Two spacecraft are embarking on a seven-year journey to decode Mercury’s mysteries

Tonight, Europe and Japan are set to launch a joint spacecraft that will travel more than seven years to one of the least-explored places in our Solar System: the planet Mercury. The launch marks the first mission to this tiny world for the two countries. The hope is to answer some lingering questions about the planet that were raised by the last probe we sent there, NASA’s MESSENGER spacecraft.

The mission, named BepiColombo, consists of two spacecraft in one package. The European Space Agency (ESA) has built its own vehicle called the Mercury Planetary Orbiter, or MPO, which is designed to study the planet from orbit with a suite of 11 instruments. The contribution from the Japan Aerospace Exploration Agency (JAXA) is also an orbiter — the Mercury Magnetospheric Orbiter, or MMO — a vehicle that will constantly spin as it revolves around Mercury. The two spacecraft will travel to Mercury together and then split apart, with each vehicle entering the planet’s orbit individually.

The pair will maintain dueling orbits at once, helping scientists paint a more vivid picture of Mercury. Planetary researchers are curious to know more about how this tiny planet formed and evolved over the 4.5 billion years of the Solar System, and studying Mercury’s core and exterior should help. Plus, Mercury exhibits some strange behavior that scientists want to know more about. For one thing, the planet seems to be shrinking as its core cools; it’s also super dark, more so than expected, and scientists want to know why that is. Additionally, the planet has other weird features that just don’t make sense considering where Mercury is located in the Solar System.

Decoding all of these mysteries could bring scientists much closer to understanding how Mercury came to be. “It will give us much more insight in the formation of the Solar System, and especially what happens to the innermost planets — the terrestrial ones,” Mauro Casale, the BepiColombo development manager at the ESA, tells The Verge. “So better understanding what happened to our system, but also we can extrapolate this to the extrasolar systems outside our Solar System.”

Image: NASA

Of the innermost rocky planets in our Solar System, Mercury has been visited the least. Only two probes have ever traveled to the planet. The first was Mariner 10, a spacecraft meant to study both Venus and Mercury. It flew by Mercury three times in 1974 and 1975. Then, in 2004, NASA launched the MESSENGER spacecraft, which successfully inserted itself into Mercury’s orbit in 2011. The probe spent four years circling the planet before NASA intentionally crashed the vehicle into Mercury’s surface.

While MESSENGER was at Mercury, NASA learned a lot. The space agency was surprised to find that on this world, where temperatures can exceed 800 degrees Fahrenheit, there may actually be water ice in the planet’s craters. But MESSENGER raised some questions, too. For instance, the spacecraft found weird surface feature depressions that are unique to Mercury known as “hollows,” but the process that forms them is unclear. MESSENGER also found that the center of Mercury’s magnetic field seems to be shifted northward — another unexplained feature. And the planet has a weird concentration of gases in its atmosphere that doesn’t make sense. “For a planet so close to the Sun, it’s not justified,” says Casale. “It looks like this would be compatible with a planet which was born at a distance from the Sun comparable with the one of Mars and then moved closer to the Sun later on.”

Now, the ESA and JAXA hope to pick up where MESSENGER left off. “MESSENGER did a great job, discovered many things, but most of these things have not been explained completely yet,” Casale says. “So the job of BepiColombo will be to go a little bit farther with what MESSENGER was able to do.” Combined, BepiColombo’s two orbiters have more instruments than NASA’s old spacecraft, and they also contain more up-to-date technology.

Additionally, the ESA and JAXA have gone to great lengths to ensure that their spacecraft remain at a working temperature while at Mercury. Since it’s so close to the Sun, the planet experiences major temperature swings, ranging from more than 800 degrees Fahrenheit to -279 degrees Fahrenheit. Solar radiation in this region is about 10 times the amount we get at Earth. And there’s very little protection from the solar wind, the swaths of charged particles constantly streaming from the Sun. Here on Earth, our thick atmosphere and magnetic field shield us from much of the solar wind. However, Mercury’s atmosphere is incredibly thin and its magnetic field is weak, so charged particles are constantly bombarding the surface of Mercury at super high speeds.

Image: ESA

It’s a harsh environment that can fry an ill-prepared spacecraft. That’s why BepiColombo’s most important parts — its solar panels, antennas, and sensors — are covered in a special protective coating meant to keep the vehicle cool. Additionally, BepiColombo’s solar panels can’t point directly at the Sun, or they’d overheat. The panels will actually have to rotate away from the Sun. They’ve been built with a large surface area, though, so they can still get enough light to power the spacecraft.

In fact, strategic pointing is a big part of the BepiColombo mission. JAXA’s orbiter is meant to spin perpendicular to Mercury, so the top and bottom of the spacecraft are never pointing directly at the planet’s extremely hot surface. Plus, the spinning will help even out the temperatures on the spacecraft during its orbit. Comparatively, the ESA’s orbit will be pointing its instruments at Mercury’s surface, but it’s equipped with a very special tool: the largest radiator the ESA could possibly fit on the vehicle. The radiator is meant to reflect heat so that the orbiter can still grab valuable data when flying low over Mercury’s hot surface.

And the ESA orbiter will be getting rather close. When MESSENGER was at Mercury, it was in an extremely elliptical orbit, one that took the spacecraft between altitudes of 124 miles and 9,420 miles above the surface. The BepiColombo orbiters will also be elliptical, but the ESA’s orbiter won’t go so far out; it’ll orbit between 300 miles and 930 miles, and it will get much closer over the southern hemisphere than MESSENGER did.

But before all this can happen, the duo has to get into Mercury’s orbit. And that’s going to be a challenge. A big reason why Mercury hasn’t been explored much is because getting to the planet is massively difficult. Because of Mercury’s close location to the Sun, spacecraft traveling to this region are being pulled into the Sun by the star’s gravity and are constantly accelerating. Vehicles must repeatedly “brake” against this pull in order to slow down enough to reach Mercury and get into its orbit. BepiColombo is equipped with ion thrusters, which propel spacecraft by accelerating ions with electricity. These thrusters will help to slow down BepiColombo once it leaves Earth.

But ion thrusters alone can’t totally do the trick. BepiColombo will also be using the inner planets for help. The spacecraft is scheduled to do nine flybys over the next seven years, using the gravity of planets to adjust trajectory and slow down to get to Mercury. First, BepiColombo will do a flyby of Earth, then two of Venus, followed by six Mercury flybys. All of this should get the spacecraft on the right course to enter Mercury’s orbit at the end of 2025.

And then it will be time for the two spacecraft to separate and get into Mercury’s orbit. Up until reaching Mercury, the two orbiters will travel together attached to a transfer module. But at the planet, the transfer module will detach, and the two orbiters will be captured by Mercury’s orbit together. Eventually, the duo will separate and travel to their own intended orbits. Once that happens, JAXA will take control of its spacecraft and the ESA will take control of its own. The two are slated to last for up to a year in Mercury’s orbit, though it’s possible the agencies may extend the missions for another year.

It’s a long way to get to that point, and it all starts with a launch. The combo BepiColombo spacecraft is set to go up tonight on an Ariane 5 rocket out of Europe’s South American spaceport in French Guiana. Liftoff is scheduled for 9:45PM ET. Check back then to watch the mission live.