The BepiColombo mission cleared a major milestone this week in the final stretch of its eight-year interplanetary voyage to Mercury. The robotic science mission, with a price tag of nearly $2 billion, is led by the European Space Agency with contributions from Japan and the United States. Since its launch in 2018, BepiColombo has spiraled closer to the Sun using a combination of plasma propulsion and a series of flybys of Earth, Venus, and Mercury.

The maneuvers changed the spacecraft's velocity and steered it toward a final encounter with Mercury later this year. Next time it reaches Mercury, BepiColombo will be traveling at just the right speed for the planet's gravity to capture the spacecraft into orbit. 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. A monitoring camera onboard the Mercury Transfer Module captured this view of Mercury's cratered surface during a flyby on January 8, 2025.

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.