1 Earth, 2 Venus, 6 Mercury: the route of BepiColombo to reach Mercury
On the 8th January 2025, the team in the Mio mission control room were gazing at temperature readings from the space probe’s solar panels. The values had just started to drop. Onboard the BepiColombo spacecraft, the Mio orbiter had dipped into the shadow of Mercury.
At the front of the control room, BepiColombo Project Scientist Murakami Go was explaining the temperature readings to the YouTube live stream. The spacecraft was performing its final Mercury swing-by—a manoeuvre that brought the spacecraft close to the planet and used Mercury’s gravity to act as a brake. After the swing-by, BepiColombo would be moving at nearly the same speed as Mercury around the Sun. When the two met again in November 2026, their relative speed would be small enough to place Mio into orbit.
Nine swing-bys with BepiColombo, including clips from the actual footage captured from the spacecraft as it passed each planet.
BepiColombo is a collaborative mission between the European Space Agency (ESA) and JAXA to explore the planet Mercury. The BepiColombo spacecraft has a unique stacked configuration that consists of an orbiter developed by each space agency, and the Mercury Transfer Module (MTM) that is providing the propulsion for the interplanetary journey. When the moment arrives for orbit insertion, the spacecraft will separate to allow Mio and the ESA MPO orbiter to be snagged by Mercury’s gravity.
Orbiting Mercury is a difficult task. If BepiColombo travelled straight towards the Sun, the spacecraft would gain too much speed for Mercury’s gravity to pull the probes into orbit. BepiColombo therefore performed nine swing-bys, passing close enough to a planet to be slowed by the planet’s gravity. In total, BepiColombo completed one Earth swing-by, two Venus swing-bys, and now six Mercury swing-bys.
While the need for a planetary braking system makes arrival at Mercury slow, the close pass to the planet’s surface provides a unique opportunity for early observations.
“The original plan for Mio was to hibernate throughout the interplanetary cruise phase,” explains Seki Taeko, Mission Manager for Mio. “But because it takes such a long time to arrive at Mercury, we realised that the swing-by operations were a good experience in many ways: practising science observations as well as team building and skill training. Without this opportunity, it would have been more difficult to prepare for the Mercury arrival.”
But while the planet’s gravity provides the force during a swing-by, which avoids the need of major spacecraft operations, each close pass is still a tense moment for the team. However, during the first swing-by, the challenge had been one for the humans, rather than the spacecraft, as BepiColombo approached Earth during the COVID-19 pandemic.
“International travel was restricted,” remembers Seki. “So we had to change our plans at the last minute. Instead of flying to join the European team, we quickly set up a configuration where we could monitor the whole operation remotely from Japan. It was challenging, but this worked so well that this became the default for all the swing-bys!”
For Mio itself, one concern is that of temperature. Once detached from BepiColombo, the orbiter will extend long antennas and mast to explore the magnetic and electric fields around Mercury, spinning continuously. The spin will stabilise the orbiter’s position, but it will also ensure that the pounding heat from the close proximity of the Sun is distributed evenly around the spacecraft.
During the interplanetary cruise, Mio is mounted on BepiColombo and cannot rotate. This risks overheating and damage to the orbiter. To mitigate this issue, Mio is tucked inside a sunshield that blocks most of the heat. But the orientation of BepiColombo during the Mercury swing-bys exposes the opening in the sunshield to the planet’s illuminated surface, allowing radiated heat to enter the sunshield. The temperature telemetry is therefore some of the most important information during a swing-by.
“The temperature is shared live on YouTube, so it can be particularly nerve-wracking!” says Murakami. “During Mercury swing-bys we do observe values close to the upper limit of the allowable range!”
During the fourth Mercury swing-by in September last year, temperatures onboard Mio did reach dangerous levels. The decision was made to put the orbiter into hibernation, a move that powers down all observation equipment onboard Mio.
“The battery temperature was higher than expected even before we reached our closest approach to Mercury,” recalls Seki. “We decided to turn off Mio early. But we also wanted to observe as long as possible so deciding on when to actually send the commands was a bit difficult.”
The protective sunshield around Mio does limit the orbiter’s observational view, and the mast and antennas are also currently folded and stored. Despite this, Mio has snapped data using all but one of its five observational instruments during the swing-by operations. Working with the European team, the attitude of BepiColombo was selected so that the sunshield opening would typically face Mercury for a few hours immediately after the spacecraft’s closest approach.
