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At twilight, streaks from Starlink satellites could ruin about one-third of Vera C. Rubin Observatory images.


Tens of thousands of communications satellites could spoil view of giant sky telescope

Starting in 2022, the Vera C. Rubin Observatory will survey the entire night sky every few nights from a mountaintop in Chile, using a giant mirror to capture faint, fast-changing objects. But something much more mundane is likely to streak into view: thousands of low-flying communications satellites glinting in the Sun. The threat, evident ever since rocket company SpaceX launched its first batch of 60 Starlink satellites in May 2019, has come into sharper focus—as has a potential way to limit the damage.

Unpublished studies, some seen by Science, suggest satellite trails could ruin about one-third of the images from the Rubin Observatory during parts of the night. Observatory staff say the problem cannot be avoided with software tweaks or shrewd telescope pointing. The best remedy seems to be to darken the satellites themselves, something SpaceX and the Rubin Observatory are already exploring.

“Darkening is the name of the game,” says Tony Tyson, chief scientist of the Rubin Observatory. “I’m cautiously optimistic we will get there with SpaceX.” But with other companies starting to build similar megaconstellations, astronomers are racing to keep ahead of the problem.

After the first launch of the Starlinks, which aim to provide internet access to remote parts of the globe, skywatchers could see strings of bright dots with the naked eye as the table-size satellites spread out 290 kilometers up. “We were totally surprised by the brightness,” says Patrick Seitzer of the University of Michigan, Ann Arbor, a member of a working group set up by the American Astronomical Society to study the problem.

Just how big the problem will become is hard to predict. Companies keep tight hold of spacecraft design details, says aerospace engineer Hugh Lewis of the University of Southampton, who has adapted a space debris model to help understand the brightness of Starlinks. “Ideally we would want to know everything about the spacecraft, but that’s not the world we live in,” he says. Astronomers also don’t know how big the constellations will be: SpaceX, for example, is aiming for an initial constellation of about 1600 but has applied to the Federal Communications Commission to loft as many as 42,000.

Brightness measurements made by Las Cumbres Observatory, a global network of small telescopes, and others showed that the Starlinks dimmed as they rose to their working orbits 550 kilometers up. They are brightest when they’re low on the horizon, around twilight. Most telescopes can avoid the trails, because they observe high in the sky and have a narrow field of view. But long exposures, a large mirror, and a wide field of view all make a telescope vulnerable. “Combine two or three of these and you’re in trouble,” says Olivier Hainaut, who is studying the problem for the European Southern Observatory.

The Rubin Observatory, with an 8.4-meter mirror that will take pictures of the sky the size of 40 full Moons in 30-second exposures, “is the perfect machine for running into these satellites,” Tyson says.

He and his team conducted simulations that suggested the track of a satellite image across their camera would saturate each camera pixel as it passes, and cause leakage into neighboring ones. The resulting artifacts “cannot be removed in software. We have failed in doing that,” Tyson says. The team looked at altering schedules to avoid satellite trails, but with such a wide field of view, avoiding thousands of satellites would end up as “a wild goose chase,” he says.

So Tyson is pinning his hopes on SpaceX darkening its future satellites. He and his team speak several times a week with engineers at SpaceX, which launched one darkened satellite in January that is just now reaching its final orbit. Tyson’s team calculated that if the company can reduce reflections by a factor of 15, the issue will be manageable. Images would still contain trails, but they wouldn’t saturate pixels and could be removed digitally. SpaceX and its chief, Elon Musk, are “totally committed to solving this problem,” Tyson says, and his team has worked with them to “narrow to a design that may work.” Several satellites with this updated dark design will be launched in coming weeks. SpaceX did not respond to requests for comment.

Starlink is, however, just the first constellation to get started. “It’s safe to say, the number will rise dramatically,” says Connie Walker of the National Optical-Infrared Astronomy Research Laboratory, who chairs an International Astronomical Union commission looking at the threat. The company OneWeb launched 34 satellites on 7 February—a first installment in an initial constellation of 648. The satellites are smaller and orbit higher than Starlinks, so they are fainter, but their altitude means they’re illuminated by the Sun most of the night. “They’ll need to darken them,” says Tyson, who is in contact with OneWeb and other operators, including Amazon, whose Project Kuiper envisions 3200 satellites.

Tyson has his fingers crossed. The constellations won’t kill the Rubin Observatory, but they will make its job harder and could jeopardize its chances of making discoveries. “It’s there that the existential threat is,” he says.