The forthcoming Proba-3 mission will generate artificial solar eclipses to examine the Sun.

Every 18 months or so, the Moon perfectly aligns with our view of the Sun, obscuring the face of the star. Although it lasts for a few minutes, a total solar eclipse is the perfect opportunity for scientists to probe the outermost part of the Sun’s atmosphere, known as the corona, which is usually hidden by the star’s light. An upcoming mission aims to recreate this naturally occurring event on a much more regular basis, with one spacecraft covering the Sun for the other.

The European Space Agency’s (ESA) Proba-3 is gearing up for launch on Wednesday at 5:38 a.m. ET. The double-satellite mission will liftoff from Satish Dhawan Space Centre in Sriharikota, India, riding on board the Indian Space Research Organization’s (ISRO) PSLV-XL rocket. The launch will be live-streamed on ESA Web TV, with the ISRO broadcast beginning around 30 minutes before liftoff.

Why are there two satellites?

While the two satellites will launch together, the pair are set to separate about 18 minutes after liftoff to begin an extremely precise demonstration of formation flying. The satellites will launch to an extremely elliptical orbit around Earth, flying at a distance of 492 feet (150 meters) from one another. The satellites must maintain that distance with an accuracy down to a single millimeter to pull off the mission.

“Proba-3 is very different because our satellites will be flying just one and a half football fields away from each other during active formation flying. And their relative positions will be maintained precisely to just a single millimetre for six hours at a time,” Damien Galano, Proba-3 mission manager, said in a statement.

The two spacecraft will form one giant virtual telescope, with one casting a precisely controlled shadow on the other, blocking out the Sun’s light from view so that the other spacecraft, equipped with an optical instrument, can view the star’s corona.

The mission’s active formation flying will take place at the top of its orbit, around 37,282 miles (60,000 kilometers) away from Earth. At that distance, Earth’s gravity won’t have as big of an effect on the spacecraft and less fuel will be required to shift their positions.

“Early simulations showed we would need to make so many positioning adjustments with our thrusters that our propellant would be exhausted swiftly; the mission would have been over in about half an hour!” ESA’s Frederic Teston said in a statement.

The spacecraft will take 19 hours and 36 minutes to complete one orbit around Earth, carrying out observations of the corona during a six-hour window of each orbit. Proba-3’s elliptical orbit takes it as close as 372 miles (600 kilometers) to Earth, going around in an elongated loop to expend as little fuel as possible during maneuvers. Ideally, the mission would be situated in one of the Sun-Earth Lagrange points, where the gravitational forces of the two bodies keeps an object in place in its orbit. But that would have required a larger budget; Proba-3 is considered a low-cost mission, according to ESA.

Why do we study the Sun’s corona?

The corona is a million times fainter than the Sun. The outermost region of the Sun’s atmosphere extends millions of miles into space, and hides the biggest mystery surrounding Earth’s host star.

The solar corona heats up to around 2 million degrees Fahrenheit (around 1 million degrees Celsius), which is roughly 200 times hotter than temperatures on the surface of the Sun. The high temperatures of the corona are rather counterintuitive, and astronomers are not sure how the outer layers of the Sun are heated.

Aside from its mysteries, the Sun’s corona also drives solar wind and coronal mass ejections, two main components that govern space weather. The charged particles in the corona escape into space, and can sometimes have an effect on satellites in orbit, as well as communication systems on Earth.

What are the Proba missions?

The Proba missions derive their name from a Latin word that implies the phrase, ‘let’s try,’ and also stands for PRoject for OnBoard Autonomy. The low-cost, small satellite missions seek to test new technologies and concepts in spaceflight.

ESA has launched three other Proba missions thus far, with one currently observing the Sun. Proba-2 launched in 2009, and was originally meant to spend two years in a Sun-synchronous low Earth orbit but is still going more than 15 years later.

Similarly, Proba-3 has an expected lifespan of two years. The limiting factor for the mission is propellant, with its cold gas thrusters required to make small pulses every 10 seconds during the satellites’ six-hour active formation flying phase.

For a technology demonstration mission, the team has high hopes for the pair of tiny spacecraft. “When I first heard about it, Proba-3 seemed like science fiction technology,” Andrei Zhukov, principal investigator of Proba-3’s main instrument, said in a statement. “But the real-life design will truly deliver excellent science.”