BepiColombo enters Mercury orbit

Seven years and nine flybys end as the ESA–JAXA spacecraft slips into Mercury orbit. Replay the whole spiral in the orbit simulator.

The moment of BepiColombo enters Mercury orbit: https://nowsky.org/e/bepicolombo-mercury-orbit — opens at the right place and time, replayable, no login.

Available views: Orbit ✓ · Sky — (not visible in the sky)

The story

BepiColombo is a joint ESA–JAXA mission to Mercury. It left Earth in October 2018 and has been flying for the better part of eight years, shedding speed through a long sequence of gravity assists — one at Earth, two at Venus, and several at Mercury itself. The reason the trip takes so long is straightforward: a trajectory that falls toward the Sun picks up speed on the way down, and all of that speed has to be thrown away again before Mercury can capture the spacecraft.

How to throw it away is essentially the whole design of the mission. During cruise, solar-powered ion engines apply a faint thrust for months at a time, while the decisive braking comes from skimming past planets. Each flyby winds the orbit a little further inward, and the point where the spiral finally closes is Mercury orbit. It is one of the most painstaking braking cruises in the history of planetary exploration.

Once insertion is complete, the stack that flew out as a single vehicle comes apart. ESA's Mercury Planetary Orbiter works the surface and interior from a low orbit; JAXA's Mio takes the magnetic field and plasma environment from a longer, more elongated one. Two orbiters watching the same planet from different altitudes means what happens on the ground and what happens in the magnetosphere can finally be read together.

The schedule has not been smooth. Lower-than-planned output from the propulsion system during cruise pushed the arrival back, and orbit insertion is now planned for late November 2026. On a route to Mercury, where there is very little margin, adjustments like that are not unusual. Move the orbit view on this page to that date and you can follow the entire eight-year inward spiral on a single screen.

Why it matters

Mercury is the hardest of the inner planets to explain. It is far too heavy for its size, with a metal core taking up a large share of the whole body. Where the missing outer rock went — stripped away in a giant impact, or never there because the planet formed in the hot inner disk — remains unsettled. Whichever answer holds changes the general story of how planets are built.

Mercury also carries its own magnetic field, which is surprising for a small, old world. The magnetosphere that field carves out of the solar wind is a miniature of Earth's, but it reacts far faster. With almost nothing that deserves the name atmosphere, the solar wind strikes the ground directly, and particles knocked loose form a thin envelope around the planet. Having Mio watch that process from inside the magnetosphere while MPO images the same surface at the same moment is the core idea of the mission.

Then there is the ice. The planet closest to the Sun has polar craters whose floors never see sunlight, and radar has long shown material there that looks like water ice. The hottest daytime surface in the solar system and some of its oldest ice sit on the same world. BepiColombo will gather the measurements needed to identify that material and work out where it came from.

How to watch

This is not an event you watch in the sky. Orbit insertion happens quietly beside Mercury and is confirmed on Earth only through radio signals. The most direct way to follow it is the orbit-view replay on this page: wind the clock back to autumn 2018 and run it forward, and you can see the trajectory tighten inward past Earth, Venus and Mercury in turn. Compare the shape of the orbit just before and just after a flyby and what a gravity assist actually does becomes obvious.

The planet itself is a naked-eye object. Even at its greatest separation from the Sun, though, that gap stays small, so Mercury shows up only low in the east before sunrise or low in the west after sunset. Find a spot with an open horizon and look for a fairly bright yellowish point in the twilight. Never sweep that part of the sky with binoculars or a telescope while the Sun is still above the horizon.

For the mission itself, follow the agencies. ESA and JAXA publish status reports around major manoeuvres and have released early images and data as they come in — the close-up views returned from each Mercury flyby during cruise are a good example. Insertion and the separation of the two orbiters unfold in stages over several days, so following a run of announcements reflects the real sequence better than waiting for a single day of live coverage.

See this for yourself — replay the moment in the orbit view

Background

Very few spacecraft have ever visited Mercury. Mariner 10 made three close passes in the 1970s and returned the first images of nearly half the surface, and then a full generation went by with nothing. In the late 2000s NASA's MESSENGER worked its way into Mercury orbit through a series of flybys and produced the first global map of the planet, and the case for ice in the permanently shadowed polar craters grew much stronger from its data.

The mission is named for the Italian mathematician Giuseppe "Bepi" Colombo. He explained the planet's unusual resonance — three rotations for every two orbits around the Sun — and proposed reworking Mariner 10's trajectory so that a single encounter could become repeated ones. The very problem of designing trajectories for Mercury is part of his legacy.

Once insertion is complete the science phase begins in earnest. The two orbiters each run a nominal mission of roughly a year from their own orbits, with extension possible if the hardware holds up. So close to the Sun, heat and radiation set the limits, and operating life depends on how the spacecraft weathers them. The next big Mercury event visible from the ground is the transit of 2032, when the planet crosses the face of the Sun.

FAQ

Can I see the orbit insertion in the sky?

No. The spacecraft manoeuvres beside Mercury and the event is confirmed on Earth only by radio. Use the orbit-view replay on this page instead: it lets you follow the eight-year cruise and its flybys step by step, while the agencies report on the manoeuvre itself.

Why does it take so long to reach Mercury?

A path toward the Sun speeds up on its own, so a spacecraft has to give that speed back before Mercury can capture it. The mission spreads that work across many gravity assists and long stretches of ion-engine thrust. The difficulty of a Mercury trip is not distance but getting rid of velocity.

What does it mean that the spacecraft splits in two?

BepiColombo flies as two orbiters joined into one stack. After insertion they separate into ESA's MPO and JAXA's Mio and settle into different orbits — MPO on the surface and interior, Mio on the magnetic field and plasma — so the same planet is observed from two vantage points at once.

How do I find Mercury with the naked eye?

Look low in the east just before sunrise or low in the west just after sunset. You need an open horizon, and Mercury appears as a fairly bright yellowish point in the twilight. Do not sweep that part of the sky with binoculars while the Sun is still up.

Related

• BepiColombo launch (2018)
• The 2032 transit of Mercury
• JUICE arrives at Jupiter
• Mercury — object index
• BepiColombo — object index
• The Sun — object index

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Times and figures are approximate — verify with official sources when planning travel. Simulation uses J2000 mean elements.