On September 3 at 12:00 UTC, more than 200 million kilometers from Earth, a set of pre-programmed commands initiated a separation sequence: springs pushed a 2.6-ton propulsion module away from the BepiColombo stack. There was no room for real-time intervention — at that distance, a radio signal takes 11 minutes one way. The math checks out: 200 million km divided by the speed of light equals 667 seconds. Yes, really.
Confirmation had to come from the ground. ESA's deep-space antennas at Cebreros (Spain) and Malargüe (Argentina) received telemetry at 13:52: separation successful, all systems nominal, and the MPO's solar wings charging its batteries. ESA's broadcast page put it well: "Executing a critical separation more than 200 million kilometres from Earth demands precise planning, navigation and commanding, with no opportunity for real-time intervention."
Note the wording: this is called "the start of the arrival phase," not arrival. Orbit insertion comes 79 days later.
Why Throw Away the Engine
Getting to Mercury is harder than getting to Pluto — that's no exaggeration. Falling toward the Sun is an acceleration process, and Mercury orbits at 47 km/s, so a spacecraft must brake the whole way or fly right past. BepiColombo's answer was a tugboat: the Mercury Transfer Module (MTM), with four xenon ion engines producing a combined 290 millinewtons of thrust — "the most powerful ion engine array ever operated in space" — accumulating over 15,000 hours of firing. Add nine gravity assists (one Earth, two Venus, six Mercury), a planned route of roughly 9 billion km, and media reports put the actual distance traveled at arrival above 10 billion km.
On June 15, the ion engines were shut down for good after completing their historic mission. The rest of the way relies on coasting plus MPO's chemical propulsion. There's no point dragging more than two tons of dead weight while decelerating.
The solution was simple. Jettison it.
The Schedule Ahead
Orbit insertion on November 21 — exactly eight years, one month, and one day after launch. The capture is a weak capture: first into a large elliptical orbit with a ~60-hour period, then refined through about 16 maneuvers into the working orbit. From December 9–10, Mio separates from MPO into its own orbit; around December 16, Mio's sunshield is discarded. In March, MPO enters its 480×1500 km science orbit, and the science phase begins around April 6.
- 2018-10-20: Ariane 5 launch
- 2020–2025: nine gravity assists, over 15,000 hours of ion propulsion
- 2026-06-15: ion engines shut down
- 2026-09-03: MTM jettisoned
- 2026-11-21: orbit insertion — eight years and one day after launch
- 2026-12-09: Mio separates into position
- 2027-04-06: science phase begins
For scale: Mariner 10's mission lasted about 506 days, MESSENGER about 3,922 days, and BepiColombo will have taken roughly 2,954 days just to reach orbit.
The mission name is a fitting loop. Bepi Colombo was a mathematician at the University of Padua who proposed the trajectory Mariner 10 used in 1974 to meet Mercury three times via Venus gravity assists. Half a century later, a gravity-assist mission to Mercury bears his nickname. There's also a footnote: a 2024 power fault reduced thrust, delaying arrival by about 11 months — the original target was last December.
What We're Waiting For
ESA's official page poses two captivating questions: "Why is there ice in the polar craters of the scorched planet? Why does Mercury have a magnetic field?" — the closest planet to the Sun has water ice in its polar craters, and among terrestrial planets, only it and Earth have a global magnetic field. Add a disproportionately huge iron core, plus its role as a precision testing ground for general relativity.
There were no fireworks when the engine was discarded. Just the muffled sound of a spring, 200 million kilometers away, as an eight-year journey begins its finale. The main event comes in 79 days.