BepiColombo

• 1. Introduction

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  BepiColombo is a joint mission between the European Space Agency and the Japan Aerospace Exploration Agency, built to study the planet Mercury. On the 20th of October 2018, an Ariane 5 rocket carried two orbiters away from Earth: the Mercury Planetary Orbiter and Mio. The mission's total cost was estimated in 2017 at two billion US dollars. Reaching Mercury would take years of gravity assists past Earth, Venus and Mercury itself, with the exact arrival date shifting along the way. What was the spacecraft named for, and why does a trip to a neighboring planet take so long? What did BepiColombo find during that long approach, and what became of a lander once planned to join it?

• 2. The Man Behind The Name

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  Giuseppe Colombo, known as Bepi, was born in 1920 and died in 1984. He worked as a scientist, mathematician and engineer at the University of Padua in Italy. Colombo was the first to propose the interplanetary gravity assist manoeuvre, a technique later used by the 1974 Mariner 10 mission to Mercury. That method, now common among planetary probes, lets a spacecraft borrow momentum from a planet's gravity instead of burning fuel to change course. Mission planners marked the coincidence when the first Mercury flyby, in October 2021, landed on the 101st anniversary of Colombo's birth.

  Mio, the name of the Mercury Magnetospheric Orbiter, was chosen from thousands of suggestions submitted by the Japanese public. In Japanese, the word means a waterway. JAXA said the name reflects both the milestones already reached and hopes for safe travel ahead. JAXA also compared the spacecraft's passage through the solar wind to a ship crossing the ocean. In Chinese and Japanese tradition, Mercury itself carries the name water star, linking the orbiter directly to the planet it was built to examine.

• 3. Nine Gravity Assists

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  After launch, the stacked spacecraft carried a hyperbolic excess velocity of 3.475 kilometers per second. It first settled into a heliocentric orbit close to Earth's own path around the Sun. After BepiColombo and Earth each completed one and a half orbits, the spacecraft returned for a gravity assist. That maneuver bent its path toward Venus. During the Earth flyby in April 2020, the probe was briefly and mistakenly given a provisional designation as a near-Earth asteroid. Two consecutive Venus flybys then pulled the orbit's closest point in near Mercury's own distance from the Sun, with almost no thrust needed. A sequence of six Mercury flybys slowed the craft's relative velocity to 1.76 kilometers per second. After the fourth of those flybys, in 2024, BepiColombo settled into an orbit similar to Mercury's own and has stayed in the planet's vicinity since.

  On the 15th of May 2024, ESA reported a problem preventing the spacecraft's thrusters from reaching full power. The issue had emerged during a scheduled maneuver on the 26th of April 2024. By the 2nd of September 2024, engineers had built a revised trajectory to compensate for the reduced thrust. That revision added eleven months to the cruise, pushing the expected arrival from the 5th of December 2025 to November 2026. Once captured, only a small maneuver will be needed to settle the craft into an orbit with an apocentre of 178,000 kilometers.

• 4. What The Flybys Revealed

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  In September 2020, scientists tentatively identified the potential biomarker phosphine in the atmosphere of Venus. ESA researchers suggested BepiColombo might detect the compound during its two Venus flybys, but it remained unclear whether the spacecraft's instruments were sensitive enough. No detection has been announced since. During the first Venus flyby in October 2020, seven science instruments and a radiation monitor on the Mercury Planetary Orbiter were active. Three instruments on Mio gathered data as well. The observations were coordinated with JAXA's Akatsuki, the only other spacecraft then orbiting Venus, plus Earth-based observatories. The second Venus flyby in August 2021 came only 33 hours after another ESA spacecraft, Solar Orbiter, completed its own gravity assist at the same planet. Both spacecraft studied Venus's magnetic, plasma and particle environment, producing unique multipoint data. The MPO's MERTIS instrument captured high resolution spectra of the Venus atmosphere, while the Mercury Transfer Module's monitoring cameras recorded black-and-white images documenting the flyby's phases.

  The first Mercury flyby, in October 2021, gave the spacecraft its first images of the target planet, taken with the M-CAM monitoring cameras on the Mercury Transfer Module. Instruments on both orbiters were also active, examining Mercury's magnetic and particle environment and measuring the planet's gravity. During the second flyby, in June 2022, the M-CAM cameras imaged a crater with a candidate volcano, later named Heaney after the poet Seamus Heaney at the request of the M-CAM team. The third flyby, in June 2023, used Mio's MPPE instrument suite to map Mercury's magnetosphere. That data revealed a low latitude layer of particles with an unusually broad energy range. It also showed energetic hydrogen ions trapped near the equator. Cold plasma ions of oxygen, sodium and potassium appeared too, likely knocked from the surface by micrometeorites or the solar wind. The same flyby picked up chirping whistler-mode emission waves, previously known from Earth's magnetosphere but never before detected at Mercury.

