After nearly eight years and billions of kilometers of travel, the European-Japanese spacecraft BepiColombo has entered the final and most hazardous phase of its journey to Mercury. Mission controllers confirmed on September 3 that the spacecraft’s propulsion module separated from its two science orbiters as planned, kicking off a monthslong approach that engineers describe as the most demanding part of the entire mission.
Eight Years, One Final Approach
BepiColombo launched in October 2018 on a long, looping trajectory designed to slow the spacecraft down enough to be captured by Mercury’s weak gravity, using a series of flybys of Earth, Venus and Mercury itself along the way. That indirect path was necessary because a spacecraft heading toward the sun picks up enormous speed from the sun’s gravity, and slowing down enough to settle into orbit around a small, close-in planet like Mercury takes far more fuel than a direct route would allow.
Over its nearly eight years in transit, the spacecraft completed one flyby of Earth, two of Venus and a series of flybys of Mercury itself, each pass using the planet’s gravity to bleed off a portion of the spacecraft’s speed. That gradual approach, spread across almost a decade, stands in sharp contrast to missions bound for outer planets, which often benefit from gravity assists that add speed rather than remove it, underscoring how unusual the physics of reaching Mercury actually are.
The mission has now shed the Mercury Transfer Module, the propulsion unit that carried the spacecraft through its multiple gravity-assist flybys. Confirmation of that separation came through signals picked up by two of the European Space Agency’s deep-space tracking antennas, located in Spain and Argentina, verifying that the maneuver executed on schedule.
Why the Arrival Phase Is Considered the Riskiest
Engineers involved in the mission have described the arrival sequence as tricky, a characterization that reflects how much can go wrong in the coming months. Mercury’s proximity to the sun means any spacecraft approaching it must contend with intense heat, strong solar radiation and a gravitational environment that leaves little room for error in timing or trajectory.
Unlike the flybys BepiColombo has already completed, the arrival phase culminates in a maneuver that must place the spacecraft precisely into Mercury’s orbit. A single significant miscalculation during that insertion could send the mission past the planet entirely or on a trajectory that cannot be corrected, after nearly a decade of travel and years of preparation before launch.
What Happens Between Now and Orbit Insertion
The two science orbiters, the Mercury Planetary Orbiter and the Mercury Magnetospheric Orbiter, are scheduled to enter orbit around Mercury on November 21, 2026, according to mission timeline reporting. That insertion will not immediately mark the end of the arrival process. The two orbiters, currently traveling together, are set to separate from each other on December 9 and 10, positioning each in the distinct orbit required for its own set of instruments.
Even after that separation, the mission is not expected to begin its primary science campaign right away. Current planning calls for science operations to start in April 2027, giving controllers several additional months to verify each orbiter’s systems and settle both spacecraft into their final working orbits before instruments begin returning research-grade data.
A Joint European-Japanese Mission
BepiColombo is a collaboration between the European Space Agency and the Japan Aerospace Exploration Agency, reflected in its two separate science orbiters built and operated by the respective agencies. Coverage of the arrival milestone notes the mission’s cost has run into the billions of dollars across both agencies, a scale that underscores how much rides on a successful orbit insertion after eight years of travel.
The partnership divides responsibilities between the two orbiters’ complementary instrument packages, one focused primarily on mapping the planet’s surface and composition, the other on studying Mercury’s unusually strong magnetic field for a planet of its size. Both are needed to fulfill the mission’s full science goals, which is part of why the December separation maneuver carries its own risk on top of the November orbit insertion.
What the Mission Aims to Study at Mercury
Mercury remains one of the least explored planets in the solar system despite being the closest to Earth’s sun, largely because it is so difficult to reach and orbit. Only two previous NASA missions, Mariner 10 and MESSENGER, have visited the planet, and BepiColombo is designed to build substantially on what those missions found, with more advanced instruments covering the planet’s surface composition, internal structure and magnetic environment.
Scientists are particularly interested in questions those earlier missions left open, including the exact makeup of Mercury’s unusually large iron core relative to its size, the origin of ice deposits detected in permanently shadowed craters near its poles, and the mechanics behind its magnetic field. Answering those questions depends entirely on BepiColombo surviving the arrival sequence that began this month and reaching a stable science orbit next spring.
The stakes of that dependence are part of why engineers have been so direct about calling the coming months the riskiest stretch of the entire mission. A launch failure or a lost flyby earlier in the journey could in some cases have been compensated for with a revised trajectory; a failed orbit insertion in November would leave two fully built, instrument-laden orbiters with no way to recover the mission’s central goal after eight years of travel and roughly a decade of planning that preceded launch.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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