BepiColombo facts for kids

Quick facts for kids
BepiColombo

The BepiColombo spacecraft stack. From bottom to top: MTM, MPO, and Mio.
Mission type	Planetary science
Operator	ESA JAXA
Website	science.esa.int/bepicolombo
Mission duration	Cruise: 8 years, 1 month Science phase: 1 year (planned)
Spacecraft properties
Manufacturer	Airbus ISAS
Launch mass	4,100 kg (9,000 lb)
BOL mass	MPO: 1,230 kg (2,710 lb) Mio: 255 kg (562 lb)
Dry mass	2,700 kg (6,000 lb)
Dimensions	MPO: 2.4 m × 2.2 m × 1.7 m (7 ft 10 in × 7 ft 3 in × 5 ft 7 in) Mio: 1.8 m × 1.1 m (5 ft 11 in × 3 ft 7 in)
Power	MPO: 150 watts Mio: 90 watts
Start of mission
Launch date	20 October 2018, 01:45 UTC
Rocket	Ariane 5 ECA (VA245)
Launch site	Centre Spatial Guyanais, ELA-3
Contractor	Arianespace
Mercury orbiter
Spacecraft component	Mercury Planetary Orbiter (MPO)
Orbital insertion	21 November 2026 (planned)
Orbit parameters
Perihermion	480 km (300 mi)
Apohermion	1,500 km (930 mi)
Inclination	90.0°
Mercury orbiter
Spacecraft component	Mercury Magnetospheric Orbiter (Mio/MMO)
Orbital insertion	10 December 2026 (planned deployment from MPO)
Orbit parameters
Perihermion	590 km (370 mi)
Apohermion	11,640 km (7,230 mi)
Inclination	90.0°
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BepiColombo is an exciting space mission sent to explore Mercury, the closest planet to our Sun. It is a joint robotic mission created by two space agencies: the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA). The mission carries two distinct scientific orbiters: the Mercury Planetary Orbiter (MPO) and Mio (the Mercury Magnetospheric Orbiter, or MMO). Together, these advanced robotic explorers study the rocky surface, deep interior, thin atmosphere, and invisible magnetic bubble around Mercury.

Getting to Mercury is one of the hardest challenges in space exploration. A spacecraft traveling toward the center of our solar system speeds up constantly due to the massive gravitational pull of the Sun. To slow down and steer safely, BepiColombo used high-tech ion thrusters and multiple planetary flybys of Earth, Venus, and Mercury. After a long journey across the inner Solar System, the spacecraft separated from its heavy cruise engine module to begin entering final orbit around Mercury.

What Is the BepiColombo Space Mission?

The four main parts of the stacked BepiColombo spacecraft

BepiColombo traveling in its cruise setup through deep space

The Four Main Parts of the Spacecraft

During its long journey across space, BepiColombo flew as a single combined stack. This combination was called the Mercury Composite Spacecraft (MCS). It was made of four main modules:

• Mercury Transfer Module (MTM): Built by ESA, this carrier module carried powerful solar wings and advanced ion engines to push and steer the spacecraft across the solar system.
• Mercury Planetary Orbiter (MPO): Built by ESA, this main probe is nicknamed "Bepi" and studies the surface, rocks, craters, and core of Mercury.
• Mercury Magnetospheric Orbiter (MMO / Mio): Built by JAXA, this Japanese orbiter studies Mercury's magnetic field, waves, and surrounding space particles.
• Magnetospheric Orbiter Sunshield and Interface (MOSIF): A heat shield built by ESA that protected Mio from scorching solar heat during the cruise.

Who Built and Controls the Mission?

The overall mission is managed by the European Space Agency in close cooperation with Japan's JAXA. Teams of mission flight controllers operate the probes from the European Space Operations Centre located in Darmstadt, Germany.

Communications across millions of kilometers of deep space are handled by large satellite dishes on Earth. The primary ground station is a giant 35-meter-wide antenna situated at Cebreros in Spain. Japanese flight scientists at the Sagamihara Campus in Japan also control the science experiments for Mio using a huge 64-meter antenna dish at the Usuda Deep Space Center.

How Did the Probes Get Their Names?

