A Spacecraft Split in Two Today, After an 8-Year Journey to the Solar System's Hardest Planet to Reach

Keypoints:

• BepiColombo's Mercury Transfer Module separated from its two science orbiters on September 3
• The mission launched in 2018 and has traveled nearly eight years to reach Mercury
• A 2024 power fault forced engineers to redesign the approach, delaying arrival by 11 months
• Full orbit insertion happens November 21, with the two orbiters separating from each other in December
• Science operations begin in April 2027

At 2:00 PM Central European time today, mission controllers at ESA's operations centre in Darmstadt, Germany watched a spacecraft that had traveled as one unit for nearly eight years finally split apart. 

BepiColombo's Mercury Transfer Module, the module that powered the entire journey, separated from its two science orbiters, a milestone ESA flight director Ignacio Tanco compared to launching an entirely new spacecraft, except one already orbiting a different planet.

Reaching Mercury is deceptively difficult. Getting to the solar system's innermost planet actually requires more energy than reaching Pluto, since spacecraft accelerate the entire way in toward the Sun's gravity and need serious braking to avoid simply flying past. BepiColombo's route has included nine planetary flybys, one of Earth, two of Venus, and six of Mercury itself, each one bleeding off speed before the final approach.

The mission nearly lost its window entirely. A 2024 fault in the transfer module's power system cut available thrust to 90%, forcing engineers to redesign the trajectory and pushing the arrival date back 11 months. Engineers ultimately used one of the Mercury flybys to extract extra gravitational braking, making up for some of the lost thrust without sacrificing the mission's science goals.

Today's separation is just the first step in a sequence ESA calls one of the most demanding it's ever attempted. The joint ESA and JAXA mission still faces a critical series of thruster burns in late November to enter Mercury orbit, followed by the two orbiters, Europe's Mercury Planetary Orbiter and Japan's Mercury Magnetospheric Orbiter, splitting from each other in December to begin studying the planet's surface, exosphere, and magnetic field from separate vantage points starting in April 2027.