BepiColombo drops its transfer module and starts the Mercury arrival sequence

The European Space Agency confirmed on 3 September that BepiColombo's Mercury Transfer Module had separated from the rest of the stack. The signal arrived at the European Space Operations Centre in Darmstadt at 15:49 CEST. Two deep-space antennas, in Spain and Argentina, locked the telemetry. The Mercury Planetary Orbiter's solar panels were already charging the batteries.

The cut took place more than 200 million kilometres from Earth. No one on the ground could send a last-second correction. The transfer module had done the work it was built for: power and ion thrust across an eight-year cruise of 9.9 billion kilometres and nine planetary flybys. Partial power loss on those thrusters in 2024 forced a one-year slip. The module is now discarded mass.

What remains is a joined pair. ESA's Mercury Planetary Orbiter still carries JAXA's Mercury Magnetospheric Orbiter, Mio. They are more than three million kilometres from Mercury. Arrival is not one burn. It is a six-month sequence. Using MPO's chemical propulsion, the stack is to refine its path, enter orbit on 21 November, release Mio around 9 or 10 December, and settle MPO into its final science orbit in March 2027. If that holds, BepiColombo becomes the first mission to park two spacecraft around Mercury.

Geraint Jones, ESA's lead project scientist, said the cruise and the nine flybys had already produced usable data, and that the point of this step is to switch on the full instrument sets on both orbiters. Mariner 10 flew past Mercury three times in 1974 and 1975. Messenger orbited from 2011 to 2015. Neither left two specialised craft in complementary polar orbits, one for the surface and one for the magnetosphere.

Mercury is a hard target because the Sun's gravity well is deep and the planet is fast. Chemical insertion from a direct Earth departure would have demanded a heavier stack than Europe and Japan were willing to fly. The transfer module's ion engines and the long chain of Earth, Venus and Mercury flybys were the cheaper way to bleed speed. Once that job ends, the ion stage is dead weight and must come off before the chemical stage can do precise work near the planet.

The next public checkpoints are narrow. November's capture burn has to put both orbiters on a path Mio can accept. December's separation has to happen without either craft colliding or drifting into a thermal attitude it cannot hold. March 2027 is when MPO's mapping orbit is supposed to begin. Each date can slip if a burn is off by metres per second.

For a general reader the fact that matters on 8 September is simpler. After eight years the propulsion barnacle is gone, the two science craft are flying on their own power budget, and Mercury is no longer a flyby target. It is a capture target with a November date on the wall at ESOC.