The ESA mission BepiColombo has been on its planned eight-year journey to the innermost planet of the Solar System since October 2018. On board are highly specialised scientific instruments that will help to explore Mercury – a celestial body that remains poorly understood to this day.
On 20 October 2018, the BepiColombo mission launched from Europe's Spaceport in Kourou, French Guiana. The joint project between the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA) has since been on an eight-year journey to Mercury, the planet closest to the Sun. The German Aerospace Center (DLR) is contributing significantly to the scientific payload of BepiColombo with two instruments: the MERTIS spectrometer (MErcury Radiometer and Thermal Infrared Spectrometer) and the BELA laser altimeter (BEpiColombo Laser Altimeter). Both instruments were developed by DLR and are being operated during the mission.
BepiColombo due to enter Mercury's orbit in November 2026
In November 2026, the BepiColombo probe is scheduled to enter orbit around Mercury. The scientific objectives of the mission are to study Mercury's surface, the influence of the solar wind, the planet's internal structure and geological history, and its interactions with the solar environment. Comparative studies of the formation of the Solar System are also planned.
Mercury's proximity to the Sun makes it a complex journey
Due to the Sun's enormous gravitational pull, BepiColombo had to perform a total of nine close flybys in order to gradually approach the innermost planet on the ideal trajectory: one of Earth, two of Venus and six of Mercury itself.
During these close flybys, also known as swing-by manoeuvres, the space probe uses the gravitational pull of the planets to adjust its trajectory and change, for example, its flight velocity. These manoeuvres are crucial for two reasons: they save fuel and are necessary for the spacecraft to enter Mercury's orbit at all.
BepiColombo successfully completed the last of its nine flybys on 8 January 2025 and is now on its final approach to Mercury, scheduled to enter into orbit in November 2026.
During these flybys, the MERTIS imaging spectrometer – developed by DLR and operated jointly with the University of Münster – was able to already make observations of Earth and Venus, as well as delivering the first detailed view of Mercury's surface in the thermal infrared, revealing the planet in a completely new light.
BepiColombo comprises three main parts
BepiColombo consists of three main parts, connected together on their journey to Mercury. The Mercury Transfer Module (MTM) uses its solar panels to generate electricity to power an ion thruster that transports the mission to Mercury. The Mercury Planetary Orbiter (MPO) is the European scientific component of the mission carrying 11 instruments onboard, including MERTIS and BELA. Finally, mounted on top of the MPO, and protected by the MMO Sunshield and Interface Structure (MOSIF), is the Mercury Magnetospheric Orbiter (MMO) – the Japanese contribution to the mission. Upon arrival, these three components will separate and begin their respective scientific investigations, in different orbits, over a period of one year.
MERTIS analyses Mercury's surface composition and heat distribution
The MERTIS spectrometer will map Mercury's mineralogy and investigate the temperature distribution of its surface in unprecedented detail. Its scientific objectives include analysing Mercury's surface composition, identifying rock-forming minerals, mapping surface mineralogy and studying surface temperatures and thermal inertia. During BepiColombo's flyby of the Earth-Moon system on 10 April 2020, and the Venus flybys on 15 October 2020 and 10 August 2021, MERTIS was successfully deployed. On 1 December 2024, during BepiColombo's fifth flyby of its ultimate destination, it observed Mercury for the first time. In total, MERTIS collected more than 1.4 million spectra of Mercury's surface from a distance of more than 37,000 kilometres. The first look at MERTIS' mid-infrared spectral data revealed a large number of surface features that can be detected at these wavelengths. During this fifth flyby, MERTIS had half of the approximately 1550-kilometre Caloris impact basin and parts of a large volcanic plain in the northern hemisphere in its field of view.
BELA to map Mercury's surface
BELA is a laser altimeter that will measure the topography of the entire planet. Among other goals, it will deliver the first elevation data for Mercury's southern hemisphere. To achieve this, BELA will emit ten laser pulses per second from a distance of up to 1400 kilometres above the surface, recording the signals reflected back to precisely measure the light's travel time – down to fractions of a nanosecond. Each pulse will provide an exact distance measurement, and using a large number of signals it will map the entire surface topography of Mercury. In conjunction with gravity field data, this survey should provide insights into the processes behind the formation of the planet's crust and surface structures. Observations of Mercury's tidal deformation could provide further evidence of a liquid outer core and help to narrow down the size of its solid inner core.
Giuseppe 'Bepi' Colombo's ingenious flight path solution
Swing-by manoeuvres were first used during the Mariner 10 mission by the American space agency NASA in 1974 and 1975 – at that time along Mercury's orbit to enable two additional close flybys after the initial encounter with the planet. The essential calculations for Mariner 10's trajectory were developed by Italian engineer and mathematician Giuseppe 'Bepi' Colombo. The joint European-Japanese Mercury mission was named BepiColombo in his honour.
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BepiColombo on the long journey to Mercury
The BepiColombo mission is a joint project of the European Space Agency (ESA), which contributes the Mercury Planetary Orbiter (MPO, centre), and the Japanese Space Agency (JAXA), which contributes the Mercury Magnetoshpheric Orbiter (MMO, top). The orbiter duo is powered by an ion engine in the Mercury Transfer Module (MTM, bottom).
Image: 1/6, Credit:
ESA/ATG medialab
BepiColombo’s eight year journey to Mercury
Travelling from Earth into the inner Solar System is a technically demanding task due to the Sun's immense gravitational pull. The European-Japanese BepiColombo mission will take eight years to complete its journey to Mercury. Over this time, the spacecraft will perform several flybys – one at Earth, two at Venus, and six at Mercury – to gradually slow down and adjust its trajectory. These manoeuvres will steer BepiColombo into an orbit around the Sun that increasingly aligns with Mercury’s. In November 2026, the spacecraft will finally enter orbit around the innermost planet.
Image: 2/6, Credit:
ESA
Fifth Mercury flyby on 1 December 2024
The joint Mercury BepiColombo mission, from the European Space Agency (ESA) and the Japanese Aerospace Exploration Agency (JAXA), flew by its target for the fifth time on 1 December 2024 at a distance of 37,630 kilometre. The spacecraft is set to enter Mercury's orbit in November 2026. During this penultimate flyby, the spacecraft was 200 times further away from the planet than during its previous flyby, when it came within 165 kilometres of Mercury's surface. This fifth flyby is the first time the probe has used its Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) instrument, which measures the temperature and composition, providing information about the types of minerals found on Mercury's surface.
