After Eight Years and 10 Billion Kilometers, BepiColombo Cut Loose Its Engine and Started Falling Toward Mercury

BepiColombo threw away its engine on Saturday. Eight years and more than 10 billion kilometers after launch, the joint European-Japanese mission to Mercury separated its two science orbiters from the Mercury Transfer Module that had been hauling them across the inner solar system, beginning the arrival sequence the European Space Agency describes as the most operationally challenging ever attempted at a planet.

Nobody on Earth could help. The one-way signal delay to the spacecraft is about 30 minutes, which means the separation had to execute autonomously on a schedule loaded in advance. Confirmation that it had worked reached the control room in Darmstadt, Germany nearly two hours after the event itself.

Getting to Mercury is counterintuitively hard. Falling toward the sun means gaining speed, and a spacecraft that simply aims at Mercury arrives far too fast to be captured by a body whose gravity is only slightly stronger than the Moon's. BepiColombo's answer was to spend eight years bleeding off velocity with nine planetary flybys — Earth, Venus and Mercury itself — combined with long stretches of solar-electric thrusting from the transfer module now discarded.

The remaining milestones are tight. Both probes are scheduled to enter Mercury orbit on November 21. On December 9 and 10 they separate from one another into their individual working orbits: ESA's Mercury Planetary Orbiter, which will map the surface and interior from close in, and JAXA's Mio, which will sit farther out and study the planet's magnetic field and the particles trapped in it. Science observations are set to begin in April 2027.

Between them the two spacecraft carry 16 instruments and represent about €1.7 billion of investment. The questions they are aimed at have been open for decades. Mercury has a magnetic field, which a small planet that has cooled for four and a half billion years arguably should not. Its iron core takes up an implausible fraction of its volume. Its crust, recent analyses suggest, may have started as floating graphite. Only two missions have been there before: Mariner 10, which flew past three times in 1974 and 1975, and MESSENGER, which orbited from 2011 until it was deliberately crashed in 2015.

The mission has already survived one serious scare. In 2024, a fault in the transfer module's power system cut the available thrust, forcing controllers to redesign the trajectory and push the arrival date back by nearly a year. That module has now done its job and been let go. What is left is two orbiters, a fixed set of burns, and eleven weeks of falling.