres at Mercury. This image was taken by the Mercury Transfer Module’s Monitoring Camera 3, when the spacecraft was 1406 km from the surface of Mercury.
Mio carries five groups of science instruments with a total mass of 45 kg (99 lb):[3][51]
Plasma Wave Investigation (PWI), studies the electric field, electromagnetic waves, and radio waves from the magnetosphere and solar wind
Mercury Sodium Atmosphere Spectral Imager (MSASI), studies the thin sodium atmosphere of Mercury
Mercury Dust Monitor (MDM), studies dust from the planet and interplanetary space
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.[16][17][18]
Mercury Surface Element (cancelled)
The Mercury Surface Element (MSE) was cancelled in 2003 due to budgetary constraints.[8] 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.[28] 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.[62]
^"BepiColombo flies by Earth". Europlanet Society. 10 April 2020. Retrieved 24 June 2022. The data collected for this image, even though it was submitted to the Minor Planet Center as artificial satellite 2018-080A (BepiColombo's official designation), led to it being mistaken for a Near Earth asteroid. The "discovery", announced by the Minor Planet Center as asteroid 2020 GL2, was retracted soon after. This was the third time a spacecraft had been mistakenly announced as a "new asteroid" during an Earth flyby, after Rosetta a.k.a. 2007 VN84 and Gaia a.k.a. 2015 HP116. Incidentally, all three of these are ESA missions.
^Hayakawa, Hajime; Maejima, Hironori (2011). BepiColombo Mercury Magnetospheric Orbiter (MMO)(PDF). 9th IAA Low-Cost Planetary Missions Conference. 21–23 June 2011, Laurel, Maryland. Archived from the original(PDF) on 23 February 2020. Retrieved 15 August 2011.
^ ab"BepiColombo". National Space Science Data Center. NASA. 26 August 2014. Retrieved 6 April 2015. This article incorporates text from this source, which is in the public domain.
^"BepiColombo flies by Venus en route to Mercury". ESA. 15 October 2020. Retrieved 15 October 2020. The flyby itself was very successful", confirms Elsa. "The only difference to normal cruise phase operations is that near to Venus we have to temporarily close the shutter of any of the star trackers that are expected to be blinded by the planet, similar to closing your eyes to avoid looking at the Sun
^"BepiColombo". Twitter/BepiColombo. 20 June 2023. Retrieved 20 June 2023. Our #BepiColombo @esaoperations team confirm all went well with our #MercuryFlyby last night! Now we wait and see what images & data our instrument teams collected
^Qualification of the T6 Thruster for BepiColomboArchived 12 August 2016 at the Wayback Machine R. A. Lewis, J. Pérez Luna, N. Coombs. 30th International Symposium on Space Technology and Science 34th International Electric Propulsion Conference and 6th Nano-satellite Symposium, Hyogo-Kobe, Japan, 4–10 July 2015
^"Strofio". Discovery Program. NASA. Archived from the original on 8 January 2017. Retrieved 7 January 2017. This article incorporates text from this source, which is in the public domain.
Missions are ordered by launch date. † indicates failure en route or before any data returned. ‡ indicates use of the planet as a gravity assist en route to another destination.
Launches are separated by dots ( • ), payloads by commas ( , ), multiple names for the same satellite by slashes ( / ). Crewed flights are underlined. Launch failures are marked with the † sign. Payloads deployed from other spacecraft are (enclosed in parentheses).
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