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BepiColombo: the (near) Impossible Mission to Mercury

Written by Morris M.


It’s the least-studied planet in the inner solar system. A mysterious world on the edge of our cosmic neighborhood that we’ve almost never visited.

From the dawn of the Space Race right through to today, one constant has been our ability to study the other terrestrial worlds. To date, humanity has sent over 40 craft to Venus; and over 50 to Mars. Heck, even Jupiter – which is both decidedly non-rocky and far harder to get to – has seen something in the region of 10 visits by NASA and the ESA. 

Yet, there’s one world we’ve constantly overlooked. The runt of the planetary litter: Mercury. 

Despite being a mere 77m km away at its closest point, Mercury remains almost woefully unvisited. Our only encounters have been Mariner 10 in 1974, and Messenger from 2011-15. At least, until now. 

As you watch this, twin craft are currently sailing across the heavens, destined for the first planet. Known collectively as BepiColombo, they represent a joint European-Japanese attempt to uncover Mercury’s remaining secrets. 

Yet visiting our inner neighbor isn’t an easy task. It requires feats of complex engineering and course-planning. Feats MegaProjects is exploring today.


So Near, So Far…

Here’s a trick question to get this post rolling: what is the closest planet to Earth?

If you have any space knowledge, you most-likely answered with Venus. It’s true that the second planet is the one that comes closest to us – peaking a mere 38 million km away. Practically close enough for a cosmic brush of the lips. 

Yet Venus also spends a good deal of its year much further afield, sometimes right on the other side of the Sun. 

That means the planet that – on average – spends the most time closest to Earth must be the one with the shortest orbit. The one that will repeatedly swing by us even as our other neighbors are off on their voyage around the Sun. 

With a year that lasts only 88 days, that planet – of course – is Mercury. 

We mention this not just because it’s a good factoid, but because it also nicely sets up the great engineering challenge at the heart of this episode: how to safely get a craft into orbit around the first planet. 

Common sense would dictate that visiting one of the closest planets is easier than visiting one of the farthest away. Surely, you just point a rocket at Mercury, then kick back and wait for it to get there?

But that just shows that common sense doesn’t know squat. 

While it really is easy to for a craft to reach Mercury, it’s nigh-on impossible for it to stop once it does so. 

The reason? That gigantic gravity sink at the center of our solar system known as the Sun. 

To understand why this is a problem, picture one of those Olympic cycling arenas, with the steeply sloped track shaped kinda like a bowl.

In this metaphor, Mercury is someone like champion cyclist Chris Hoy, whizzing around on those tilted sides at high speed. Humanity is the crowd above him in the spectator stands; and the Sun’s immense gravity is the bottom of the bowl, where the slope ends. 

Now, let’s say you wanted to go from the stands at the top and visit Chris Hoy for a chat mid-race. You technically could just point your bike straight down the slope and intercept him, but you’re not going to be able to stop if you do so. 

At best you’ll just go zooming past to the bottom of the bowl. At worst, you’ll go crashing into him and have to deal with all the angry Chris Hoy fans being mad at you. 

In a super-simplified way, that’s the problem engineers face with getting a craft to Mercury. Point your rocket straight at Planet One, and watch it go sailing past and into oblivion. 

No, if we want to get close to cosmic Chris Hoy, we’re going to have to take a roundabout route. One that involves cycling around the track ourselves until we’re moving parallel to him. 

But it’s here that our metaphor breaks down. Because a probe can utilize something no Earthly cyclist can: gravity assists. 

Theoretically, it would be possible to fire a forward-facing thruster as a probe approached Mercury, slowing it so much it could enter the planet’s orbit.

But counteracting the Sun’s gravity like that would require unbelievable amounts of energy. In fact, it would require more energy than it would take to send a craft outside our solar system. 

That means scientists need to get creative when trying to explore the Sun’s closest companion. 

And it’s this wild creativity that in part makes Bepicolombo – like Messenger before it – so utterly fascinating. 


Gravity Assist

Ever since The Martian came out, the concept of a gravity assist has been embedded in pop culture. The idea that a craft can use the massive gravity of a planet to speed itself up on its journey is now something even the layest of laymen understands. 

But The Martian only covered using gravity assists – also known as ‘gravity slingshots’ – to gain speed. They can also be used for slowing craft down. 

