From Earth to the innermost planet
BepiColombo, a joint mission led by the European Space Agency (ESA) with contributions from Japan and the US, is finally entering its endgame at Mercury after an 8‑year cruise. Launched in 2018, the nearly $2 billion robotic mission has been spiraling inward toward the Sun using a mix of plasma (ion) propulsion and a complex sequence of nine gravity‑assist flybys of Earth, Venus, and Mercury.
Reaching fast‑orbiting Mercury is surprisingly harder than flying past Pluto: a probe must shed a huge amount of delta‑v to be captured by Mercury instead of falling deeper into the Sun’s gravity well. To do this, BepiColombo carried the most powerful electric propulsion system ever flown in deep space, with four gridded ion thrusters on its Mercury Transfer Module (MTM).
Shedding the transfer module in a harsh environment
With arrival now just months away, BepiColombo no longer needs the MTM. At a distance of only 63 million km from the Sun—less than half Earth’s distance—mission control commanded a critical separation. In this extreme thermal and radiation environment, the rest of the spacecraft had to take over power generation, attitude and thermal control in one step, a maneuver ESA likened to “launching a new spacecraft”.
BepiColombo is actually a three‑spacecraft stack:
- Mercury Transfer Module (MTM) – provided power and propulsion during cruise, now discarded;
- Mercury Planetary Orbiter (MPO) – ESA’s orbiter, with cameras and spectrometers to study Mercury’s surface and composition;
- Mercury Magnetospheric Orbiter (MMO / Mio) – JAXA’s orbiter, tuned for magnetic fields, plasma and dust.
Telemetry after separation confirmed that MPO’s solar arrays are generating power and recharging batteries, and that previously obscured instruments are finally seeing “first light”.
Two orbiters, one mysterious world
If all remaining maneuvers go to plan, BepiColombo will be captured into Mercury orbit on 21 November, with MPO and Mio separating in December. The European orbiter will fly in a low orbit to map the cratered surface, investigate dark polar craters that may harbor water ice, and build global high‑resolution datasets that go beyond NASA’s earlier MESSENGER mission.
Japan’s Mio will operate higher up, sampling the magnetosphere, solar wind interaction, plasma environment and micrometeoroid flux. Having two spacecraft taking coordinated measurements is a game‑changer for understanding how the solar wind sculpts airless, weakly magnetized bodies like Mercury.
Routine science operations are expected to begin around April next year, opening a new chapter in our knowledge of Mercury’s origin, interior, and its place in Solar System evolution.
Source: Ars Technica
