The ESA's Proba-3 Satellites Will Fly in Tandem to Block Out the Sun

Along with the Gemini constellations, the Piccard twins ballooning their way into the stratosphere, and Mars’s pair of moons, outer space is going to get another cosmic duo.

The European Space Agency, led by the Spanish engineering company SENER, plans to launch a pair of satellites that will operate as one. Though they may be as far apart as 150 meters, the satellites will be precisely placed with “relative position accuracy to within less than one millimetre," according to Salvador Llorente, the director of this project, which is called PROBA-3.

The plan is to launch the two satellites sometime in 2017, and have them orbit the Earth in an off-center orbit. At their closest pass to the Earth, the satellites will be roughly 375 miles up. At their farthest, they’ll be some 37,000 miles away, and this is where the two really tango.

At their peak, or apogee, where the influence from the Earth’s gravity is the weakest, the ESA hopes to arrange the satellites with the utmost of precision. Their initial plans revolve around using one satellite—nicknamed the ‘Blocker’—to block out the Sun for the other, creating an artificial eclipse that allows the satellite twin—called the ‘coronagraph’—to record the event.

While the artificial eclipse isn’t exactly new territory—NASA and the Soviets pulled one off in 1975 during the Apollo-Soyuz odd coupling—this level of precision is unprecedented. If such incredibly scrupulous spacecraft placement and formation flying can be achieved and maintained, then the possibilities get really exciting: two-satellite telescopes, zipping around creating a focal length that’s hundreds of meters long, staring deeply into space, for instance.

Just gaining agility and precision when moving spacecraft around from Earth (without crashing them into each other) holds a lot of promise. The more things that can be assembled in space, the easier it is to get them into orbit, since you’re spared the agony of launching one big, heavy payload all at once.

The details of the PROBA 3 mission were just published in the journal Acta Astronautica. With the launch still four years away, the ESA hasn’t even chosen how they’ll get the satellites into space, much less all that they can achieve while there.