BepiColombo Begins Its Final Approach to Mercury

BepiColombo’s planned September 3, 2026 separation of the Mercury Transfer Module (MTM) is a handoff between two very different parts of the mission. The MTM powered the spacecraft across the inner Solar System; after separation, ESA’s Mercury Planetary Orbiter (MPO) and JAXA’s Mio orbiter must complete the difficult sequence of manoeuvres that will place them around Mercury. 56

The event is therefore a major arrival milestone, but it is not the moment BepiColombo enters Mercury orbit. Orbit insertion is planned for November 21, after which the spacecraft will continue through several separations and orbit-raising and lowering manoeuvres before routine science operations begin in April 2027. 57

Why reaching Mercury takes eight years

BepiColombo launched on October 20, 2018. Rather than flying directly to Mercury, the mission combined solar-electric propulsion with a carefully designed series of planetary flybys: one of Earth, two of Venus and six of Mercury. 1210

The unusual route is dictated by the Sun. A spacecraft falling inward toward the Sun gains speed, so simply aiming for Mercury is not enough. BepiColombo had to continually reduce its heliocentric energy and match Mercury’s motion closely enough to be captured. Gravity assists provided much of that braking and trajectory shaping, while the MTM’s low-thrust electric propulsion supplied gentle acceleration over long periods. 41013

This approach is slow, but it reduces the amount of propellant that a conventional high-thrust route would require. ESA describes the cruise as involving thousands of hours of challenging solar-electric-propulsion operations. 15

The propulsion problem—and the revised route

In April 2024, a power-related issue prevented BepiColombo’s electric thrusters from operating at full power. Mission teams adapted the trajectory and operations rather than abandoning the mission. 11

The solar-electric propulsion system completed its final thrust arc at 15:24 CEST on June 15, 2026. That shutdown ended the long cruise phase and prepared the spacecraft for the arrival campaign. 6

After MTM separation, the MPO’s chemical propulsion system will support the arrival manoeuvres. The remaining spacecraft must still execute a carefully timed series of burns and separations in the harsh environment near Mercury. 5

What happens on September 3

During the planned separation, the MTM will detach from the composite carrying MPO, Mio and the Mio sunshield. The transfer module has completed the job for which it was designed: providing solar power and electric thrust during the journey to Mercury. 5

The separation is operationally important because it leaves the mission’s science spacecraft in their arrival configuration. It also begins a sequence in which each component must be released or repositioned at the correct time. ESA planned to livestream the event from 13:45 CEST, giving the public a view of the first major step in one of the mission’s most challenging phases.

The livestream does not make the spacecraft arrive more safely or generate scientific data. Its significance is communicative: it brings attention to a decisive, largely irreversible engineering milestone after years of cruise operations.

BepiColombo’s arrival timeline

The planned sequence is:

• September 3, 2026: The Mercury Transfer Module separates from the MPO–Mio–sunshield composite. This marks the formal start of the Mercury-arrival phase. 5
• November 21, 2026: MPO and Mio, still connected, begin Mercury orbit insertion. ESA describes the campaign as involving 16 chemical-propulsion burns that continue into March 2027. 5
• December 9, 2026: Mio separates from MPO and begins moving toward its own highly elliptical orbit for observations of Mercury’s magnetic environment. 5
• December 2026: The Mio sunshield, known as MOSIF, is jettisoned after protecting Mio during the cruise and arrival configuration.
• March 2027: MPO is expected to reach its planned science orbit after the final orbit-shaping manoeuvres.
• April 2027: Coordinated routine science operations by the two orbiters are scheduled to begin. 7

Each milestone depends on the previous one. Separating the MTM too early, inserting into the wrong orbit or releasing Mio at the wrong point could compromise the spacecraft’s ability to reach its dedicated science orbits.

Why the new silica finding matters

Recent laboratory experiments and analyses of observational data estimate that silicon dioxide may account for about 37% of Mercury’s surface by mass—up to 25% less than earlier estimates suggested.

That result matters because silica abundance helps researchers infer how Mercury’s crust formed. A lower proportion can be consistent with volcanic rocks produced by a greater degree of partial melting in the mantle. The interpretation under discussion is that some of Mercury’s volcanic material may have formed from mantle melts that were deeper and hotter than previously assumed.

The finding is an interpretation, not a complete reconstruction of Mercury’s interior. Surface composition can vary from region to region, and measurements made from Earth have limits. The key question for BepiColombo is whether the low-silica estimate is representative of Mercury as a whole or reflects particular terrains and measurement uncertainties.

How BepiColombo can test the idea

MPO and Mio will examine different parts of that problem. MPO’s instruments are designed to study Mercury’s surface composition, geology, interior structure and topography. Those observations can connect elemental abundances with volcanic plains, crustal thickness, gravity variations and the planet’s geological history.

Mio will focus on Mercury’s magnetic field and surrounding space environment. Its measurements can provide context for the planet’s magnetosphere, plasma and exosphere while MPO investigates the surface and interior. 5

Together, the orbiters can strengthen or complicate the deeper-melting explanation. They may show that Mercury’s low silica content is widespread, identify regional chemical differences or reveal that some of the apparent deficit comes from the limitations of earlier observations. In each case, the mission will move the debate from indirect estimates toward direct, global measurements.

The bigger significance of the separation

The MTM separation is easy to mistake for a routine spacecraft breakup. It is better understood as the mission’s transition from transport mode to precision planetary operations.

For eight years, the priority was to deliver the combined spacecraft to Mercury using an exceptionally energy-efficient route. From September onward, the priority changes: the mission must navigate orbit insertion, separate its two science spacecraft, discard protective hardware and establish the specific orbits needed for long-term observations.

If that sequence succeeds, BepiColombo will provide a much more complete picture of Mercury’s surface, interior, magnetic field and geological past. The September 3 event is the first visible sign that the mission has reached the final—and most demanding—stage of that effort.