  In May 2024, computers on BepiColombo, along with those on ESA's Mars Express, recorded a sharp rise in memory errors. The spike coincided with a massive solar flare from the active region AR3664, then facing away from Earth. ESA's Solar Orbiter observed the same event in detail. The fourth flyby, in September 2024, gave the spacecraft its first clear view of Mercury's south pole. The M-CAM cameras imaged the Vivaldi crater and a newly named crater called Stoddart, honoring the painter Margaret Olrog Stoddart. During the fifth flyby, in December 2024, the MERTIS instrument let BepiColombo become the first spacecraft ever to observe Mercury in mid-infrared light. The sixth and final Mercury flyby, in January 2025, saw the M-CAM 1 camera image four permanently shadowed craters near the north pole: Prokofiev, Kandinsky, Tolkien and Gordimer.

• 5. Two Spacecraft, One Stack

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  The Mercury Transfer Module carries a mass of 2,615 kilograms, including 1,400 kilograms of xenon propellant, and sits at the base of the spacecraft stack. Built by ESA, it carries the two science orbiters to Mercury and supports them during the cruise. Four QinetiQ-T6 ion thrusters, each 22 centimeters across, can operate singly or in pairs. Together they produce a combined maximum thrust of 290 millinewtons, the most powerful ion engine array ever flown in space. Two 14-meter solar panels supply power to the thrusters and to the two hibernating orbiters. Output ranges between 7 and 14 kilowatts, depending on the probe's distance from the Sun. Each T6 thruster needs between 2.5 and 4.5 kilowatts, depending on the desired thrust level. Moments before Mercury orbit insertion, the module will be jettisoned from the stack. The MPO will then power and support Mio until it reaches its own mission orbit.

  The Mercury Planetary Orbiter has a mass of 1,150 kilograms and relies on a single-sided solar array capable of producing up to 1,000 watts. Optical Solar Reflectors keep the array's temperature below 200 degrees Celsius. The array rotates continuously to hold the Sun at a low incidence angle while limiting heat. Eleven instruments make up its payload. They include cameras and spectrometers covering infrared, ultraviolet, X-ray, gamma-ray and neutron wavelengths. The suite also carries a radiometer, a laser altimeter, a magnetometer, particle analysers, a Ka-band transponder and an accelerometer. Most instruments sit on the spacecraft's nadir side for lower detector temperatures. The MERTIS and PHEBUS spectrometers are the exception, mounted at the main radiator for a better field of view. A 1.0 meter high-gain antenna, built to resist high temperatures, communicates on the X-band and Ka-band. It averages 50 kilobits per second, moving 1,550 gigabits of data each year.

  Mio, built mostly by Japan, takes the shape of a short octagonal prism, 180 centimeters long and 90 centimeters high. Its mass is 285 kilograms, including a 45 kilogram scientific payload, spun stable at 15 rotations per minute. Solar cells on its sides supply 90 watts. An 0.8 meter X-band phased array antenna and two medium-gain antennas link it to Earth. Once in Mercury orbit, Sagamihara Space Operation Center will run Mio using Japan's 64 meter Usuda Deep Space Center antenna in Nagano. ESA, meanwhile, oversees the mission overall from mission controllers based in Darmstadt, Germany. The 35 meter Cebreros ground station in Spain serves as the primary communications link. Elsa Montagnon served as the spacecraft operations manager until 2021, when Ignacio Clerigo took over the role.

• 6. Answers Mercury Might Give

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  Mercury is too small and too hot to hold on to any significant atmosphere for long, yet it keeps what scientists call a tenuous surface-bounded exosphere. That thin envelope contains hydrogen, helium, oxygen, sodium, calcium, potassium and other trace elements, constantly lost and replenished from different sources. BepiColombo's objectives begin with the planet's origin and evolution so close to its parent star. The mission will study Mercury's form, interior structure, geology, composition and craters, along with the exosphere's makeup and dynamics. It will also examine the planet's magnetosphere, its structure and dynamics, and investigate the origin of Mercury's magnetic field.

  One objective tests Einstein's theory of general relativity, measuring the parameters gamma and beta of the parameterized post-Newtonian formalism with high accuracy. The orbiters' instruments will also characterize Mercury's solid and liquid iron core and determine the size of each region. They will map the planet's gravity and magnetic field as well. Russia contributed gamma ray and neutron spectrometers specifically to verify whether water ice exists in polar craters that stay permanently shadowed from sunlight.

• 7. The Lander That Never Was

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  Budget constraints ended the Mercury Surface Element in 2003, cancelling what would have been a small lander riding along with BepiColombo. At 44 kilograms, the lander was designed to operate on Mercury's surface for about one week. Shaped as a 0.9 meter diameter disc, it was meant to touch down near latitude 85 degrees, close to the terminator region between Mercury's day and night sides. Braking maneuvers would have brought the lander to zero velocity at an altitude of 120 meters. At that point the propulsion unit would eject and airbags would inflate. The module would then fall to the surface at a maximum impact velocity of 30 meters per second. Scientific data would have been stored onboard and relayed through a cross-dipole UHF antenna to either the MPO or Mio. The lander's 7 kilogram payload was to include an imaging system with a descent camera and a surface camera, plus a heat flow and physical properties package. Rounding out the instruments were an alpha particle X-ray spectrometer, a magnetometer, a seismometer, a soil-penetrating device known as a mole, and a micro-rover.