Two days after Elon Musk's SpaceX launched 60 satellites in May as part of a mission to bring quick Internet service to people worldwide, astronomers noticed something different.
As some of the satellites zipped past the Lowell Observatory in Flagstaff, Arizona, telescopes trained on the night sky captured streaks of reflected sunlight that marred their view of a far-off star system.
Astronomers now worry that the vast number of communications craft planned, including nearly 12,000 of Musk's Starlink fleet, will shine so brightly that they'll interfere with research that depends on delicate visual observations of distant galaxies and nearby asteroids. The new satellites will fly lower than many traditional craft, and will arrive in unprecedented numbers - all told, more than double the roughly 5,000 satellites that are circling Earth now.
"We just happened to be pointed in the right direction, and Starlink flew right through it" on May 25, two days after launch, said Jeffrey Hall, director of the Lowell Observatory. The unexpected appearance helped to signal that, as Hall put it, "this is potentially a problem."
Musk's Space Exploration Technologies Corp. is authorized to launch 11,943 satellites for its Starlink fleet, making it by far the leader in a total of nearly 13,000 low-Earth orbit satellites currently approved by the Federal Communications Commission, which coordinates trajectories and radio-frequency use. In addition, Amazon.com's Jeff Bezos on Thursday filed to place 3,236 Internet-beaming satellites into low-Earth orbit.
The lower trajectories offer minimal lag time for data to bounce between the ground and the spacecraft, overcoming the signal lethargy that's limited Internet-from-space schemes dependent on traditional communications satellites. Those older craft are parked some 22,000 miles above the Earth, an altitude that lets them appear to hover in one spot.
At low-Earth orbit — altitudes of just 112 to 1,200 miles — satellites need to race around the globe to stay aloft, completing orbits in as little as 90 minutes. As one moves toward the horizon it will pass signal duties off to the next satellite coming by.
Many satellites are needed if continuous, widespread coverage is the goal — thus the constellations planned by Musk and others.
There are 1,338 satellites in low-Earth orbit, according to a database compiled by the Union of Concerned Scientists. NASA, the U.S. space agency, tallied 4,972 satellites in its most recent count of payloads that are active and defunct.
The number of stars visible to the unaided human eye isn't much more than 1,628, which is how many are registered at the 5th magnitude of a brightness scale used by scientists, Robert Zinn, an astronomer at Yale University, said in an email. Abnormally favorable conditions could yield more.
Plans for low-flying satellite fleets have been around for years. The realization that they might startle sky-watchers seems novel. A video of the Starlink satellites floating in a train across the sky has attracted more than 1.3 million views on the Vimeo video-sharing site. AndMusk's public statements have varied.
"Sats will be in darkness when stars are visible," Musk tweeted May 25.
Later he tweeted, "We'll make sure Starlink has no material effect on discoveries in astronomy." Musk added that he'd sent a note to the Starlink team about "albedo reduction," or cutting the proportion of light reflected from the spacecraft.
Astronomers are studying the extent of the problem, said Pat Seitzer, former chair of the Committee on Light Pollution, Radio Interference, and Space Debris at the American Astronomical Society.
The satellites may be less bright once moved into planned higher orbits, and their visibility may vary with the seasons: their altitude means they'll stay out of the Earth's shadow and remain in sunlight even after dusk for a longer period in the summer than the winter.
"Our concern is just how bright they might be," said Seitzer, an astronomer at the University of Michigan.
Astronomers who use radio telescopes that rely on the non-visible spectrum also may be affected. They'll need to adjust to a sky full of low-orbiting satellites, said Harvey Liszt, spectrum manager for the National Radio Astronomy Observatory based in Charlottesville, Virginia.
"We will have to learn how to operate our electronics to detect weak cosmic signals in the presence of satellite signals at other frequencies that will be millions of times stronger," Liszt said by email.
The proposals for fleets serving up broadband from space fit a terrestrial policy imperative: to expand high-speed Internet service to people and places left poorly served by traditional communications providers.
It's not clear who can help if scientists determine the fleets of tomorrow will interfere with multimillion-dollar telescopes that can detect objects millions of times dimmer than those visible with the naked eye.
NASA doesn't regulate orbits or the spacecraft that enter them, said a spokesman . The Federal Aviation Administration regulates the safety of commercial launches and doesn't regulate satellites, said a spokesman.
The legal arena "is really the Wild West" and "international space law doesn't really deal with this use of outer space at all," said John Barentine, public policy director for the International DarkSky Association, which works to protect night vistas from light pollution.