The mission honors the famous Italian mathematician and engineer Giuseppe Colombo (1920–1984), who had the nickname "Bepi." He discovered how to use the gravity of planets like Earth and Venus to redirect spacecraft paths toward other worlds. This clever technique is called a gravity assist. He also explained how Mercury rotates three times on its axis for every two orbits around the Sun.

The Japanese orbiter name Mio was chosen from thousands of public suggestions in Japan. In the Japanese language, the word mio refers to a clear water channel followed by ships navigating rivers or oceans. JAXA chose this name to symbolize smooth sailing through space and the stream of particles from the Sun known as the solar wind. In Japanese and Chinese astronomy, Mercury is historically known as the "water star."

Main Science Goals at Mercury

Mercury is one of the least explored rocky worlds in the solar system. The spacecraft was designed to solve several major mysteries:

• Discover how rocky planets form and evolve very close to their parent stars.
• Map all the craters, volcanoes, and surface landforms on Mercury in high definition.
• Examine the thin layer of gas surrounding the planet, called the exosphere.
• Understand the size and state of Mercury's giant iron core, which takes up nearly three-quarters of the planet's radius.
• Study the structure of Mercury's magnetic field and how it shields the planet from solar particles.
• Test Albert Einstein's theory of general relativity by tracking precise radio signals near the heavy gravity of the Sun.

How BepiColombo Traveled to Mercury

Venus seen by BepiColombo during a flyby in October 2020

A volcanic crater on Mercury seen during a flyby in June 2022

Launch into Space

Engineers spent many years designing and building the spacecraft to withstand extreme conditions. The mission was launched into space on 20 October 2018 from the European spaceport in Kourou, French Guiana. A heavy Ariane 5 rocket blasted the combined 4.1-ton spacecraft into space with tremendous speed.

The Art of Gravity Assist Flybys

Flying straight toward Mercury requires a massive amount of rocket fuel because the Sun constantly pulls the spacecraft faster and faster. To slow down without running out of fuel, flight controllers used nine planetary gravity assist flybys:

• One flyby past Earth in April 2020.
• Two flybys past Venus in October 2020 and August 2021.
• Six close flybys past Mercury between October 2021 and January 2025.

During each close approach, the gravitational field of the planet robbed orbital energy from the spacecraft, slowing it down. This allowed BepiColombo to match the fast orbit of Mercury naturally.

Discoveries Made During Flybys

Even before entering orbit, the scientific instruments on board took measurements and photos during their flybys:

• Venus Flybys: In 2020 and 2021, the spacecraft flew close to cloud-covered Venus. Its infrared sensors studied the dense Venusian clouds, while particle detectors examined how the solar wind strips away parts of Venus's upper atmosphere.
• First Mercury Views: In October 2021, monitoring cameras captured their first close-up pictures of Mercury's cratered surface.
• Volcanoes and South Pole: In June 2022, cameras spotted the Heaney crater, showing evidence of past volcanic eruptions. Later flybys in September 2024 provided the first detailed views of the craters at Mercury's south pole.
• Infrared Views and Ice Craters: In December 2024, the MERTIS instrument became the first sensor ever to map Mercury in mid-infrared light. In January 2025, cameras imaged the dark, permanently shadowed craters near the north pole, which may hold deep deposits of hidden water ice.

Thruster Glitch and Orbit Arrival

In May 2024, space engineers noticed an electrical glitch that prevented the ion thrusters from working at 100 percent full power. The smart mission navigation team quickly redesigned the flight path. The new route used extra gravity maneuvers, gently shifting the final arrival date to November 2026. In September 2026, the spacecraft safely separated from its MTM carrier module, preparing both orbiters for scientific orbital insertion.

Timeline of Key Mission Events

Spacecraft Engineering and Science Instruments

The science instrument layout on the European orbiter

The fully stacked spacecraft inside cleanrooms with its white sunshield

Mercury Transfer Module (MTM)

The MTM was the powerhouse that pushed the probe through space. It weighed about 2,615 kilograms, including 1,400 kilograms of xenon gas. Xenon is an inert noble gas used as rocket fuel for ion thrusters.