It’s this sort of gravity assist that’s essential for any probe wishing to orbit Mercury. 

The basic idea is that BepiColombo has quite a lot of ‘orbital energy’. In other words, it’s orbiting the sun at a fairly high speed. 

To interact with Mercury’s weak gravity, it needs to reduce that energy and slow down. 

By approaching something with massive gravitational pull – like a planet – in the right direction and at the right time, it can exchange some of that orbital energy, reducing its excess velocity.

But just doing this once wouldn’t be enough. Instead, visiting Mercury requires multiple flybys of planets, until enough energy has been robbed from the craft to stop it whizzing off to the bottom of our stellar cycling track. 

As the European Space Agency (or ESA), one of the two organizations behind BepiColombo, puts it: 

“The spacecraft left Earth with a hyperbolic excess velocity of 3.475 km/s.” The goal of the multiple flybys is to “lower the relative velocity to 1.84 km/s.”

By the way, the original scientist to realize any craft sent to Mercury would need to utilize gravity assists? The Italian mathematician Giuseppe Colombo, better known to his friends as “Bepi”. 

So, for those of you who’ve been wondering why the probe is called BepiColombo, there’s your answer.

Now, BepiColombo isn’t the first probe forced to undertake a super-complicated route to get to Mercury. NASA’s Messenger craft was launched in August 2004 and spent seven years looping around the inner solar system in a series of complex maneuvers, before finally settling into orbit in March 2011. 

Yet the fact it’s been done once before doesn’t make it any less difficult. 

Launched on October 20, 2018, from the EU’s space port in French Guiana, BepiColombo’s journey has involved some truly headache-inducing calculations. 

In 2020 alone, it performed flybys of both Earth and Venus – the first of these undertaken while the ground team were having to operate within the strict global lockdown caused by covid’s first wave.

2021 saw a second flyby of Venus, followed by an important step. On October 1, BepiColombo at last visited Mercury – on the 101st anniversary of Giuseppe Colombo’s birth. 

But this was just a flying visit to slow the craft. Not the beginning of orbital insertion. 

Since then, the probe has conducted a further flyby of Mercury, swinging past and capturing new images on June 23, 2022. 

Another four flybys are still required before it can enter orbit: one in 2023; two in 2024; and one in 2025.

Finally, on December 5, 2025, BepiColombo will have slowed so much that Mercury’s weak gravity will at last be able to capture it. 

Known as a “weak stability boundary capture,” the process will see the probe basically drift into Mercury’s embrace, ending its complex journey after seven long years. 

Yet, while most of the slowing to ensure it reaches Planet One will be carried out by gravity assists, it’s not complex calculations alone that will get BepiColombo where it’s going. 

Across its journey, the team will have to make constant tiny course adjustments, plus one final maneuver when the craft is at last captured by Mercury. But this won’t be accomplished with traditional chemical reactions, oh no. 

Instead, BepiColombo will be redirected using another of the probe’s impressive innovations.

The most-powerful electric ion propulsion system ever sent into space.


Ion Engines 

Although BepiColombo mostly consists of two probes – the ESA’s Mercury Planetary Orbiter (or MPO), and the Japan Aerospace Exploration Agency’s Mercury Magnetospheric Orbiter (or Mio) – there’s a third piece of kit included. 

The Mercury Transfer Module, designed and built by the ESA, is what will get the two probes to Mercury in the first place. The groovy space bus both will hitch a ride on. 

And while MPO and Mio get most of the attention, the Transfer Module is also a pretty funky feat of engineering. Not least because of its groundbreaking solar-powered electric ion thrusters.

Despite literally sounding like something from sci-fi – the name TIE Fighter in Star Wars is an acronym for “Twin Ion Engine” – ion propulsion has been around a long time. 

NASA first tested the technology in space way, way back in 1964. In 1998, the Deep Space 1 probe became the first to be primarily powered by ion thrusters.

But it was really in the 2010s, with the Dawn mission to the underrated dwarf planet Ceres, that ion propulsion came into its own; allowing Dawn to execute maneuvers that would otherwise have been impossible. 

Now, just a handful of years later, BepiColombo is demonstrating the most-powerful version yet.

A grid of four bunched together on one side of the craft (although only a maximum of two can be used at once), the ion thrusters are electric powered: fed by the huge, 15m solar array on the Module. 

