内容摘要：Most deep space missions send spacecraft hurtling far from the sun, into the frozen unknown hundreds

Most deep space missions send spacecraft hurtling far from the sun, into the frozen unknown hundreds of millions of miles beyond Earth. But on Friday, the European Space Agency (ESA) will blast two orbiters to heavily cratered Mercury, the planet closest to the sun.

Liftoff of the two craft — collectively called BepiColombo — is scheduled for 9:45 p.m. ET on Friday. It can be watched live here.

A European Ariane 5 rocket will lift BepiColombo into space. The two craft will then spend seven years traveling through the void before arriving at their Mercury destination, a metallic outpost in a relatively warm part of the solar system.

Read on for the vital facts about this monumental excursion.

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1. Why they're visiting Mercury

Radiation-blasted Mercury is still a little explored place.

Only two NASA spacecraft, Mariner 10 and Messenger, have visited Mercury before, with Messenger intentionally crashing into the planet in 2015.

By improving our understanding Mercury's composition, atmosphere, and magnetism, scientists can better understand how rocky-Earth-like planets came to be — and provide insight into whether other far-off solar systems may have formed similar planets.

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"Exploring Mercury is therefore fundamental to answering important astrophysical and philosophical questions such as 'Are Earth-like planets common in the Galaxy?'," the ESA writes.

2. Why the name is "BepiColombo"

Like many great spacecraft — here's looking at you, Hubble and Kepler — BepiColombo is named after a scientist.

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"The mission is named after the Italian mathematician and engineer Giuseppe (Bepi) Colombo (1920–84)," ESA said in a statement.

"He is known for explaining Mercury’s peculiar characteristic of rotating about its own axis three times in every two orbits of the Sun."

Now Colombo's namesake is heading to the planet he helped us all get to know a little better.

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3. Why there are two spacecraft involved

BepiColombo is made up of two different spacecraft with two very different jobs.

One craft, called the Mercury Planetary Orbiter, will carry 11 scientific instruments to the small planet; they're all focused on mapping Mercury and the space environment that surrounds it.

The other spacecraft, called Mercury Magnetospheric Orbiter, was built by Japan's space agency and will focus on studying the planet's magnetic environment.

4. How BepiColombo will deal with being so close to the sun

BepiColombo's sensitive instruments will encounter temperatures rivaling those in a pizza oven. But ESA has a plan to keep the two spacecraft nice and cool.

"To cope with this, the spacecraft’s external items, such as the antennas, solar arrays, Sun sensors, and multilayer insulation, have temperature-resistant outer layers and protective coatings, which were individually qualified to prove their capability," ESA said in a statement.

The Mercury Planetary Orbiter will also come equipped with a radiator designed to reflect heat from the spacecraft, keeping it functioning even when close to the sun.

5. How long it will take

It's not exactly a quick trip to the closest planet to the sun.

Once BepiColombo launches, it should travel through space for a little over 7 years before making it into orbit around Mercury.

Before beginning its orbit of Mercury in 2025, the spacecraft will flyby Earth in 2020, flyby Venus in 2020 and 2021, and then perform six different flybys of Mercury before making it into orbit.

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