BepiColombo drops propulsion module near Mercury

The spacecraft’s most dramatic cut happened far from Earth: 39 million miles (63 million kilometers) from the Sun, BepiColombo released the propulsion module that had carried it through eight years of interplanetary travel. Four springs pushed the two sections apart, and early telemetry showed the Mercury Planetary Orbiter’s solar arrays generating power.

That discarded section, the Mercury Transfer Module, supplied propulsion and power during the cruise. Its route used plasma propulsion and nine planetary flybys of Earth, Venus and Mercury to reshape its path and reach the speed needed for the planet’s gravity to capture it. The spacecraft has covered more than 6 billion miles (10 billion kilometers) since leaving Earth.

The mission is a three-part stack. Europe built the Mercury Planetary Orbiter, Japan built the Mercury Magnetospheric Orbiter, nicknamed Mio, and the transfer module supported both during the journey. The two science orbiters are due to reach their initial orbit around Mercury in November, then separate from one another in December.

The separation was also a high-risk systems test. Near Mercury, the remaining spacecraft had to take over power generation, propulsion, pointing and thermal control while facing intense solar radiation and extreme temperatures. Several instruments, including the best cameras, had been hidden behind the propulsion module and can now begin seeing Mercury directly. The mission’s ion thrusters also lost partial power in 2024, extending the cruise by a year.

So what changes, concretely? BepiColombo is now on track to maneuver into Mercury orbit on November 21, bringing two dedicated orbiters into the planet’s vicinity for scientific observations. Early signals are encouraging, but orbit insertion and the later separation of the orbiters are still ahead.