BepiColombo: A Spacecraft Has Been Falling Toward Mercury for Eight Years — and It Finally Arrives This Week

Mercury is 77 million kilometres from Earth at its closest point. BepiColombo left in October 2018. It arrives next week.

On 3 September 2026, the ESA/JAXA BepiColombo mission will complete one of the most complex journeys in the history of spaceflight — shedding its transfer module and releasing two science orbiters into Mercury orbit. After nine planetary flybys, roughly 8.5 billion kilometres of travel, and nearly a decade of patient falling through the inner solar system, humanity is about to get its sharpest look yet at the most overlooked world in our cosmic neighbourhood.

Why does Mercury — the closest planet to the Sun — take eight years to reach?

Mercury is closer to Earth than Mars by a factor of three. You'd think you could get there in months. The problem is pure physics.

Earth orbits the Sun at 30 km/s. Mercury orbits at 47 km/s. To go inward — toward the Sun — a spacecraft has to slow down. Dramatically. You're fighting solar gravity pulling you in too fast. A spacecraft aimed directly at Mercury arrives moving so quickly it either flies straight past or crashes into the surface. Stopping requires more fuel than a mission to Pluto.

The solution BepiColombo's engineers devised: nine years of free braking. One Earth flyby, two Venus flybys, six Mercury flybys — each one stealing a little more speed from the spacecraft using nothing but gravity. It's the only approach that works. And it takes the better part of a decade.

Who built BepiColombo — and who is it named after?

This is a joint mission between the European Space Agency and the Japan Aerospace Exploration Agency. That's unusual. Most deep-space planetary missions are solo efforts. This one needed two agencies, hundreds of engineers across two continents, and a combined budget of over €1.65 billion.

It's named after Giuseppe "Bepi" Colombo — an Italian mathematician who in 1970 worked out how to use planetary gravity assists to reach Mercury at all. Before his equations, the mission was physically impossible. NASA used his technique for Mariner 10's Mercury flybys in 1974. We still use his math. We named the spacecraft after him.

What is BepiColombo actually made of?

Three spacecraft stacked together like a tower, flying as one unit for eight years. On 3 September, the stack separates for the last time.

• MTM (Mercury Transfer Module) — ESA's propulsion unit. It provided the electric ion thrusters that kept BepiColombo on course across the inner solar system. On September 3, it separates and is discarded — its job done forever.
• MPO (Mercury Planetary Orbiter) — ESA's science spacecraft. Will orbit tightly over Mercury's poles, mapping its surface and measuring its magnetic field at close range.
• Mio / MMO (Mercury Magnetospheric Orbiter) — JAXA's spacecraft. Will study Mercury's magnetic environment and its interaction with the solar wind from a wider orbit, simultaneously with MPO.

Two spacecraft, two orbits, one planet, measured at the same moment from two different distances. No previous Mercury mission could do this. It's the scientific point of the whole arrangement.

What do we know about Mercury — and why does none of it make sense?

Barely anything. Only one spacecraft has ever orbited Mercury before — NASA's MESSENGER, which arrived in 2011 and deliberately crashed into the surface in 2015. Before that, Mariner 10 did three flybys in 1974-75 and mapped less than half the planet. In the entire history of space exploration, Mercury has had roughly four years of orbital science. Compare that to Mars, which has had continuous coverage since 1997.

What we found in those four years was deeply strange.

Mercury is actively shrinking. As its iron core slowly cools, the planet contracts — squeezing its crust into massive cliff-like scarps, some stretching hundreds of kilometres across the surface. Some of those scarps are still forming today. A planet physically collapsing in on itself, right now.

Mercury has ice at its poles. On a planet where dayside temperatures hit +430°C — hot enough to melt lead — permanently shadowed craters at the poles hide water ice that has never once seen sunlight. It may have been delivered by ancient comets. It has sat there, untouched, for billions of years. Ice. On the hottest planet's surface.

Mercury has a magnetic field it should not have. Small, rocky planets don't generate global magnetic fields — they're geologically dead. Mercury is tiny. It should be cold and inert. Yet it has a global magnetic field, tilted off its axis, mysteriously offset from the planet's geometric centre in a way no current model fully explains. MESSENGER found it. BepiColombo was built to understand it.

What will BepiColombo actually discover?

That depends on what Mercury is hiding. Here's what the science teams are specifically chasing:

• The core mystery. Why is Mercury's iron core proportionally enormous? Did the planet form that way, or was its outer rocky mantle stripped away by a catastrophic impact billions of years ago?
• The magnetic field. MPO and Mio will map Mercury's magnetic field simultaneously from two different distances — creating a 3D picture no single orbiter could make. If the field's structure matches a specific model, we'll know which formation theory is right.
• The polar ice. Is it still there? How thick? What's its chemical composition? Could organic molecules be preserved alongside it?
• The surface chemistry. MESSENGER found unexpectedly high levels of sulphur and potassium — elements that shouldn't survive in Mercury's formation environment. BepiColombo's spectrometers are more sensitive. They'll finally measure why.
• A test of Einstein. Mercury's elliptical orbit was the first observational evidence that Einstein's general relativity was correct — it precesses in a way Newtonian gravity couldn't explain. BepiColombo will measure tiny deviations in its own orbit to test gravitational theory to new precision, nearly 120 years after Einstein wrote down the equations.

When exactly does the arrival happen?

The Mercury Arrival Phase has already begun. As of today, BepiColombo is in its final approach. On 3 September 2026, ESA's Mercury Transfer Module separates — its eight-year job complete. The two science orbiters then begin their own orbit insertion sequences: a choreographed series of engine burns spanning weeks, each one carefully trimming speed until MPO and Mio settle into their final science orbits.

This is not like parking a car. A miscalculation means flying past Mercury and losing the mission entirely. Every burn matters. ESA mission controllers will be watching every telemetry packet in real time.

You can watch ESA's live coverage on their website. SkyLens will have updates as the arrival unfolds — check the blog for breaking coverage.

Why does Mercury matter at all?

Because it keeps getting ignored. We sent 12 humans to the Moon. We have active rovers on Mars. We've flown past Pluto, we're on our way to Jupiter's ocean moons, and we've studied Saturn's rings for decades. Mercury — the planet that finishes a full orbit every 88 days, the one closest to the Sun, the one whose interior structure defies every model we have — has had barely four years of close scientific observation across all of human history.

That's not a gap. That's a blind spot.

Earth teaches us how a rocky planet at the right distance from its star can support life. Mercury teaches us the limits of that understanding — what happens to a planet too close, too small, too stripped, with a core too large and a magnetic field that shouldn't exist. Understanding Mercury is understanding the extreme end of the spectrum that Earth sits on.

You can track real-time spacecraft positions — including the inner solar system — on the SkyLens live tracker. And if BepiColombo's magnetic field mystery has you curious about how planets generate them, the SkyLens learn section has the explainer.

Eight years. Nine flybys. 8.5 billion kilometres. Next week, two spacecraft finally answer the questions MESSENGER left open. Whatever Mercury is hiding — we're about to find out.