Laser altimeter BELA ready to explore Mercury’s surface

Launched on Oct. 20, 2018, the joint ESA/JAXA BepiColombo mission has traveled for almost eight years, using a carefully planned but eventful sequence of planetary flybys and solar-electric propulsion to gradually approach its destination, Mercury. The mission is now entering the final stage of its journey to the smallest and innermost planet of our solar system.

The BepiColombo spacecraft consists of the Mercury Transfer Module (MTM) and two orbiters: ESA’s Mercury Planetary Orbiter (MPO) and JAXA’s Mercury Magnetospheric Orbiter (Mio). The spacecraft are flying to Mercury together as a coupled system, but once there, they will be placed into separate orbits.

Following the first separation maneuvers of the MTM from Mio and MPO, preparations are underway for the spacecraft’s arrival at Mercury and the start of its scientific mission in 2027. Aboard the space probe is an instrument designed and built at the Physics Institute of the University of Bern: the laser altimeter BELA—the heaviest instrument on the MPO probe. On Sept. 14, 2026, the instrument team successfully activated BELA’s laser for the first time in space and for the first time since 2017.

BELA ready to map Mercury’s surface

On Sept. 14, 2016, the laser altimeter BELA left Bern for ESA’s European Space Research and Technology Centre ESTEC to be integrated onto the MPO spacecraft. Ten years later to the day, the instrument team from the University of Bern and the German Aerospace Center (DLR) Berlin fully activated the BELA laser altimeter for the first time since the spacecraft began its journey—a crucial step toward the start of BepiColombo’s scientific mission.

BELA was developed by an international consortium under the leadership of the University of Bern and the German Aerospace Center (DLR), in cooperation with the Max Planck Institute for Solar System Research and the Institute of Astrophysics of Andalusia (IAA-CSIC). The aim is to measure the shape, topography and morphology of Mercury’s surface.

“With BELA, we will be able to create a three-dimensional representation of Mercury with unprecedented accuracy,” explains Antoine Pommerol from the Space Research and Planetary Sciences Division at the Physics Institute of the University of Bern, co-principal investigator for BELA.

A cautious step-by-step activation

The activation of BELA followed a carefully controlled sequence. First, the team commanded the laser to emit a 10-second burst. During this initial test, more than 100 ultrashort pulses of infrared light were sent toward open space from the spacecraft, which is roughly 200 million kilometers (124 million miles) away from Earth.

Pommerol explains, “The laser pulses were only sent toward open space this time, not yet toward Mercury. But they could still be characterized by the instrument, providing crucial data on the health of the laser system after eight years in space and nine years since its last firing. When the first data packet reached the ground station about 10 minutes after the test, it confirmed that the instrument had performed as expected.”

BELA was then given the first “go” for the next step. The team repeated the 10-second sequence and, after receiving a second confirmation, extended the laser operation to 10 minutes. Only after this test had also been successfully completed did they run the laser for more than an hour.

This step-by-step procedure allowed the team to check the instrument’s performance at each stage while keeping the risk as low as possible. “In particular, the test confirmed that the laser was operating reliably and that its thermal behavior remained within the expected limits,” says Pommerol.

“Commissioning is meant to be undramatic,” says Dr. Liliane Burkhard, instrument scientist for BELA at the University of Bern’s Physics Institute. “Each step is carefully designed to reveal any potential problems while the laser is operating for only 10 seconds, rather than for an hour. Seeing the first data packet arrive exactly as predicted was incredibly rewarding, especially after the instrument had spent so many years traveling in space just waiting to be switched on.”

The final steps toward Mercury science

“It was very exciting to fully activate BELA again,” says Pommerol. The last time the team fully tested the functionality of the instrument was in 2016, in a dedicated laboratory in Bern. “Although we were cautiously optimistic that BELA would perform as intended, it was a great relief—and a source of satisfaction—to confirm that all subsystems are working nominally. We are now eager to begin the science phase and investigate some of Mercury’s enduring mysteries.”

The next major milestone is scheduled for Dec. 9, 2026, when JAXA’s Mio will be deployed into its operational orbit around Mercury. Further maneuvers will then prepare ESA’s Mercury Planetary Orbiter (MPO) for its own science orbit, which it is scheduled to reach in March 2027.

On April 6, 2027, the long wait will finally be over: BepiColombo’s science phase will begin. After more than eight years in space, the mission will start its detailed investigation of Mercury, offering scientists an unprecedented opportunity to study the planet closest to the sun.

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Gaby Clark

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Robert Egan

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