BepiColombo

BepiColombo

Company type	Space mission
Industry	Space exploration
Founded	October 20, 2018 (2018-10-20)
Founder	European Space Agency (ESA), Japan Aerospace Exploration Agency (JAXA)
Headquarters	Darmstadt , Germany
Area served	Mercury
Key people	Airbus Defence and Space (main contractor)
Products	Mercury Planetary Orbiter, Mercury Magnetospheric Orbiter (Mio), Mercury Transfer Module
Parent	European Space Agency, Japan Aerospace Exploration Agency

BepiColombo represents a collaborative effort between the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA) focused on exploring Mercury. The mission includes two spacecraft launched simultaneously: the Mercury Planetary Orbiter (MPO) and Mio (the Mercury Magnetospheric Orbiter, MMO). Its primary objective is to perform an extensive investigation of Mercury’s magnetic environment, magnetosphere, internal structure, and surface characteristics. The launch occurred on 20 October 2018 aboard an Ariane 5 rocket, with orbital insertion around Mercury scheduled for November 2026 following multiple flybys of Earth, Venus, and Mercury. This mission was authorized in November 2009 as part of ESA's Horizon 2000+ programme, marking the final launch within this initiative.

On 15 May 2024, ESA announced a "glitch" that reduced the thruster power available during a maneuver conducted in April. To compensate, mission planners adjusted the trajectory, lengthening the cruise phase by nearly eleven months and postponing the arrival date from December 2025 to November 2026.

Names

The name BepiColombo honors Giuseppe "Bepi" Colombo (1920–1984), an Italian engineer and scientist who pioneered the interplanetary gravity assist technique utilized by the 1974 Mariner 10 mission—a method now standard in planetary exploration.

The designation Mio for the Mercury Magnetospheric Orbiter was selected from thousands of suggestions submitted by the Japanese public. In Japanese, Mio translates to "waterway," symbolizing both key research achievements and safe passage ahead. JAXA compares the spacecraft’s journey through the solar wind to a vessel navigating a vast ocean. Additionally, in both Chinese and Japanese traditions, Mercury is referred to as the "water star" (水星), reflecting its classification in wǔxíng.

Following its Earth flyby in April 2020, BepiColombo was temporarily misidentified as a near-Earth asteroid, receiving the provisional label 2020 GL2 before this mistake was rectified. This incident marked the third time an ESA mission was confused with an asteroid during Earth flybys.

Mission

The mission architecture consists of three major elements that will separate into distinct spacecraft upon reaching Mercury:

• The Mercury Transfer Module (MTM), providing propulsion capabilities, developed by ESA.
• The Mercury Planetary Orbiter (MPO), constructed by ESA.
• The Mercury Magnetospheric Orbiter (MMO), also known as Mio, built by JAXA.

During launch and cruise phases, these components collectively form the Mercury Cruise System (MCS), with the Magnetospheric Orbiter Sunshield and Interface (MOSIF) positioned between Mio and MPO.

Airbus Defence and Space serves as ESA’s main contractor. ESA is responsible for mission design, development, integration, testing of propulsion systems and MPO modules, as well as overseeing launch operations. Control of both orbiters is managed from Darmstadt, Germany. The spacecraft were launched together on 20 October 2018 via Ariane flight VA245 from Europe’s Spaceport in Kourou, French Guiana. Over an eight-year transit utilizing solar-electric propulsion (ion thruster technology) combined with gravity assists from Earth and Venus, they will eventually enter orbit around Mercury. The primary communications link is maintained through ESA’s Cebreros ground station located in Spain.

Originally planned for December 2025, arrival was deferred to November 2026 due to thruster anomalies detected prior to the fourth planetary flyby. Upon reaching Mercury, Mio and MPO will separate to perform synchronized scientific observations over at least one year, with potential mission extensions. Instruments provided by European nations and Japan will investigate Mercury’s solid inner core and liquid outer core—comprising approximately three-quarters of its radius—as well as its gravitational and magnetic properties. Russian contributions include gamma-ray and neutron spectrometers designed to confirm the existence of water ice within permanently shadowed polar craters.

