The tough fight of astronomers to protect themselves from satellite constellations

A new attraction appeared in the night sky in 2019 after the first launches, in clusters of sixty, of machines from Starlink, Elon Musk’s company which provides Internet from space via a megaconstellation of satellites. The latter, settling into their low orbit, a few hundred kilometers from Earth, followed one behind the other and formed dotted lines on the celestial jet. Even though the Sun had just set, it was still close enough to the horizon for its rays to reflect off Starlink’s satellites. The spectacle was striking, but it dismayed astronomers.

Indeed, their telescopes produced photos scratched by the passage of spacecraft trains. With the prospect of Starlink putting up to 42,000 satellites into orbit, not to mention the scheduled launch of other megaconstellations, the danger was real of losing the so-called “astronomical” sky, spoiled by light pollution falling from space. Although most scientists operate instruments focusing on tiny parts of the cosmos, with little risk of a satellite crossing the field of view, the community was concerned about a major future facility: the Vera-Rubin Observatory, whose construction cost, in Chile, is around 600 million dollars (around 570 million euros), must enter service in 2025.

Equipped with a large field, it will “scan” the entire southern sky every three days. The Vera-Rubin’s main mission is, in fact, to identify everything that changes or moves in the cosmos, from the supernova – the explosion of a star – which lights up in another galaxy to the asteroid which approaches the Earth. A fortiori, it will detect all artificial satellites. Simulations have shown that, taking a thousand photos per night with a pause time of 30 seconds, the Vera-Rubin will see 10% of its photos spoiled by the passage of spacecraft. And this rate will exceed 50% at dusk and before dawn. However, these simulations are only based on a number of 40,000 satellites, which risks being much lower than the reality of the future, given all the megaconstellations planned.

Reduce shine

Since 2019, the community of astronomers has therefore mobilized to try to keep this pollution within acceptable proportions. “We reached out to the constellation operators to discuss”, explains Eric Lagadec, astrophysicist at the Côte d’Azur Observatory in Nice. The first idea was to reduce the brightness of the satellites as much as possible. Starlink has thus modified the coating of its machines and the orientation of their solar panels so that less light is reflected towards the ground. But these laudable efforts are not enough, quite simply because of the extreme sensitivity of modern telescopes: “darkened” satellites remain millions of times brighter than the targets tracked by observatories.

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