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

Tonight, Europe and Japan are set to launch a joint venture between Solar System and Planet Mercury. The launch marks the first mission to this tiny world for the two regions. The hope is to answer some lingering questions about the planet that were raised by 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 Mercury Planetary Orbiter, or MPO, which is designed to study the planet from orbit with a suite of 11 instruments. The contribution of the Japan Aerospace Exploration Agency (JAXA) is also an orbiter – the Mercury Magnetospheric Orbiter, or MMO – a vehicle that will be constantly spinning 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 dueling orbits at once, helping scientists paint a more vivid picture of Mercury. Planetary researchers are curious to know more about this tiny planet and more than 4.5 billion years of the Solar System. More, 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 than expected, and scientists want to know why that is. In addition, the planet has other weird features that just do not make sense when Mercury is located in the Solar System.

Mercury came to be. "It will give us much more insight into the formation of the Solar System, and especially what happens to the innermost planets – the terrestrial ones," Mauro Casale, the BepiColombo Development Manager at 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. NASA launched the MESSENGER spacecraft spacecraft.

While MESSENGER was at Mercury, NASA learned a lot. The space agency can be found in the world, where temperatures can exceed 800 degrees Fahrenheit, there may actually be some water in the planet's craters. Goal 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 does not 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 that was born at a distance from the Sun, comparable to 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 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 greater 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 sun, the swaths of particles constantly streaming from the Sun. Here on Earth, our thick atmosphere and magnetic field shield However, Mercury's atmosphere is incredibly thin and its magnetic field is weak, so it is constantly being bombarded with 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 designed to keep the vehicle cool. Additionally, BepiColombo's solar panels can not be directly linked to the Sun, or they'd overheat. The panels will actually rotate 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 direction is a big part of the BepiColombo mission. JAXA's orbiter is meant to spin perpendicular to Mercury, so it is not directly on the planet's extremely hot surface. Plus, the spinning will help you 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 is equipped with a very special tool: the largest radiator ESA could possibly fit the vehicle. The radiator is meant to reflect heat orbit when it is still moving.

And the ESA orbiter will be getting rather close. When MESSENGER was at Mercury, it was 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 will not go so far; it'll orbit between 300 miles and 930 miles, and it will get much closer to 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 has not been explored. Because of Mercury's close location to the Sun, spacecraft traveling to this region is being pulled into the Sun by the star's gravity and is constantly accelerating. Vehicles must repeatedly "brake" against this pull in order to slow down 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 not totally do the trick. BepiColombo will also be using the inner planets for help. The spacecraft is scheduled to fly over the next seven years, using the gravity of planets to adjust trajectory and slow down to get to Mercury. First, BepiColombo will flyby of Earth, then two of Venus, followed by six Mercury flybys. All of this should get the spacecraft on the right 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 to a transfer module. But at the planet, the transfer module will be detach, and the two orbiters will be captured by Mercury's orbit together. Eventually, the duo will be separated into their own orbits. Once that happens, JAXA will take control of its spacecraft and the ESA will take control of its own. Mercury's orbit, but 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 9:45 PM ET. Check back then to watch the live mission.