Due to Mercury’s weak gravitational pull, it cannot sustain a dense atmosphere but supports a thin surface-bound exosphere composed of elements such as hydrogen, helium, oxygen, sodium, calcium, potassium, along with trace species. The mission will examine this exosphere’s composition, dynamics, formation mechanisms, and loss processes.

Objectives

The mission aims to achieve the following key goals:

• Investigate the formation and developmental history of a planet situated near its star
• Examine Mercury’s morphology, internal structure, geological features, chemical makeup, and impact craters
• Study the composition and behavior of Mercury’s exosphere
• Analyze the structure and dynamics of Mercury’s magnetosphere
• Understand the origins of Mercury’s magnetic field
• Validate aspects of Einstein’s general relativity by accurately measuring the parameters gamma and beta within the parameterized post-Newtonian framework

Design

The spacecraft stack is planned to take eight years to enter orbit around Mercury, utilizing solar-electric propulsion combined with nine gravity assist maneuvers: Earth and Moon (April 2020), Venus (twice in 2020 and 2021), and six flybys of Mercury between 2021 and 2025.

After launch with a hyperbolic excess velocity of , the spacecraft initially follows a heliocentric orbit akin to Earth’s. Completing one and a half orbits, it returns for an Earth gravity assist before being redirected toward Venus. Two Venus flybys serve to lower the perihelion close to Mercury’s orbit with minimal propulsion. Subsequently, six Mercury flybys progressively reduce relative velocity to . Following the fourth flyby at Mercury, the spacecraft enters an orbit proximate to Mercury’s trajectory.

Four final propulsion arcs further decrease velocity, enabling a “weak” capture into Mercury orbit in November 2026 with a polar orbital inclination. A minor maneuver adjusts the orbit to an apocentre of . The two orbiters then separate and refine their orbits using chemical thrusters.

History

ESA selected the BepiColombo mission proposal in 2000. A call for proposals for scientific instruments was issued in 2004. By 2007, Astrium was appointed as prime contractor with Ariane 5 designated as the launch vehicle. The originally scheduled July 2014 launch was postponed primarily due to delays in developing the solar electric propulsion system. By 2017, total mission costs were estimated at approximately US$2 billion.

Schedule

As of , important mission milestones include:

Date	Event	Notes
20 October 2018, 01:45 UTC	Launch
10 April 2020, 04:25 UTC	Earth flyby	Occurred about one and a half years after launch
15 October 2020, 03:58 UTC	First Venus flyby	ESA confirmed successful encounter; potential detection of phosphine as a biosignature candidate in Venus’ atmosphere was considered, though instrument sensitivity remained uncertain
10 August 2021, 13:51 UTC	Second Venus flyby	Achieved a closest approach within 552 kilometres (343 mi) of Venus’ surface
1 October 2021, 23:34:41 UTC	First Mercury flyby	Passed at approximately 199 kilometres (124 mi) altitude; coincided with Giuseppe Colombo’s 101st birth anniversary
23 June 2022, 09:44 UTC	Second Mercury flyby	Closest approach near 200 kilometres (120 mi) altitude
19 June 2023, 19:34 UTC	Third Mercury flyby	Closest approach around 236 kilometres (147 mi) altitude
4 September 2024, 21:48 UTC	Fourth Mercury flyby	Passed within roughly 165 kilometres (103 mi) altitude
1 December 2024, 14:23 UTC	Fifth Mercury flyby	Closest approach approximately 37,626 kilometres (23,380 mi) altitude
8 January 2025, 05:58:52 UTC	Sixth Mercury flyby	Closest approach near 295 kilometres (183 mi) altitude
November 2026	Insertion into Mercury orbit	Spacecraft separation occurs; roughly seven Mercury years after sixth flyby
2027	MPO achieves final science orbit	About 1.13 Mercury years following orbit insertion
April 2028	Nominal mission concludes	Nearly six Mercury years after orbit insertion
April 2029	Extended mission ends	Close to ten Mercury years post-orbit insertion

Mercury Transfer Module

QinetiQ T6	Specifications
Type	Kaufman Ion Thruster
Number of units	4
Diameter	22 cm (8.7 in)
Maximum thrust	145 mN per unit
Specific impulse (Isp)	4300 seconds
Propellant used	Xenon
Total power consumption	4628 W

The Mercury Transfer Module (MTM), with a mass of and carrying xenon propellant, facilitates the journey of the two scientific orbiters to Mercury. Located at the bottom of the spacecraft stack, it utilizes a solar electric propulsion system powered by four QinetiQ-T6 ion thrusters, representing the most potent ion engine assembly ever flown in space, delivering a combined thrust up to 290 mN.