Instead of burning fuel with fire, ion engines use electricity from solar panels to give xenon atoms an electric charge (ionization). The thrusters then shoot these charged particles out the back at extreme speeds. While the push (thrust) feels very gentle, these engines can run for months continuously, reaching huge speeds over time while using very little fuel.

Mercury Planetary Orbiter (MPO)

The MPO is a sleek, three-axis stabilized satellite weighing 1,150 kilograms. It carries a large radiator and special reflective mirrors to prevent overheating from the intense sunlight. It will fly in a low polar orbit around Mercury, between 480 and 1,500 kilometers above the ground.

The MPO carries 11 advanced scientific instruments:

• BELA (Laser Altimeter): Shoots laser pulses at the surface to measure heights of mountains and depths of craters.
• ISA (Accelerometer): Measures non-gravitational forces acting on the spacecraft with extreme precision.
• MPO-MAG (Magnetometer): Measures the intensity and direction of Mercury's planetary magnetic field.
• MERTIS (Thermal Infrared Spectrometer): Analyzes rock minerals on the surface and maps ground temperatures.
• MGNS (Gamma-ray and Neutron Spectrometer): Detects chemical elements on the surface and looks for water ice hidden in dark polar craters.
• MIXS (X-ray Spectrometer): Identifies metals like iron, magnesium, aluminum, and silicon on the ground.
• MORE (Radio Science Experiment): Uses radio links to calculate Mercury's gravity field and test theories of gravity.
• PHEBUS (Ultraviolet Spectrometer): Explores the thin gases in Mercury's upper exosphere.
• SERENA (Particle Analyzers): Studies neutral atoms and ions knocked off the surface by space radiation.
• SIMBIO-SYS (Imaging System): Captures high-definition 3D stereo color photos of the planet's surface features.
• SIXS (X-ray and Particle Spectrometer): Monitors energetic rays and particles coming directly from the Sun.

Mercury Magnetospheric Orbiter (Mio)

The Japanese orbiter Mio is shaped like an eight-sided box (octagonal prism) weighing 285 kilograms. Unlike MPO, Mio spins like a top at 15 revolutions per minute. Spinning keeps the spacecraft stable and ensures that all sides share the solar heat evenly.

Mio flies in a wider, elliptical orbit ranging from 590 to 11,640 kilometers above the planet. This higher orbit is perfect for exploring the wider magnetic bubble around Mercury.

Mio carries five specialized instrument packages:

• MPPE (Plasma Particle Experiment): Detects fast electrons, charged ions, and solar wind particles.
• MMO-MGF (Magnetometer): Accurately tracks changes in the planet's outer magnetic boundaries.
• PWI (Plasma Wave Investigation): Listens to natural electromagnetic radio waves produced in space plasma.
• MSASI (Sodium Atmosphere Imager): Maps the glowing clouds of sodium gas around Mercury.
• MDM (Dust Monitor): Detects tiny grains of interplanetary dust and micrometeorites hitting the spacecraft.

Images for kids

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  Mercury's south polar craters seen during the fourth flyby in September 2024

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  Mercury observed in mid-infrared light in December 2024

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  Shadowed craters near Mercury's north pole imaged in January 2025

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  Diagram illustrating the flight path and planetary flybys

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  Animation showing how the spacecraft enters orbit around Mercury

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  Diagram showing the science orbits of MPO and Mio around Mercury

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  Testing the transfer module in a space simulator chamber

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  Testing the unfolding of the giant solar wings

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  The transfer module prepared in the cleanroom

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  Close-up view of the high-tech ion thrusters

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  Placing the MPO into a thermal vacuum chamber for space testing

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  Testing radio signals and antennae on the MPO

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  The cooling radiator panel and science instruments of MPO

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  The MPO orbiter prepared for stacking at the space center

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  The Japanese orbiter Mio at the space facility

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  Lifting Mio with a crane during spacecraft assembly

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  Mio mounted on top of MPO without its protective sunshield

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  Model showing how Mio separates in orbit

Related Pages

In Spanish: BepiColombo para niños

• Exploration of Mercury
• MESSENGER (NASA mission that orbited Mercury)
• Mariner 10 (First probe to fly past Mercury)
• European Space Agency
• JAXA
• Solar wind