How BepiColombo Reached Mercury’s Arrival Phase

Reaching Mercury is less straightforward than simply flying toward the Sun. A spacecraft leaving Earth is already moving rapidly around the Sun, so it must shed a great deal of solar-orbital energy before Mercury can capture it. BepiColombo solved that problem with a long, carefully sequenced combination of solar-electric propulsion and gravity assists—and on September 3, 2026, it completed a key transition into its arrival phase. 3311

The route to Mercury: slow down, don’t speed up

BepiColombo launched on October 20, 2018 as a joint mission of the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA). Its cruise configuration paired two science orbiters with the Mercury Transfer Module (MTM), which supplied power and electric propulsion for the journey. 157

The mission did not take a direct path. Instead, it used nine planetary flybys: one of Earth, two of Venus, and six of Mercury. Each encounter adjusted the spacecraft’s solar orbit and helped reduce its velocity relative to Mercury without requiring an impractically large propellant load. 1533

Between flybys, the MTM’s solar-electric propulsion system supplied low but sustained thrust along arcs around the Sun. ESA says this combination of electric propulsion and flybys was designed to bring BepiColombo to Mercury at a sufficiently low relative velocity for orbit capture. 45

A propulsion problem changed the timeline

The cruise was complicated by an electrical-power issue that limited how much power the MTM could provide to its thrusters. ESA reported that the spacecraft could not operate its thrusters at full power; teams restored thrust to about 90% of its previous level, while available power remained reduced. 44

Mission planners revised the trajectory and operations around that lower performance. The mission remained viable, but Mercury orbit insertion moved from the earlier 2025 plan to November 2026. 111

That adjustment illustrates why the long approach mattered: BepiColombo’s route was not merely transit time. It was an extended campaign to reshape its orbit while preserving the conditions needed for a safe arrival.

Why releasing the transfer module started the arrival phase

On September 3, 2026, BepiColombo successfully separated from the MTM. ESA defines this event as the beginning of the Mercury arrival phase. Signal acquisition by ESA deep-space antennas confirmed the separation. 11

The MTM had completed its purpose: it powered and propelled the stack during the interplanetary cruise. The remaining composite spacecraft consists of ESA’s Mercury Planetary Orbiter (MPO), JAXA’s Mio magnetospheric orbiter, and their protective interface structure. They will remain together initially as they begin the demanding sequence of maneuvers required for capture and deployment. 738

What happens between now and science operations

The remaining schedule is a sequence of planned milestones rather than a single arrival moment:

• November 21, 2026: The MPO–Mio composite is scheduled to enter orbit around Mercury. 1115
• November 2026 to March 2027: A sequence of 16 burn maneuvers is planned to establish the spacecraft in their intended trajectories and orbits. 38
• December 9–10, 2026: MPO and Mio are scheduled to separate. 1115
• April 2027: Routine science operations are scheduled to begin. 1115

The dates remain planned mission events, so the final outcome depends on the successful execution of the arrival maneuvers and spacecraft deployments.

What the two orbiters will study

BepiColombo’s scientific design relies on two complementary spacecraft observing Mercury and its environment at the same time.

MPO, developed by ESA, is focused on the planet itself: its surface, chemical composition, gravity field, magnetic field, and internal structure. Mio, developed by JAXA, is designed to study Mercury’s magnetic field, magnetosphere, exosphere, and surrounding plasma environment. 2021

Together, the orbiters are intended to provide a more complete view of how Mercury’s surface and interior relate to its unusually active space environment. After a journey of roughly eight years and billions of kilometres, the MTM separation marks the end of the long cruise—and the start of the mission’s highest-stakes operational phase. 115