BepiColombo drops its cruise stage and aims at Mercury orbit

BepiColombo dropped its Mercury Transfer Module on 3 September and began the last two million miles of an eight-year trip to the innermost planet. Controllers at ESA's centre in Germany waited through a two-hour light-time delay before antennas in Spain and Argentina confirmed the split. Telemetry showed the Mercury Planetary Orbiter's solar wings charging batteries. Lead scientist Geraint Jones called it a brilliant moment.

The stack that left Earth in 2018 was three craft bolted together: ESA's transfer module with the ion engines, ESA's Mercury Planetary Orbiter, and JAXA's Mercury Magnetospheric Orbiter, nicknamed Mio. The engines had already done their work. A power shortfall in the solar-electric system in 2024 forced a one-year slip, pushing orbit insertion from December 2025 to 21 November 2026. Mio is due to separate on 9-10 December. The European orbiter then walks down to a 480 by 1,500 kilometre science orbit. Full science starts in April 2027.

The cruise covered more than 10 billion kilometres, with flybys of Earth, Venus and Mercury used as brakes. Mercury's gravity well is shallow and the Sun's pull is strong, so arriving is harder than leaving. The transfer module is now dead mass by design. Keeping it would have made insertion more expensive in fuel the orbiters do not have.

The European orbiter will map topography and interior structure and run a relativity experiment in a strong gravitational field. Mio will sit in a different orbit and measure the magnetosphere. Mercury has a magnetic field it should not have if it were a simple cooled stone, and polar craters that hold ice a few hundred kilometres from ground that sits near 400 degrees Celsius in sunlight. Those two facts are the reason the mission exists.

Only two earlier spacecraft have visited the planet up close: Mariner 10 in 1974-75, on a path designed with Giuseppe Colombo's gravity-assist arithmetic, and NASA's Messenger, which orbited from 2011 to 2015. BepiColombo is the first two-orbiter attack on the same target.

The remaining risks are insertion in November and the December split. Both are clockwork if the 3 September separation is as clean as the Doppler says. If it is not, April 2027 science slips again. For a mission already a year late, that is the only date that now matters.

Until 21 November the public evidence is a confirmed jettison, live solar arrays on the European orbiter, and a trajectory that still aims at Mercury orbit. That is enough to say the cruise is over. The science has not started.

Messenger mapped the surface. It could not sit in two orbits at once. The Japanese craft is the reason this mission can ask how a small planet keeps a magnetosphere. That question is why the 3 September split had to work.