To move the craft, they accelerate xenon gas inside an electric field, before blasting it out at high speeds.

As the xenon atoms go whizzing off into space, they create a push on the craft. Not much: equivalent to about the force required to hold up a piece of paper (around 125 milliNewtons). 

But, in the vacuum of space, even that push inevitably has an effect on the Module. And, if the ion thrusters are fired for a long time, they can significantly alter BepiColombo’s course.

Each of these sustained periods of ion propulsion is known as a “thrust arc”. The ESA plans to use 22 of them across the mission’s lifetime, to make course adjustments, and to facilitate the final insertion into Mercury’s orbit. 

If you’re thinking this is all just an excuse to show off some advanced tech, though, you’re missing the broader point. 

The fuel required to perform these adjustments with chemical propulsion would weigh so much that it would send the costs of the mission spiraling. By investing in advanced technology, the ESA is saving money.

Still, that’s not to say there weren’t significant engineering challenges.

Perhaps the biggest was dealing with the extreme cold the thrusters would experience during periods of inactivity.

It seems counterintuitive, right? BepiColombo is heading close to the Sun, so you’d expect it to get hotter and hotter. 

Of course, though, nature doesn’t quite work like that. While the side of the craft exposed to sunlight will have to deal with extremely high temperatures, the bits in shadow will shiver at -150C.

For those of you used to thinking in Fahrenheit, that’s extremely cold. So cold, that even your friend from Canada who insists on wearing t-shirts in winter would be all like “its chilly here, eh?”

More to the point, it’s cold enough to freeze xenon. Yet the ESA managed to get around this, creating ion thrusters that would function at “cold even for a Canadian” temperatures. 

In fact, the entire craft had to be engineered to withstand insane conditions. Not just the Mercury Transfer Module, but the MPO too.

The result was a complex system of sealed pipes, spreading through the craft like blood vessels.

Inside, liquid gets heated up on the sun-facing side, until it evaporates. It’s then carried to the shadow side, where radiating plates warm the equipment. 

Back in the cold, the liquid cools and condenses. Whereupon its cycled through to the sun-facing side, and the process starts all over again. 

Described like that, it sounds simple. But to make sure it works, scientists had to begin running tests all the way back in 2001. 

It was worth it, though. Fast forward to today, and both the MPO and Mio are headed for their target on schedule. Barring some unforeseen catastrophe, they should begin their scientific missions within the next couple of years. 

Which brings us to the next part of today’s post. Exploring the probes themselves.


The Twin Explorers  

As the first major joint effort between Japan and the supra-national European Space Agency, BepiColombo is a vast project. By one count, over 1,200 specialists from sixteen separate nations are involved.

That being said, both the ESA and JAXA have worked relatively independently. Hence the ESA taking charge of the transfer module and MPO, while the Japanese focus on Mio. 

So, let’s examine them one by one. 

The biggest of the two probes is undoubtedly the ESA’s Mercury Planetary Orbiter.

Weighing 1,150 kg (or 2,540 lbs), the MPO carries a whopping 11 scientific instruments. 

Some of these will replicate the investigations undertaken by NASA’s Messenger craft, simply at higher resolutions. Yet others, by contrast, will aim to answer longstanding questions. 

Perhaps the most interesting of all involves water ice. 

Despite being so close to the Sun that its surface roasts at over 400 C, several of Mercury’s craters are permanently in shadow, where temperatures drop – once again – to “crying Canadian” zones.

Deep in these craters, we’ve known for a long time that ice exists. Messenger all but confirmed that it’s water ice. Now, the MPO is intended to remove the “all but” from that sentence, and bring back final confirmation.

In this, it will be aided by two special instruments: the Mercury Gamma-ray and Neutron Spectrometer, and the Probing of Hermean Exosphere by Ultraviolet Spectroscopy. 

While both have multiple uses – the first will also investigate Mercury’s composition, while the second will try to understand the planet’s exosphere – the real hope is that they confirm Planet One as yet another world in our solar system where some form of water exists.

To do this, MPO will be getting seriously close to the surface. So close, that it will loop the planet once every 2.3 hours (a big contrast to Mio’s 9.3-hour orbit).

And that closeness will be useful for another instrument, too: the BepiColombo Laser Altimeter (BELA). 

In effect a super hi-res mapping device, BELA will scan the surface in extreme detail, creating a better model than even Messenger achieved. Yet this is still far from the most-interesting thing MPO will do.

