BepiColombo: Mercury orbit insertion planned for 21 November

What happens on 21 November and afterwards

BepiColombo is now two spacecraft travelling as one: ESA's Mercury Planetary Orbiter (MPO) and JAXA's Mercury Magnetospheric Orbiter, called Mio. A third craft, the Mercury Transfer Module, carried and pushed them with its ion engines until mid-June; it was released on 3 September, and since then MPO's own thrusters have steered the pair.

On 21 November the two still-joined spacecraft are to enter a polar orbit, passing over both poles so that, as Mercury turns, its whole surface comes into view. ESA's published sequence after that is:

• 9–10 December: MPO releases Mio into its own orbit.
• 16 December: MPO jettisons Mio's sunshield and begins descending to its own orbit.
• 10 March 2027: MPO reaches its final orbit.
• April 2027: science operations begin.

How firm is the date? ESA's update of 6 October 2026 still gives 21 November, but its timeline adds that exact dates may change for operational reasons. It has moved before: in April 2024 a power fault kept the ion engines below full thrust, and the orbit insertion planned for December 2025 slipped to November 2026. Some ESA pages give December 2026 as the arrival, apparently counting from 9–10 December, when the two orbiters fly as separate spacecraft.

Two orbiters, two jobs

MPO will circle 480 to 1,500 km above the surface every 2.3 hours. Most of its eleven instruments will study the planet itself: a laser altimeter will map surface elevation, X-ray and gamma-ray spectrometers will measure the surface's composition, and radio tracking will map the gravity field and, from it, reveal the size and state of the core.

Mio will follow an elongated orbit, from 590 to 11,640 km above the surface, once every 9.3 hours. Its five instruments will measure Mercury's magnetic field, the charged particles trapped in it, the planet's thin exosphere (including its sodium) and the dust along Mercury's orbit.

Among ESA's questions: why a planet this small still has a magnetic field, why there is ice in craters near the poles of a world this close to the Sun, and how the surface features called hollows formed. BepiColombo will be the second mission to orbit Mercury, after NASA's MESSENGER (2011–2015), and the first to put two orbiters there.

Why it took eight years

Mercury is never more than about 220 million km from Earth, yet BepiColombo has travelled 9.9 billion km to get there. The problem is speed, not distance. A spacecraft falling inwards towards the Sun gathers speed all the way: coasting down from Earth's orbit on the cheapest path, it would reach Mercury's distance at about 57 km/s, some 10 km/s faster than the planet itself moves there. Mercury's gravity is weak (its escape speed is 4.3 km/s), so to be captured a spacecraft must approach with nearly the planet's own speed and direction.

So BepiColombo spent eight years shedding that excess speed. Its ion engines pushed gently for weeks at a time, and nine flybys changed its speed and direction: Earth (April 2020), Venus twice (October 2020 and August 2021) and Mercury six times (October 2021 to January 2025). Each pass brought its orbit around the Sun closer to Mercury's own, until the planet could capture it.

Mercury in the sky the same week

Mercury also reaches its greatest angular distance west of the Sun late on 20 November (UTC), so the mornings around orbit insertion are a good time to see the planet from mid-northern latitudes: from London it stands about 10° above the southeastern horizon at civil dawn, when the Sun is 6° below the horizon. Our story "Mercury in the morning sky: use Venus to find it before sunrise" shows where to look. From around 34° S (Sydney or Cape Town), Mercury is only about 4° up at the same point of twilight and hard to catch.

The spacecraft itself cannot be seen. A craft about ten metres across, 150 million km away and only about 20° from the Sun, is far beyond the reach of any telescope.