BepiColombo: a mission to Mercury

20.6.2019
The European Space Agency probe was sent to study Mercury and solar flares. The researchers are expecting data in six years.

With nearly three hundred travel days behind it, BepiColombo has already sent a large number of space selfie broadcasts to delight its audience on Earth. Researchers are relieved, as are many physicists and programmers: BepiColombo’s (@BepiColombo) journey is going according to plan.

Last October, at the launch site in French Guiana, about a hundred kilometres north of the Brazilian border, tensions were high. Shooting a carrier rocket always comes with certain risks. However, this time the stakes were particularly high, as the expedition to Mercury is the most extensive endeavour ever conducted by the European Space Agency ESA. The preparations for the expedition begun before the turn of the millennium.

A hole in space

Even though the most difficult part is now over, the probe will not reach its destination for another six and a half years. There are still many twists and turns to come on the way to the most valuable data: the probe’s route to the innermost planet in our solar system is a winding one.

Next autumn BepiColombo will still fly past Earth before embarking on the first interplanetary leg of its journey. After that, it will perform two flybys of Venus, and loop around Mercury six times before being able to place its equipment in suitable positions for research purposes. The probe will travel considerable distances, even though the direct route to Mercury is not much longer than to Mars.

One of the challenges of the trip has to do with Mercury’s greater rotational velocity in comparison to that of Earth. The probe will have to overcome a significant change in speed. Astronomers and physicists describe the probe reaching Mercury as popping into a gravitational hole.

The flight route has been planned in a way that allows the probe to adjust its speed with the help of the gravitational pull of the planets it flies by. This will keep the amount of energy spent on the voyage within reasonable limits. BepiColombo is also equipped with the strongest ion motor ever launched into space, which propels the probe forward by shooting out xenon ions.

It takes two

In fact, BepiColombo is not a single probe but a close-knit duo of probes: Bepi from the ESA and Mio from the Japan Aerospace Exploration Agency, which are both travelling on the same satellite and working in concert. Once at the destination, Bepi will be positioned slightly closer to Mercury than its Japanese counterpart.

The duo is carrying a total of sixteen research instruments. One of them, an instrument called SIXS, was constructed almost entirely in Finland. SIXS measures the Sun's X-ray radiation and electron and proton emissions. It was built under the supervision of Juhani Huovelin, docent of astronomy and space physics, who leads the Finnish efforts in the BepiColombo project.

Another central figure in the project is Karri Muinonen, professor of astronomy at the University of Helsinki. Together with the principal investigator at the University of Leicester, Muinonen has been working on the MIXS spectrometer that observes the surface of Mercury.

The instruments packed in BepiColombo will be able to measure nearly the full electromagnetic spectrum, from radio waves to X-rays. Many of the instruments travelling with BepiColombo are improved versions of the equipment attached to Earth’s previous explorer to Mercury, NASA’s Messenger.

Another strength of the current expedition is the power of cooperation: both the Mio-Bepi probe duo and the MIXS-SIXS instrument duo are designed to grasp the cause and effect relations of various phenomena in space. Two is better than one.

Heavenly short circuit

With its weak magnetic field and proton-bombarded surface, Mercury is the perfect natural laboratory for plasma physics. The solar wind changes the surface of the planet at a dizzying rate while pushing huge amounts of particles into space.

The mission of Bepi and Mio is not just restricted to Mercury. If all goes according to plan, they will also accrue observations about the heliosphere, the Sun’s immediate sphere of influence. In particular the hope is to gain data about solar flares, which occur when the electromagnetic field of our closest star “short circuits”.

When we learn to understand the continuum of solar flares as distinct eruptions, we will also be able to better analyse their impact on Earth.

What on Mercury might appear to be balmy solar weather or just a grey day may become a destructive anomaly by the time it reaches Earth’s magnetic field. Magnetic storms result in impressive Northern Lights, but they can also break satellites and disrupt radio communications and satellite navigation.

“We will be able to glean entirely new information about the Sun’s corona, as this will be the first time we will have a long measurement series from such close quarters,” Huovelin says.

Map of elements

Muinonen is especially interested in the planet itself. In particular, he is looking forward to news on Mercury’s elemental composition, which is being analysed specifically through MIXS and SIXS. Before 2026, when they will be able to start putting together a map of the elements, any guesses about the minerals on Mercury will be complete shots in the dark.

We do not even know how the planet came to be. With its rough cratered surface and next to no atmosphere, Mercury is reminiscent of our Moon – they are even of a similar size. However, the structures of these two celestial bodies are radically different. Most of the Moon consists of a stone crust while Mercury has a massive iron core.

Why does Mercury lack a crust? We do not know. Neither do we have any idea why Mercury has a magnetic field, which smaller planets usually do not have.

Muinonen also has some specific questions about the elements. “One interesting question is whether we will find any organic material on Mercury. Will it have carbon from comets or asteroids?”

To conduct the elemental analysis, the probes will have to record images of every part of the planet several times. "Although if there's a particularly strong eruption, we might be able to pull together the entire map at once," the professor muses.

The probes store the information they collect for a month, during which the researchers will request that it transfers the data on any interesting moments back to Earth. It would be impossible to transfer all of the data, as the radio transmitter is much slower than a mobile phone.

1 + 1 years?

"The idea is to conduct measurements on-site for one year, but it would be possible to continue for a second one. And we might not even stop then if the equipment still works," Juhani Huovinen says of the lifespan of BepiColombo.

The mission has taken up just over €1.5 billion’s worth of work and technology in Europe, with Finland investing €20 million. The goal has been to make the best devices possible.

However, extreme conditions reduce the potential use-time of the equipment: the intensity of the barrage of particles and solar radiation on the innermost planet is nearly ten-fold to that on Earth. Temperatures on Mercury vary between roughly -200 and +400 centigrade, depending on the geology and time of day.

BepiColombo will try to beat these temperatures with mirrors that reflect solar radiation, but solar winds will be a significant strain on the equipment already during the approach flybys.

So perhaps Huovelin and Muinonen might do well to postpone for a few years the party they solemnly announced during the tense moments in September. Since there was no firestorm of exploding equipment when the probes were launched into space from French Guiana, they have vowed to set up a real, planned firework display. 

This article was published in Finnish in the Y/05/19 issue of the Yliopisto magazine.