Two large solar arrays produce between 7 and 14 kW of electrical power depending on proximity to the Sun, which powers both the propulsion system and the orbiters while they remain in hibernation. Each T6 thruster draws between 2.5 and 4.5 kW depending on thrust output.

This propulsion system’s high specific impulse and low-thrust capability enable extended braking maneuvers over several months, assisted by gravity assists from planets, to gradually lower velocity for orbital capture around Mercury. The MTM is discarded shortly before orbit insertion, after which the Mercury Planetary Orbiter (MPO) takes over power and data management until deployment of Mio via spin-ejection into its operational orbit.

Mercury Planetary Orbiter

The Mercury Planetary Orbiter (MPO) has a mass of and is equipped with a single-sided solar array that can provide up to 1000 watts. The array includes Optical Solar Reflectors to keep temperatures below , and it continuously rotates to maximize power generation and thermal regulation.

MPO hosts eleven scientific instruments such as cameras and spectrometers covering infrared, ultraviolet, X-ray, gamma-ray, and neutron wavelengths, alongside a radiometer, laser altimeter, magnetometer, particle analyzers, a Ka-band transponder, and an accelerometer. Most instruments are installed on the nadir side to maintain low detector temperatures; exceptions include MERTIS and PHEBUS spectrometers placed near the main radiator for improved viewing angles.

A heat-resistant 1.0 m high-gain antenna is mounted on a boom on the zenith side. Communication operates via X- and Ka-band frequencies with an average data rate of 50 kbit/s and an annual data volume of approximately 1550 Gbit. The primary ground station for communications is ESA’s 35 m Cebreros facility in Spain.

Science payload

The MPO carries eleven scientific instruments:

• BepiColombo Laser Altimeter (BELA), developed jointly by DLR, University of Bern, Max Planck Institute for Solar System Research (MPS), and Instituto de Astrofísica de Andalucía
• Italian Spring Accelerometer (ISA)
• Mercury Magnetometer (MPO-MAG or MERMAG), developed by Germany and the United Kingdom
• Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS), developed in Germany
• Mercury Gamma-ray and Neutron Spectrometer (MGNS), developed by Russia
• Mercury Imaging X-ray Spectrometer (MIXS), developed by University of Leicester, MPS, and Max Planck Institute for Extraterrestrial Physics
• Mercury Orbiter Radio-science Experiment (MORE), developed through collaboration between Italy and the United States
• Probing of Hermean Exosphere by Ultraviolet Spectroscopy (PHEBUS), developed by France and Russia
• Search for Exosphere Refilling and Emitted Neutral Abundances (SERENA), consisting of four particle analyzers:
  • ELENA (Emitted Low-Energy Neutral Atoms) from Italy
  • STROFIO (Rotating Field Mass Spectrometer) from the United States
  • MIPA (Miniature Ion Precipitation Analyser) from Sweden
  • PICAM (Planetary Ion Camera), developed collaboratively by Russian Space Research Institute, CETP/IPSL, ESTEC, KFKI-RMKI, and MPS
• Spectrometers and Imagers for MPO BepiColombo Integrated Observatory System (SIMBIO-SYS), featuring high-resolution stereo cameras plus visual/near-infrared spectrometer developed by Italy, France, and Switzerland
• Solar Intensity Alpha Particle X-ray Spectrometer (SIXS), developed jointly by Finland and the United Kingdom

Mio (Mercury Magnetospheric Orbiter)

Primarily designed by Japan, Mio, also known as the Mercury Magnetospheric Orbiter (MMO), is a compact spacecraft shaped as a short octagonal prism, measuring approximately 1.4 meters in length and 1.1 meters in height. It has a total mass of around 53 kilograms, which includes a scientific payload composed of five instrument suites—four dedicated to plasma and dust analysis led by Japanese teams, and one magnetometer provided by Austria.