There are also a ton of instruments devoted to Mercury’s magnetosphere. 

Unique among the non-Earth terrestrial planets, Mercury generates its own global magnetic field – something neither Mars nor Venus currently does. 

Now, this magnetic field is much weaker than Earth’s, and unevenly distributed. But it’s still there, and so scientists naturally want to study it. 

Hence MPO coming with something called the Magnetic Field Investigation (MPO-MAG), which does exactly what it says on the tin. 

Over the course of the mission, MPO-MAG will run tests on Mercury’s magnetic field; trying to figure out how the solar wind impacts it, and also how it got there in the first place. 

Again, we already have a pretty good idea thanks to Messenger. We know Mercury’s core makes up around 85% of its mass, and is likely either liquid or at least part-molten. 

But MPO-MAG is intended to confirm this. Just as the Mercury Orbiter Radio science Experiment (MORE) is intended to unearth the planet’s internal structure; while another instrument named SERENA will examine how the magnetosphere interacts with gasses in the exosphere. 

And these are just a handful of the instruments onboard MPO!

We haven’t even touched on the oddly named SYMBIO-SYS, which will examine the planet for signs of volcanism – a big deal since currently only a handful of worlds are confirmed to have active volcanoes. Nor have we covered the ISA, which will use Mercury to run tests on Einstein’s Theory of General Relativity. 

But time is short, alas, and we still need to pay attention to Japan’s probe: Mio. 


Magnetic Personality 

Clocking in at a mere 275 kg (or 606 lbs in obstinate imperial), Mio is the small, shrimpy one of the BepiColombo mission. The Edward Elric to MPO’s mighty Alphonse. 

Yet small doesn’t necessarily mean “useless”. And Mio packs a powerful science punch.

Designed to orbit high above Mercury, the long-form version of its name also gives away the craft’s primary goal. 

As the Mercury Magnetospheric Orbiter, JAXA’s probe is dedicated to teasing out the secrets of Mercury’s magnetic field. 

The intent here is to go beyond the separate readings MPO is taking, and build an in-depth understanding of the wider magnetosphere, including how it interacts with particles. 

That means Mio has been kitted out with five separate instruments: from a regular magnetometer; to two devices designed to investigate plasma; to one that will track the distribution and makeup of the dust swirling around Mercury. 

There’s even a Sodium Atmospheric Spectral Imager to investigate the sodium content of the exosphere. Because, frankly, what’s a good space mission without a probe sniffing around for traces of sodium? 

But the coolest part is that – since MPO and Mio have some similar instruments – we’ll be able to take similar readings for different sides of the planet at the same time. 

This will be like the difference between hearing music in mono or stereo. A means for rounding out our understanding and appreciation of Planet One. 

And all being well, the two probes should keep this up for over a year. 

As we end this post, then, it’s with an exciting mission underway in the skies far above us. A vast, brilliant, complex megaproject that will – hopefully – illuminate Mercury in whole new ways. 

Doubtless, BepiColombo lacks something of the sexy glamor around a true first like the James Webb Space Telescope. Lacks, too, the air of mystery that will accompany the Europa Clipper’s voyage to Jupiter’s ocean moon. 

Yet BepiColombo still represents a marvelous leap forward for our understanding of the solar system. A chance to, once again, get up close and personal with our least-visited neighbor. 

In doing so, we may yet uncover things beyond even our wildest imaginings.



ESA overview (multiple pages): https://sci.esa.int/web/bepicolombo/-/59934-spacecraft-duo 

ESA, Why gravity assists are needed: https://www.esa.int/Enabling_Support/Operations/Why_is_BepiColombo_back 

NASA, in-depth: https://solarsystem.nasa.gov/missions/bepicolombo/in-depth/ 

NYTimes, overview: https://www.nytimes.com/2018/10/19/science/bepicolombo-mercury-launch.html 

NASA, Mercury in-depth: https://solarsystem.nasa.gov/planets/mercury/in-depth/ 

Discover Magazine, the ion thrusters: https://www.discovermagazine.com/the-sciences/how-these-glowing-blue-thrusters-will-get-bepicolombo-to-mercury 

NASA, history of ion propulsion: https://solarsystem.nasa.gov/news/723/a-brief-history-of-ion-propulsion 

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