Final preparations have begun for a landmark space mission that will use satellites flying in close formation to create artificial solar eclipses high above the Earth.
The Proba-3 mission is the European Space Agency’s first attempt at precise formation flying in orbit and calls for two spacecraft to loop around the planet in an arrangement that never deviates by more than a millimetre, about the thickness of a human fingernail.
All being well, the spacecraft will blast off from India’s Satish Dhawan Space Centre in Sriharikota, on the Bay of Bengal coast, at 4.08pm local time (10.38am UK time) on Wednesday. After a four-month voyage, the probes will reach a highly elliptic orbit that swoops as close as 370 miles to Earth before swinging out for more than 37,000 miles.
“It’s an experiment in space to demonstrate a new concept, a new technology,” said Damien Galano, the Proba project manager at ESA. “It’s very challenging because we need to control very well the flight path of the two spacecraft.”
If the satellites operate as intended, they will line up with the sun such that the lead spacecraft casts a carefully controlled shadow on its partner, allowing instruments on the latter to measure the sun’s corona, the outer layer of its atmosphere.
Traditionally, scientists have studied the sun’s ring-like corona during solar eclipses, when the moon blocks enough of the sun’s glare to make the corona visible from Earth. The work requires scientists to chase eclipses around the world, often for only minutes of observation time, or none at all if the view is obscured by cloud.
The €200m (£166m) Proba-3 mission promises to transform scientists’ understanding of the corona by producing 50 artificial solar eclipses a year, each lasting six hours. The lead spacecraft carries a 1.4-metre wide occulter disc to block the sun as seen from the second spacecraft, turning the pair into a 150-metre-long instrument called a coronagraph.
Data from the mission should shed light on the longstanding mystery of why the corona is so much hotter than the sun itself; the sun’s surface is about 5,500C, but the corona can exceed 1mC.
By better understanding the corona, scientists hope to improve their predictions of solar weather, coronal mass ejections – where pulses of plasma and magnetic field burst into space – and solar storms, which can damage spacecraft and cause power outages and communications blackouts on Earth.
The Proba-3 spacecraft will swing around the planet once every 19.7 hours for two years. For six hours in every orbit the satellites will fly in formation, drawing on optical sensors and flashing LEDs to locate one another, and a precision laser system to automatically finesse their distance and orientation. The first images from the mission are expected as soon as March 2025.
Beyond the mission’s main goals, ESA scientists have set aside time to test manoeuvres that may be helpful in the future to service faulty satellites or remove “uncooperative” hardware and debris from orbit.
Formation flying could usher in a new era of space-based observatories and instruments by having multiple spacecraft work together in precise configurations. “If we would be able to have several satellites close to each other in an absolute, accurate, precise formation, we would be able to assemble larger instruments that are composed out of several satellites,” said Dietmar Pilz, the director of technology at ESA. These multi-satellite instruments could be used to study the climate crisis, objects in the solar system, and more distant planets around faraway stars.
“We all know that the launchers have been increasing in their power and the masses that they can bring into space,” said Pilz, in reference to the heavy payloads carried by modern rockets. “But no matter what you do, there’s always a limit.”