Why Stopping at Mercury Takes More Energy Than Flying to Pluto

Shortly after 1 p.m. BST on Sept. 3, the flight control team at ESA's European Space Operations Centre in Darmstadt was not watching a spacecraft. It was listening for a signal. More than 200 million kilometers away, a separation sequence had either worked or it had not, and the first hint would arrive as a Doppler shift in the spacecraft's radio signal. The same effect, as ESA explained it at the time, makes an ambulance siren change pitch as it moves toward or away from an observer. At 13:41 BST, the preliminary Doppler signal appeared, consistent with the planned separation sequence.

Just over an hour later, the separation was confirmed. Two of ESA's Estrack deep-space antennas, at Cebreros in Spain and Malargüe in Argentina, acquired the signal and confirmed that BepiColombo's Mercury Transfer Module had separated from the spacecraft stack. ESA reported that all systems were nominal and that the Mercury Planetary Orbiter's solar panels were charging the spacecraft's batteries.

The event sounds procedural, but it was anything but. Executing a separation at that distance, ESA noted, “demands precise planning, navigation and commanding, with no opportunity for real-time intervention.” The commands had to be right before they were sent because nothing could be corrected once the sequence was underway. And the module now left behind is the piece that did nearly all of the work.

Its ion thrusters were never there to make BepiColombo go faster. They were there to slow it down. Mercury is the hardest planet in the Solar System to settle into orbit around, and the reason is the Sun. A spacecraft approaching Mercury must shed a tremendous amount of orbital energy to match the planet's motion around the Sun, and ESA's mission pages put the cost plainly: the Sun's enormous gravity "makes it difficult to place a spacecraft into a stable orbit around Mercury – even more energy is needed than sending a mission to Pluto." Pluto sits at the far edge of the planetary system; Mercury is next door. Reaching the first is mostly a question of going. Reaching the second is a question of stopping.

So the mission spent nearly eight years shedding speed rather than gaining it. Since launch, it has made nine flybys, one of Earth, two of Venus and six of Mercury, using each gravitational encounter to reshape its trajectory and reduce the energy it needed to shed before reaching Mercury. The ion engines ran for long stretches in between. Electric propulsion is what made the journey possible at all. A conventional chemical rocket, ESA has said, would have required far too much fuel to carry two orbiters of this mass to Mercury, and by the end of the cruise the thrusters are expected to have accumulated a record total impulse.

Then came a major problem. BepiColombo was originally scheduled to enter Mercury orbit in December 2025. In May 2024, ESA reported a fault in the transfer module's power system that prevented the ion engines from operating at full power, and the mission settled on running them at reduced output. JAXA's Institute of Space and Astronautical Science set out the consequences that September: at the lower thrust, the original insertion date could no longer be reached. A new trajectory was announced on Sept. 2, 2024, and began with the fourth Mercury flyby two days later. The arrival moved to November 2026, about a year later than originally planned. The institute was explicit that neither orbiter would lose any of its planned science because of the delay.

The next date is Nov. 21, when the two orbiters, still joined, are captured into orbit around Mercury together. NASA's MESSENGER was the first spacecraft to enter orbit around the planet, making BepiColombo only the second mission ever to enter orbit around the planet, and the first mission to orbit Mercury with two spacecraft at the same time.

On Dec. 9-10, MPO releases Mio into its final elliptical polar orbit, which ranges from 590 to 11,640 kilometers above Mercury's surface. MPO then continues through its own arrival sequence, releasing Mio's sunshield, known as MOSIF, on Dec. 16 before descending toward its final polar orbit of 480 by 1,500 kilometers. Sending two is the whole point. Mio, JAXA's magnetospheric orbiter, will sample the vast region around Mercury where Mercury's magnetic field meets the solar wind, while MPO, the surface-focused half of the mission, will work much closer to the planet. MPO is scheduled to reach its final orbit on March 10, and science operations for both orbiters begin April 6.

Until December, MPO and Mio remain joined as part of the same composite spacecraft, with MPO supplying power for both orbiters. The braking is not finished: the critical Mercury orbit insertion maneuver that turns nearly eight years of approach into an orbit is still ahead, in November.