The spacecraft employs spin stabilization at a rate of 15 revolutions per minute, with its spin axis oriented perpendicular to Mercury's equatorial plane. It follows a polar orbit at roughly 590 kilometers altitude, positioned outside the orbit of the Mercury Planetary Orbiter (MPO). Both the top and bottom octagonal surfaces function as radiators equipped with louvers for thermal regulation, while its lateral panels are covered with solar arrays producing 90 watts of power. Communications are maintained via an X-band phased array high-gain antenna complemented by two medium-gain antennas, enabling data transmission of approximately 160 gigabits annually (about 5 kilobits per second). The mission is planned to last beyond one year. Attitude control is achieved through cold gas thrusters. Following orbit insertion, mission operations are conducted from the Sagamihara Space Operation Center, utilizing the Usuda Deep Space Center’s antenna located in Nagano, Japan.

Science payload

The scientific instruments aboard Mio consist of five groups totaling about 15 kilograms:

• Mercury Plasma Particle Experiment (MPPE): This suite examines plasma and neutral particles originating from Mercury’s magnetosphere and the solar wind. It includes:
  • Mercury Electron Analyzers (MEA1 and MEA2)
  • Mercury Ion Analyzer (MIA)
  • Mass Spectrum Analyzer (MSA), developed collaboratively by the Laboratory of Plasma Physics (LPP), Max Planck Institute for Solar System Research (MPS), Technical University of Braunschweig, and ISAS
  • High-Energy Particle detectors for electrons (HEP-ele) and ions (HEP-ion)
  • Energetic Neutrals Analyzer (ENA)
• Mercury Magnetometer (MMO-MGF): Focuses on characterizing Mercury’s magnetic field, magnetosphere structure, and interplanetary solar wind interactions
• Plasma Wave Investigation (PWI): Measures electric fields along with electromagnetic and radio wave emissions from both the magnetosphere and solar wind
• Mercury Sodium Atmosphere Spectral Imager (MSASI): Studies the composition and dynamics of Mercury’s tenuous sodium atmosphere
• Mercury Dust Monitor (MDM): Detects dust particles originating from both Mercury’s environment and interplanetary space

Magnetospheric Orbiter Sunshield and Interface

To ensure proper thermal management during transit to Mercury and to facilitate mechanical connections with the MPO, the European Space Agency developed the Magnetospheric Orbiter Sunshield and Interface (MOSIF). This component is a white conical cover designed to provide adequate clearance as Mio spins up during separation procedures scheduled for 2026 before it separates from the MPO spacecraft.

Mercury Surface Element (terminated)

The Mercury Surface Element (MSE) project was discontinued in 2003 as a result of financial limitations. This compact lander, weighing approximately 44 kg, was intended to operate for roughly one week close to Mercury’s terminator zone at an 85° latitude. The landing strategy involved deceleration maneuvers, airbag cushioning, and propulsion-assisted ejection to limit the touchdown speed to a maximum of 30 m/s. Scientific measurements would have been recorded onboard and transmitted via a UHF antenna to either the MPO or Mio spacecraft. The scientific payload, with a mass of 7 kg, was to comprise imaging equipment, instruments for heat flow and physical property analysis, an alpha particle X-ray spectrometer, magnetometer, seismometer, a soil-penetrating mole tool, and a small micro-rover.

See also

• Exploration of Mercury
• MESSENGER – the inaugural spacecraft to orbit Mercury

External links

• BepiColombo official site by the European Space Agency
• BepiColombo Operations portal provided by ESA
• BepiColombo official page from JAXA
• BepiColombo information by JAXA's Institute of Space and Astronautical Science
• BepiColombo overview from NASA's Solar System Exploration program
• BepiColombo data archive hosted by the National Space Science Data Center
• Comprehensive coverage of the BepiColombo mission to Mercury, edited by Johannes Benkhoff et al., in Space Science Reviews, 216–217 (2020–2021)
• Detailed BepiColombo article on eoPortal by ESA