But the propulsion module was no longer required, and BepiColombo needed to get rid of it before slipping into orbit in November. BepiColombo is unusual in that it is actually a stack of three spacecraft: the Mercury Transfer Module and the Mercury Planetary Orbiter, built by Europe, and the Mercury Magnetospheric Orbiter from Japan, nicknamed Mio.

The separation of the propulsion module Thursday leaves the European and Japanese orbiters still traveling together. The two spacecraft will release from one another in December, soon after they arrive in their initial orbit around Mercury. The orbiters each have their own instruments for scientific observations at Mercury, while the transfer module is dead mass.

A punishing environment

Nothing like the controlled disassembly of BepiColombo has ever been attempted in such a harsh environment as the region of space near Mercury. Separation of the Mercury Transfer Module occurred 39 million miles (63 million kilometers) from the Sun, less than half the distance of Earth, with extreme temperatures and intense solar radiation.

Ignacio Tanco, head of inner Solar System mission operations at ESA, told reporters the challenge is “equivalent to launching a new spacecraft” with “considerable risk” of something going wrong. Letting go of the transfer module required the rest of the spacecraft take over power generation, propulsion and pointing, and thermal control, a significant concern flying so close to the Sun.

“This is something that we will only see after separation if it has worked, and we will be using several new sensors and mechanisms as part of the attitude control system,” Tanco said.

Several of the mission’s science instruments, including its best cameras, were obscured by the propulsion module during the transit to Mercury. “It will be only upon release of the transfer module that these instruments will see first light,” Tanco said.