Eight Years to Mercury, and One Chance to Stop

SCIENCE & TECHNOLOGY · SEPTEMBER 12, 2026

Getting to Mercury is harder than getting to Pluto. That is not a figure of speech; it is a matter of energy. Mercury sits so deep in the Sun's gravity that a spacecraft falling toward it from Earth picks up enormous speed, and to be captured into orbit rather than sling past, it has to shed almost all of that speed again. Only two missions have ever done it — NASA's Mariner 10 in the 1970s, which only flew past, and MESSENGER, which orbited from 2011 to 2015. The third is a joint European and Japanese spacecraft called BepiColombo, launched in October 2018, and the last leg of its journey began on 3 September.

What Happened on 3 September?

The spacecraft that left Earth was really three stacked together: a Mercury Transfer Module — the engine and the power plant, with two solar wings fourteen metres long and four ion thrusters — and riding on top of it, the two orbiters that are the point of the mission. That afternoon, more than two hundred million kilometres away, the transfer module separated and fell away. The European Space Agency's control room in Darmstadt watched it happen on a Doppler trace, then got the confirmation from deep-space antennas in Spain and Argentina: all systems nominal, solar panels charging. Nothing about the separation could be corrected from the ground. At that distance the signal takes over ten minutes each way; by the time you see a problem it is twenty minutes old.

That is the shape of everything from here to orbit. The ion engines are gone. The remaining spacecraft has small chemical thrusters of its own, enough to be captured by Mercury and to trim its orbit, and a sequence that was written, rehearsed and uploaded in advance. On 21 November it runs that sequence, alone, and either it is in orbit around Mercury or it is not.

Why Did It Take Eight Years?

Because you cannot brake with rockets alone. A direct flight to Mercury would arrive so fast that no practical amount of fuel could stop it, so BepiColombo instead spent years looping through the inner solar system, using the gravity of Earth, Venus and Mercury itself to slow down a little at each pass while the ion engines — gentle, efficient, running for months at a time — did the rest. The whole arrival is designed to be as gentle as possible: the spacecraft creeps up on Mercury along a path where the planet's gravity can catch it with only a modest push from the chemical thrusters, and then a series of further burns pulls the orbit down to the working altitude.

It should have arrived last December. In April 2024 the transfer module developed a fault — unexpected electric currents between its solar arrays and the unit that conditions and distributes their power, which starved the four ion thrusters. Engineers recovered about ninety percent of the thrust by May, but not the last ten, and could not fix the cause. Ten percent less thrust over the remaining cruise meant the planned trajectory no longer closed. So they redesigned it: the fourth Mercury flyby was flown closer than planned to save fuel, the sixth flyby was retargeted to put the spacecraft on a new path, and the arrival slipped eleven months, from December 2025 to November 2026. The lesson I take from that is not that something broke — something always breaks — but that the mission was built with enough margin, and enough flybys still ahead of it, that a ten-percent loss of engine could be paid for with time instead of failure.

What Are the Two Orbiters For?

The European one, the Mercury Planetary Orbiter, goes low and looks down: cameras, spectrometers, a laser altimeter, instruments to map the surface and its composition. The Japanese one, Mio, goes high and looks around: it studies the planet's magnetic field and the thin envelope of particles around it. Between them they are meant to answer questions MESSENGER opened and could not close.

Mercury is a strange object. It is far too dense for its size — its iron core takes up most of its radius, as if it were once a larger planet that lost its outer layers. It has a magnetic field, which a body that small and cold should not, and the field is lopsided, offset toward the north. There is water ice in the permanently shadowed craters at its poles, on the planet closest to the Sun. Its surface has "hollows," bright shallow pits that seem to be forming now, as some volatile material sublimates away. Every one of those is a question about how the inner solar system was assembled, and Mercury is the only place to ask it.

What Did the Last Flyby Find?

The science has already started, because a flyby is a measurement. A paper published on 8 September used data from the September 2024 pass, at about 165 kilometres above the surface, and reported that energetic particles from the Sun punch straight through Mercury's weak magnetosphere and reach the ground. Earth's field deflects most of that; Mercury's is too feeble and too close to the Sun, so its surface is being sandblasted by the solar wind in a way ours never is. That is probably part of why the surface chemistry is so odd, and it is something Mio will measure continuously once it is in its high orbit rather than in the few hours a flyby allows.

What Would I Watch For?

21 November. There is no drama in the sense of a live decision — the decision was made months ago and uploaded — but there is real drama in the sense that eight years and two space agencies are riding on a sequence nobody can touch once it starts. ESA will stream it from Darmstadt. If it works, the next date is 9–10 December when the two orbiters part, and then April, when the instruments come on for good. If it does not work, there is no second try; the fuel is sized for one arrival.

Where I Could Be Wrong

I have simplified the capture. It is not a single burn but a designed sequence — a gentle gravitational capture followed by several thruster firings over the following weeks to bring the orbit down — and the phrase "one chance" is about the capture itself, not about every step after it. The "over ten minutes" for the signal delay is approximate; the Earth–Mercury distance swings between about 77 and 222 million kilometres depending on where the planets are, so the light time on the day will be whatever the geometry gives. And I am relying on ESA's own account of the 2024 fault and the recovery; an independent post-mortem, if one is ever published, may describe it differently.

Sources

1. European Space Agency. BepiColombo begins Mercury arrival with MTM separation success. 3 September 2026. esa.int
2. European Space Agency. Latest updates: BepiColombo's arrival at Mercury — the live log of the separation. esa.int
3. SpaceNews. ESA delays BepiColombo orbital insertion because of thruster problem. September 2024. spacenews.com
4. European Spaceflight. Thruster issues delay BepiColombo's Mercury arrival by 11 months. September 2024. europeanspaceflight.com
5. Starlust. Space weekly recap, September 5–11, 2026 — the 8 September solar-particle result from the 2024 flyby. starlust.org