Why Does a Spacecraft Take Eight Years to Reach Mercury When It Is So Close?

Heading Toward the Sun Means Braking the Whole Way

Imagine you want to visit a house at the bottom of a steep hill. Going far is not the problem. The problem is that every step downhill makes you faster, and you want to arrive slowly enough to knock on the door instead of running past it.

Space works a bit like that around the Sun. Earth, and anything launched from Earth, already travels around the Sun at high speed. To reach Mercury, a spacecraft has to move inward toward the Sun, and the Sun’s gravity keeps pulling it faster. ESA explains that, going from Earth toward the Sun, BepiColombo “needs to constantly brake against the gravitational pull of the massive star” so that it reaches Mercury with just the right speed to enter a stable orbit [1]. Mercury itself moves very fast, so a probe that arrives too quickly would simply shoot past or be unable to be caught by Mercury’s weak gravity.

That is why scientists compare the energy needed to reach Mercury with the energy needed to reach much more distant planets. ESA’s pages say a Mercury orbiter needs more energy than a mission to Pluto, and its mission page says Mercury is harder for an orbiter to reach than Saturn [1][2]. A JAXA article quotes the Japanese project team saying that the same energy could carry a probe to Saturn, or even Neptune [3]. The hill picture above is my own, not JAXA’s. The exact planet in the comparison changes from source to source and with the mission design, so the safe takeaway is this: going to Mercury is in the same league as going to the outer planets, not a short hop.

Nine Planet Flybys and a Gentle Ion Engine Do the Braking

A rocket could brake by burning fuel, but ESA says normal chemical propulsion would have needed far too much fuel for such a trip. Instead, BepiColombo uses two tools together [4].

The first is a set of nine gravity-assist flybys: one at Earth, two at Venus and six at Mercury itself. In a gravity assist, a spacecraft passes close to a planet and swaps a little motion with it, which changes the spacecraft’s speed and direction without using fuel. Done the right way around, it slows the spacecraft down. The flybys happened on these dates: Earth in April 2020, Venus in October 2020 and August 2021, and Mercury on October 1, 2021, June 23, 2022, June 19, 2023, September 4, 2024, December 1, 2024 and January 8, 2025 [4][2].

JAXA engineers describe the final approach with a skiing picture: a beginner does not go straight down a steep slope but traces zigzag turns, and each pass by Mercury is like one turn that scrubs off speed [3]. The JAXA staff member in that article adds that this is a simple image, not a strict description.

The second tool is electric propulsion. A module attached to the spacecraft, the Mercury Transfer Module, carries ion thrusters. They push very gently but for very long stretches, which suits a trip of years. ESA says the thrusters will add up to the highest total push (thrust times hours of operation) of any space mission so far, and that without them two spacecraft as heavy as these could not have been taken to Mercury with a standard launcher [4].

Why All That Braking Takes About Eight Years

Gentle braking is slow braking. Compare it with another ESA spacecraft: Solar Orbiter, which flies even closer to the Sun than Mercury does, needed just under two years to get to its destination, because it did not have to be captured into orbit around Mercury [1].

Flybys cannot be scheduled whenever you like either. The probe and the planet have to meet at the same place at the same time, and Earth, Venus and Mercury all circle the Sun at different speeds. A simple way to picture it is a set of runners on circular tracks: you must wait until the right runners are side by side. That waiting is my own explanation from the flyby dates above, not a statement quoted from ESA, but the dates do show the pattern: after leaving Earth, the probe needed nearly three years just to reach the first Mercury flyby, and the six Mercury passes were spread over more than three years.

Together, this makes ESA’s numbers: launch on October 20, 2018, nine flybys and a cruise of about eight years, with a launch mass of 4,100 kilograms (about 9,000 pounds) [2]. (One news report counts the cruise itself as 7.2 years, which is a different starting point, not a different trip [6].)

A Power Fault in 2024 Pushed the Arrival Back by About a Year

The original plan was to enter orbit around Mercury in December 2025. In May 2024, ESA reported a problem in the electrical system of the Mercury Transfer Module, which meant the ion thrusters could not run at full power. After the cause was studied, the thrusters were set to work at a lower output [5].

The institute that runs Japan’s space science missions (ISAS, part of JAXA) wrote that this lower thrust could no longer reach Mercury orbit in December 2025, so engineers designed a new route. It was announced on September 2, 2024, and it moves the arrival to about one year later, November 2026, without changing the science the two orbiters will do once there [5]. The new route lowered the altitude of the fourth Mercury flyby in September 2024 to save thruster effort for later [5].

This shows how little extra thrust the mission had: a slow, gentle engine leaves little to spare. A weaker thruster means a longer wait, not just a bit more fuel burned.

What Happens at Mercury When It Finally Arrives

ESA lists arrival at Mercury for December 2026 and the start of routine science operations for April 2027, after the transfer module separates and the two orbiters are captured together into a polar orbit around the planet [2][4]. ISAS gave November 2026 for orbit arrival when it announced the new route, so the exact day depends on how the final steps are counted [5]. As this note is written, in October 2026, the spacecraft has not yet arrived: a science news site reported in September 2026 that it had begun its final approach [6].

There are two orbiters, because the two space agencies split the work. ESA built the Mercury Planetary Orbiter, which studies Mercury’s surface and inside. JAXA built the Mercury Magnetospheric Orbiter, named Mio, which studies the magnetic bubble around the planet [6]. This is the first European mission to Mercury and the first to send two spacecraft there at once [6].

The Mission Is Named After the Italian Mathematician Who Found Mercury’s Shortcut

The name honors Giuseppe “Bepi” Colombo, an Italian mathematician and engineer. He helped explain that Mercury turns on its axis three times for every two trips around the Sun. He also suggested to NASA a gravity-assist pass by Venus that put the American spacecraft Mariner 10 into an orbit where it could fly by Mercury three times in 1974 and 1975 [6]. Our note on Mariner 10 tells that story in full. Colombo’s idea, using planets to steer a spacecraft to Mercury, is the same trick BepiColombo uses fifty years later.

Feel the Sun’s Pull With a Bowl and a Marble

You need a large smooth bowl, a marble and a spoon. Hold the bowl steady on a table and roll the marble gently around its inside so that it circles instead of dropping straight down. Stop if the marble is about to fall out. Notice how it speeds up as it sinks closer to the bottom.

Now try to make the marble settle on a low circle without letting it race past: you will find you have to nudge its speed down at the right moments. That is a rough model of what BepiColombo does, with the Sun’s gravity as the bowl. It is only a picture, since real orbits are not bowls, but it shows why going down is faster than stopping.

Then open ESA’s “Where is Bepi?” tool and check the real flyby dates yourself. If you want a question to carry away, ask: if the probe slowed too little at Mercury, where would it end up?

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