Back in October, 2018, the BepiColombo mission launched. It intrigued me then, and I made a note to learn more and write about it at the time. What piqued my interest most was the journey BepiColombo would have to take, a journey of over seven years to reach a planet that’s only an average 48 million miles away. Mars, by contrast, is 38 million miles away at its closest approach, and it only takes seven to eight months to get there. Interesting…

Well, one thing led to another and my investigation took a hiatus. But here again a couple weeks ago (January 8), BepiColombo made the news when it made its most recent flyby of Mercury – the last, in fact, before the orbiters will settle into orbit around the tiny planet in November 2026 (originally planned for December 2025). This triggered a memory, and so I dug back in.

BepiColombo is a joint project by the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA), comprised of two separate orbiters have been sharing a very long, complex journey to the innermost planet. When they finally reach orbit, ESA’s Mercury Planetary Orbiter (MPO) will study the surface and interior, while JAXA’s Mercury Magnetospheric Orbiter (MMO) will establish a higher orbit focused on measuring Mercury’s magnetic field. We’ve been there before – Mariner 10 flew by Mercury in 1974 and 1975 and did it in only 147 days, but it didn’t stop. More recently, NASA’s MESSENGER started orbiting Mercury in 2011 to map the surface, and it took six and a half years to make the trip. So, clearly actually getting to orbit around Mercury takes a lot of time… but why?

Like most things involving space travel, it’s complicated. It seems intuitive that falling toward the Sun is fairly straightforward, but it’s actually easier to send a spacecraft to outer planets than it is to travel inward.

Leaving Earth, you may have noticed during a typical rocket launch, requires a lot of speed. Low Earth Orbit typically takes around 7.8 kilometers per second (km/s), or 17,000 miles per hour (mph), with respect to the Earth. With respect to the Sun, however, the Earth itself is orbiting at about 29.8 km/s (66,700 mph). In other words, we’ve spent a lot of energy to be orbiting the Sun along with the Earth. Now, a conundrum – we have to go even FASTER to get away from the home planet, but if we want to fall inward toward the Sun, we have to slow down.

Anything we launch into deep space has to reach a speed of around 11.2 km/s (25,000 mph) to escape the gravity well and truly get away from Earth. This is our planet’s escape velocity, so we have to be going at least this fast with respect to the Earth to go anywhere else. We can be clever about it though, since this is an Earth relative speed – if we fire thrusters at a point in the orbit when we’re moving around Earth in a direction opposite to Earth’s travel (typically, orbiting eastward, but on the daylight/sunward side of the planet), we can accelerate with respect to Earth and decrease our velocity around the Sun at the same time. In other words, we’ll accelerate away from Earth and fall into a solar orbit. But we will be falling quite a distance, and the pull of the Sun will be accelerating us until we pass around the Sun and head back out again. Without any adjustments, by the time we get to the orbit of Mercury, a mere 42 million miles from the Sun, well under half the average radius of Earth’s orbit, our spacecraft will be travelling INCREDIBLY fast, and trying to drop into orbit around a very small target, while doing so in the vicinity of the very large gravity well of the Sun itself.

Now, Mercury is fast too. In order to maintain its orbit, it has to travel an average of 47.9 km/s (107,200 mph). Meanwhile, its escape velocity (this speed relative to Mercury) is a very low 4.3 km/s (9,600 mph). So, we need to achieve an orbital speed matching that of Mercury, with a relative speed less than its escape velocity while moving in the right direction to drop into a stable orbit (all while looking over our shoulder at that massive Sun trying to pull us away).

So, the trick here is to an establish a solar orbit that matches the orbit of Mercury as closely as possible when at its orbital distance, crossing its orbit at an angle that is as shallow as possible, with speed as close to that of Mercury as possible, and to make that intersection at a point in space and time that Mercury also happens to occupy. However, our initial solar orbit extends all the way out to Earth, since that’s where we’re starting from.

The most energy effective way to do this is… gradually. And that has been BepiColombo’s experience. After launching in 2018 and establishing a solar orbit, BepiColombo made a pass by Earth in 2020 and used Earth’s gravity to slow down and lower the overall orbital altitude. Later in 2020, and again in 2021, BepiColombo performed flybys of Venus to further shape the orbit into something tighter (closer to the Sun) and more circular. Then, starting in 2021, the probe made 6 passes by Mercury itself, each time using the planet’s gravity to adjust the shape of the orbit. All told, BepiColombo will have made 18 orbits around the Sun before it finally starts orbiting Mercury.

At the same time, ESA and JAXA scientists have been able to use these passes to test and calibrate instrumentation and equipment. In April 2024, engineers realized that an electrical problem was preventing effective power distribution between the solar panel array and the rest of the systems, and limiting the amount of electric propulsion available. This effectively meant that the final approach to Mercury would have to hit an even narrower window of space and time, because it would not have sufficient power to overcome large errors. In order to compensate, BepiColombo’s trajectory was altered so that its fourth pass was closer to Mercury, reducing the amount of power needed to adjust for the fifth and six passes. Those final two passes would also make more significant changes to the overall trajectory – in effect leaning on Mercury more, realizing that we could then rely on thrusters less. As a result, BepiColombo’s arrival has been delayed by 11 months, but will be a more energy-efficient insertion with a planned orbital insertion now scheduled for November 2026.

This is a very rough, one-dimensional analogy, but pretend you have to throw a tennis ball to a friend standing on a balcony eight floors up. And he’s got a bucket to catch it in. It takes a LOT of energy to get the ball moving fast enough, but if you do it right, the ball will essentially stop right in front of him, and he can reach out and easily snag it in the bucket.

Now, imagine the opposite. Your friend has to toss you the same ball from the balcony to the ground, and you have a small Styrofoam coffee cup (representing Mercury’s smaller gravity well) to catch it. Worse, he can’t just hold it out and let go, he’s got to throw it UP a little first so it’s already moving when it goes by him on the way down. You’re going to have a much easier time if you let the ball bounce a few times, losing energy, getting lower and slower on each bounce, until you can finally snag it.

It’s a long, complicated journey, and I admire the skill and precision of the flight dynamics crews that have planned and executed the complicated dance with three planets (and the Sun of course) to make it successful. I’ll be looking forward to hearing that the two probes have separated and achieved their respective orbits in November, 2026.

Get Out There!

Further reading: 
ESA's BepiColombo Mission Page

NASA's BepiColombo Mission Page