Mercury is a real oddball – and still barely researched. It is smaller, denser and older than any other planet in the solar system and rotates unusually quickly. Mercury also has a surprisingly large core and appears to be shrinking to this day. But what lies behind these peculiarities is so far only partially known
The problem: The innermost planet is difficult for space probes to reach because of its proximity to the sun. The enormous gravitational pull of the sun ensures high speeds during approach. In order to enter Mercury’s orbit, the probes must brake sharply and fly numerous loops on approach. This is one of the reasons why only two space probes have ever visited Mercury: Mariner 10 in the 1970s and MESSENGER from 2011 to 2015.
BepiColombo: In a double pack to the innermost planet
Now a third Mercury mission is on its way to its destination: BepiColombo. The European-Japanese Mercury mission, launched on October 20, 2018, is scheduled to collect new data about the innermost and smallest planet in the solar system from the end of 2026. BepiColombo is the first Mercury mission in which two space probes fly together to the planet and explore it. This allows them to simultaneously carry out complementary measurements of the magnetic field and the interactions of planetary processes between Mercury and the nearby Sun.
To date, the two Mercury probes – ESA’s Mercury Planetary Orbiter (MPO) and JAXA’s Mercury Magnetospheric Orbiter (Mio) – have traveled together to their destination using a transfer module. With the help of this module, BepiColombo controlled its multiple flybys of Earth, Venus and Mercury, which were necessary for braking. The initial propulsion was provided by an ion drive, which was switched off as planned on June 15, 2026.
This afternoon: Separation from the transfer module
This afternoon at 2:00 p.m. there will be a crucial step for the Mercury mission BepiColombo: The two Mercury orbiter probes will separate from their transfer module. “The journey to Mercury was difficult. Arriving there is even more difficult. This is the first important step in one of the most technically demanding arrival sequences ever undertaken by ESA,” explains the European Space Agency. ESA transfers this separation in the live stream from 1:45 p.m.
Separation of the transfer module (in the foreground) from the coupled orbiter probes MMO and MPO. — © NASA/JPL; ESA/ATG media lab
From today, September 3rd, the two orbiter probes will continue to fly to Mercury alone – or rather, fall. The sun’s gravitational pull ensures that the space probes fly towards Mercury without any further propulsion. If braking maneuvers and course corrections become necessary, the engine of ESA’s Mercury Planetary Orbiter takes over.
What’s next for BepiColombo
But today’s separation is just the first step in the complex and risky approach to Mercury. It continues on November 21, 2026 with another milestone: the coupled Mercury probes slow down in order to enter an orbit around Mercury. This first deceleration maneuver puts the two probes into a highly elliptical orbit that is between 670 and 178,000 kilometers from Mercury’s surface.
Over the next three weeks or so, the Mercury probes will carry out another five braking maneuvers. The final separation will finally follow on December 9, 2026: the Japanese Mercury Magnetospheric Orbiter will decouple from its European partner probe. Both Mercury probes then maneuver independently into their target orbits around the planet. From April 2027, the probes will deliver scientific data from the innermost planet in our solar system.
Source: European Space Agency (ESA)