“Although the attitude profile is similar, Mio’s field of view has been different for each Mercury swing-by because the trajectory of BepiColombo varied,” says Seki. “A few observations are also of areas that we will only pass over during a swing-by, and results from these can be compared with the observations we will make after arrival.”
“My favourite was the third Mercury swing-by!” adds Murakami. “We captured a very different view inside the planet’s magnetosphere!”
The observations gathered by Mio have stoked the team’s excitement for what Mio might uncover after arrival. One of the team’s favourite results was tracing the origin of Mercury’s X-ray aurora. First seen by the NASA MESSENGER spacecraft, Mercury’s surface is known to ripple with fluorescent X-rays. But the origin of this phenomenon had never been observed. During the first Mercury swing-by, Mio spotted electrons being accelerated by Mercury’s magnetic field to rain down on the planet.
Mercury is the only rocky planet other than the Earth in our Solar System which generates its own magnetic field. Electrically charged ions and electrons that stream from the Sun get caught in the magnetic fields of both planets and spiral inwards. On Earth, these solar particles collide with molecules in our atmosphere to create the distinctive greens and reds of our aurora. However, Mercury lacks a substantial atmosphere. Mio instead observed the electrons being accelerated around Mercury’s magnetic field before falling directing to the planet surface in high energy collisions that generated a burst of X-rays.
The long journey to Mercury has also helped to knit the Japan and European teams together.
“We often hear both in Japan and internationally, that BepiColombo is a team with a family-like atmosphere,” says Seki. “This really speaks to the relationship that has been built between us over a long period of time!”
Back in the Mio control room for the sixth Mercury swing-by, the temperature was rising as BepiColombo headed for Mercury’s north pole. It takes about ten minutes for the data to reach Earth, so BepiColombo was just emerging from Mercury’s shadow. By sharing the spacecraft telemetry directly to the live stream in the mission control room, Murakami hopes that all the viewers would also feel that they were part of that family.
“We really want to make our viewers feel close, and become familiar with Mio and our operations,” explains Murakami, who answers questions from the live stream chat throughout the operation. “We’d like everyone to feel that Mio was their own mission.”
The next time that BepiColombo approaches Mercury, it will be time for Mio and MPO to enter orbit. It will be the first time that two space probes of this size have been placed into planetary orbit at the same time. The team face a number of critical operations as Mio separates from MPO, and extends the mast and wire antennas that will enable to detailed measurements of Mercury’s magnetic field. Preparations are being conducted using a spacecraft simulator to verify all procedures, prepare any additional tools or software, and train the team.
“The operations that follow right after separation are the most critical and challenging,” says Seki. “A few of Mio’s subsystems will be activated for the first time since launch so this is very nervous and exciting too.”
On the live stream for the sixth Mercury swing-by, the temperature values from Mio had stopped climbing and were starting to return to normal values. BepiColombo has passed Mercury for the final time. When we see the planet again in November next year, we will all be ready.
BepiColombo Mio website
ESA BepiColombo website (external site)
BepiColombo on Cosmos:
[Aug. 2021] BepiColombo grazes Venus in a second swing-by, at an altitude of just 550 km
[Oct. 2020] BepiColombo is set for science, as the spacecraft performs a Venus swing-by
[April 2020] Ready for Swing-by! BepiColombo will pass close to the Earth on April 10
[Sept. 2018] Save the date! Q&A ahead of the launch of BepiColombo to Mercury in October
[Dec. 2017] Facing the furnace: BepiColombo is getting ready to depart for Mercury
BepiColombo news releases at ISAS
Web releases on results from BepiColombo at ISAS:
[Sept. 2024] Discovery of local chorus waves that unlock the source of Mercury’s electron acceleration and aurora!
[Oct. 2024] A glimpse of Mercury’s magnetosphere! ~ Mercury’s plasma environment revealed during BepiColombo’s third flyby ~
[July 2023] Mio witnesses electrons precipitating towards the planet Mercury during the BepiColombo flyby
[Oct. 2021] The BepiColombo spacecraft completes the Venus swing-bys and finally approaches Mercury
[Nov. 2020] Results from Mercury Magnetospheric Orbiter “Mio” captured during Venus flyby
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