BepiColombo launch — to Mercury
Launched on 20 October 2018, the European–Japanese BepiColombo has spent years slowing down past planets and is due to enter orbit around Mercury in late 2026.
Available views: Orbit ✓ · Sky — (not visible in the sky)
The story
On 20 October 2018 BepiColombo, built jointly by the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA), left for Mercury on an Ariane 5 rocket. The spacecraft is really three parts stacked together: Europe’s Mercury Planetary Orbiter (MPO), Japan’s Mercury Magnetospheric Orbiter, called Mio, and an electric-propulsion module that carries them both. It is named after the Italian scientist Giuseppe “Bepi” Colombo, who in the 1970s suggested the orbit that let Mariner 10 pass Mercury several times.
Going to Mercury means travelling downhill toward the Sun. The closer a spacecraft gets to the Sun, the more the Sun’s gravity speeds it up, and if it arrives too fast, Mercury cannot capture it. So BepiColombo flew past planets nine times to slow down: once past Earth, twice past Venus and six times past Mercury. In terms of fuel, this trip is harder than going all the way to Jupiter.
In April 2024 a power problem in the electric-propulsion module weakened the engines. The team redesigned the remaining route, and arrival slipped by about eleven months. The spacecraft is now due to enter orbit around Mercury in late 2026. Once there, the two orbiters will separate: MPO will study the surface and interior from a low orbit, while Mio will study Mercury’s magnetic surroundings from a long, stretched orbit.
BepiColombo will be the first spacecraft to orbit Mercury since NASA’s MESSENGER, which worked there from 2011 to 2015. Having two orbiters watching the planet from different heights at the same time is a key feature of the mission. The orbit view replays how its path tightened toward the Sun each time it passed a planet.
Mission goals
Mercury is the closest planet to the Sun, yet one of the least explored. Only Mariner 10 and MESSENGER have visited it. MESSENGER flew closest over the northern half, so the southern half is known in less detail. Mercury’s iron core fills about 85 percent of the planet’s radius, an unusually large share, and why it is so big is still an open question.
Measuring with two orbiters at once is the heart of the mission. When the solar wind strikes Mercury’s magnetic field, MPO near the surface and Mio farther out can record the same moment together, revealing the whole picture in a way a single spacecraft cannot. It is the same reason several satellites are flown together to study the magnetic field around Earth.
Mercury was also the first planet to support Einstein’s general theory of relativity. Its orbit slowly turns in a way that Newton’s theory could not fully explain, and relativity accounted for the difference. BepiColombo plans to measure the distance to Mercury very precisely, which will allow a fresh test of the theory.
Mission chronology and background
During its six passes of Mercury, BepiColombo photographed the surface up close with its monitoring cameras. On the fourth flyby, on 4 September 2024, it got its first clear view of Mercury’s south polar region. Most of the main science instruments cannot be used while the three parts are stacked together, so full science work begins only after orbit insertion.
The electric-propulsion module has four ion engines and can run up to two at once for a maximum of 290 millinewtons of thrust. That is a very weak push, about the weight of a few coins resting on a hand, but running for thousands of hours it can change the orbit a great deal. The module will be dropped just before Mercury orbit insertion, and the two orbiters will then use chemical rockets to settle into their own orbits.
Mercury’s surface heats up to 430 °C by day, and sunlight there is far stronger than near Earth. So the orbiters’ ability to handle heat is a key limit on how long the mission can last. Mio shares out the heat by spinning, so that the side facing the Sun keeps changing. ESA plans MPO’s main mission to run until April 2028, with an extension to the end of 2029.
Other milestones of this mission
- BepiColombo enters Mercury orbit(planned)
Where to see it — sky and simulator
BepiColombo is near Mercury and cannot be seen from the ground. Select it in the orbit view to see its current position and its track over the last 120 days, winding along Mercury’s orbit. Speed up the playback to follow its orbit tightening from the 2018 launch, through nine planetary flybys, to the planned orbit insertion.
Mercury never appears more than 28 degrees from the Sun in the sky. That is why it can only be seen briefly, low in the east just before sunrise or low in the west just after sunset. Select Mercury in the sky view to see when and in which direction it is visible from your location, and its phase, which changes like the Moon’s.
See this for yourself — replay the moment in the orbit view
FAQ
Why does the trip take so long?
The speed gained by falling toward the Sun has to be reduced with nine planetary flybys before the spacecraft can enter orbit around Mercury.
When does it arrive?
It is due to enter orbit around Mercury in late 2026, about eleven months later than first planned because of a power problem in 2024.
Are there two spacecraft?
Yes. Europe’s MPO studies the surface and interior, and Japan’s Mio studies the magnetic field. They fly as one stack and separate on arrival.
Who is it named after?
Giuseppe “Bepi” Colombo, the Italian scientist who suggested the orbit that let Mariner 10 pass Mercury several times.
Related
- BepiColombo arrives at Mercury
- Parker Solar Probe launch
- Transit of Mercury (2032)
- BepiColombo — object index
- Mercury — object index
Times and figures are approximate — verify with official sources when planning travel. Simulation uses J2000 mean elements.