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A report warns that fleets of new communication satellites in low-Earth orbit will spoil some astronomical observations, even if all known mitigation strategies are taken.

NOIRLab/NSF/AURA/P. Marenfeld

Report suggests ways to avoid satellites ruining telescope images but ‘there is no place to hide’

Astronomers and the operators of new, thousands-strong constellations of low-orbiting satellites will have to work together to prevent them from having a devastating impact on ground-based observations of planets, stars, and other celestial objects, says a report released today. Even then, there is no escaping some harm from the fleets of commercial orbiters. “All optical and infrared observatories will be affected to some degree,” astronomer Anthony Tyson of the University of California, Davis, said at a briefing on the report. “No combination of mitigation will eliminate their impact,” added astronomer Connie Walker of the U.S. National Optical-Infrared Astronomy Research Laboratory. 

The report is the product of a virtual workshop that brought together 250 astronomers, engineers, and satellite operators in late June and early July. The satellites, designed to provide internet access in remote areas, now number in the hundreds. But following the launch of the first batch by SpaceX in May 2019, astronomers were alarmed by how bright they appear in the sky. If all the planned constellations go ahead, the number of satellites will grow beyond 100,000. Observatories, already sited in remote locations to avoid light pollution, will have no way to avoid their effects, Phil Puxley of the Association of Universities for Research in Astronomy says. “There’s no place to hide.”

Since last year, there have been a number of independent studies of constellations’ possible impact. But the workshop’s report is the first time the satellite companies and those who would be affected, from astronomers to the agencies that fund them and their telescopes, have pooled their results and worked out a strategy for the future.

Once SpaceX’s first Starlink satellites reached orbit, astronomers quickly realized the upcoming Vera C. Rubin Observatory in Chile, set to begin operations next year, would be worst affected. With its combination of a wide field of view and sensitive 8.4-meter mirror, “It’s a perfect machine to run into these things,” says Tyson, the project’s chief scientist. The satellite trails are very bright and out of focus slightly, so they’re wide and cover several pixels on images taken with a test version of the Rubin Observatory’s camera, he notes. “It complicates data analysis and reduces discovery.”

Other observatories will likely be less affected but the report from the workshop, known as Satellite Constellations 1, said telescopes that need to work during twilight will also run into problems. Because the satellites are in low orbits, they will often be seen close to the horizon and will be most visible when they are still in sunlight but the observer is not. Searches for asteroids that potentially threaten Earth and fast-changing astronomical phenomena, such as visible signals from gravitational wave events, often must operate in those conditions.

If, however, satellite operators choose orbits above 600 kilometers, the situation gets worse because then their spacecraft are visible for more of the night, and in summer, all night long. One operator, OneWeb, planned to use 1200-kilometer orbits. Although the company filed for bankruptcy earlier this year, plans are afoot to revive it. According to Patrick Seitzer of the University of Michigan, Ann Arbor, if OneWeb launches its full 47,000-strong constellation, then every 30-second exposure of the Large Magellanic Cloud, the largest satellite galaxy around our Milky Way, would have at least one satellite flare in it.

The workshop participants made a number of recommendations to reduce the impact of constellations, including keeping orbits below 600 kilometers, darkening satellites, or controlling their orientation in space to reduce reflections (something SpaceX is already testing). Astronomers should also develop software tools to remove satellite trails from images, and the companies should make accurate orbital data available for their orbiters so telescopes can try to avoid them. The only measure the report could offer to totally eliminate the damage to astronomy was to launch fewer or no low-orbiting satellites—likely not an option given the financial investments the companies are making in the constellations and the lucrative market they foresee.

Tyson and his colleagues have been working with SpaceX engineers to modify their satellites to reduce their brightness. “We have explored all of the parameter space allowed by physics,” he says. “Now we have to work out which is the most effective.” His team has also modeled trying to steer the Rubin Observatory telescope to avoid passing satellites, but Tyson says there are simply too many. “It gets jammed. You can’t avoid them.” Astronomers are already actively pursuing image processing solutions but, he says, “the jury’s out,” on how much that can help.