BepiColombo blew the bolts on its transfer module and committed itself to Mercury

Eight years after launch and nine gravity-assist flybys later, the spacecraft on its way to Mercury severed its last connection to the propulsion stage that carried it across the inner solar system, a step mission controllers had been planning toward since 2018. The separation, confirmed earlier this month, closes out the long cruise phase of the mission and opens a shorter, more delicate final approach that still has several major milestones left before the spacecraft settles into orbit around the solar system’s smallest planet.

Cutting loose after eight years and nine gravity assists

The mission, a joint effort between the European Space Agency and the Japanese Space Agency, launched in 2018 and has spent the years since using flybys of Earth, Venus and Mercury itself to bleed off speed rather than gain it, an approach necessary because falling toward the sun accelerates a spacecraft far faster than its engines alone could ever slow it back down. Over that period the mission completed nine such flybys: one of Earth, two of Venus, and six of Mercury.

The workhorse behind those maneuvers was the Mercury Transfer Module, an ion-drive propulsion stage that fired repeatedly between flybys to adjust the spacecraft’s speed and trajectory across billions of kilometers of travel. With orbital insertion now approaching, that module had done everything it was built to do, making its separation the next required step rather than an emergency measure.

Pyrotechnics, springs and an eleven-minute radio delay

On September 3, operators at the European Space Operations Center in Darmstadt, Germany, transmitted the command to separate, a signal that took eleven minutes to cross the roughly 200 million kilometers still separating Earth from the spacecraft. That delay meant the team on the ground had no way to watch the separation happen in real time; they could only send the command and wait for confirmation to come back.

The spacecraft carried out the separation using pyrotechnic charges to burn through the physical connections holding the transfer module in place, then relied on a set of springs to push the module away cleanly and avoid any risk of collision with the remaining stack, according to a detailed account of the maneuver. Mission controllers confirmed the split had worked by detecting a slight Doppler shift between the radio signals of the departing module and the spacecraft that remained, and within minutes the surviving section of the mission had deployed its own systems and begun generating power from its own solar arrays.

Two orbiters, one iron core to study

What remains after the separation is not a single spacecraft but two orbiters flying together for now: the European Space Agency’s Mercury Planetary Orbiter, nicknamed Bepi, and the Japanese Space Agency’s Mercury Magnetospheric Orbiter, known as Mio. The two will continue traveling as a combined unit through the next phase of the approach before eventually splitting apart to pursue separate science missions once they reach Mercury.

Bepi is tasked with mapping the planet’s surface at high resolution and studying its mineral composition, along with investigating the unusually large iron core that makes up a disproportionate share of Mercury’s total volume compared with other rocky planets. Mio, flying in a wider elliptical orbit once the two separate, will focus on Mercury’s magnetic field and how it interacts with the solar wind, along with the planet’s extremely thin, tenuous atmosphere, sometimes called an exosphere.

The road to orbit still runs through November and December

The September separation is only the first of several remaining milestones before the mission reaches full operational status. The combined spacecraft is scheduled to perform its orbital insertion maneuver at Mercury on November 21, locking itself into orbit around the planet after the long cruise phase. Roughly three weeks later, on December 9 and 10, the Bepi and Mio orbiters are due to separate from one another and begin settling into their own distinct orbital paths, a process detailed in the mission’s official arrival updates.

Science operations are not expected to begin in earnest until the following spring, once both orbiters are fully deployed and calibrated in their respective positions. In the meantime, the spacecraft still has to survive the scorching thermal environment near Mercury’s orbit and the unpredictable space weather thrown off by the nearby sun, conditions that have already caused technical difficulties earlier in the mission’s long journey, including periods when the transfer module’s power output dropped below what controllers had planned for.

Even with those earlier setbacks, the mission has continued to hit its scheduled milestones, and the September separation stands out because it removed the team’s ability to change course. Once the transfer module was gone, there was no propulsion system left capable of altering the combined orbiter’s path in any major way before the November insertion, making the coming months less about maneuvering and more about monitoring instruments and waiting out the remaining distance.

This article was created with the assistance of AI and reviewed by an editor.

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