Astronomers renew concerns about Starlink satellite brightness

PASADENA, Calif. — As SpaceX gears up for another launch of Starlink satellites, astronomers are concerned the company maybe backsliding in its efforts to reduce the brightness of those satellites.

A Falcon 9 is scheduled to lift off at 12:08 p.m. Eastern June 17 from Kennedy Space Center’s Launch Complex 39A, placing 53 Starlink satellites into orbit. This mission will bring the total number of Starlink satellites launched to more than 2,700, with more than 2,450 in orbit.

These satellites, like several hundred before it, are version 1.5 of the Starlink design. Those satellites lack visors that SpaceX installed on satellites in 2020 to keep sunlight from reaching reflective surfaces on the satellites and thus reduce their brightness as seen from the ground. The visors were not compatible with the laser intersatellite links installed on the V1.5 satellites.

Astronomers say they’ve noticed the V1.5 satellites are brighter than the earlier “VisorSat” Starlink satellites. During a panel discussion at the 240th Meeting of the American Astronomical Society (AAS) here June 13, Pat Seitzer, an astronomer at the University of Michigan who studies satellite brightness, said the VisorSats were at magnitude 6.5. That brightness was near the recommendation set by astronomers of being no brighter than magnitude 7 to minimize interference with astronomical observations.

However, the V1.5 Starlink satellites are about half a magnitude brighter than the VisorSats. “In a real sense, we’re going backwards here,” he said. “We have to talk to SpaceX and see what their eventual plans on this are.”

A bigger concern is the second generation of Starlink satellites. Those satellites, designed to be launched on SpaceX’s Starship, will be significantly larger and, thus, potentially brighter. “It’s anybody’s guess what the brightness will be,” Seitzer said. “Hopefully they can incorporate all of the lessons they’ve learned so they don’t end up a factor of four brighter than they are now.”

In a presentation last month to the Federation of Astronomical Societies, David Goldstein, principal engineer at SpaceX, said the company was working on new technologies to mitigate the brightness of the second-generation Starlink satellites. That includes development of a “dielectric mirror sticker” to place on the satellites to reflect sunlight away from the Earth.

He said that approach would make the satellites 10 times dimmer than if they were coated with vantablack, one of the darkest commercially available paint. That paint also erodes in the space environment and has poor thermal performance.

Other panelists at the AAS event acknowledged that SpaceX and other companies were making efforts to reduce the brightness of their satellites. “SpaceX has put in a lot of money and person-power into solving this problem,” said Connie Walker, co-director of the International Astronomical Union’s Centre for the Protection of the Dark and Quiet Sky from Satellite Constellation Interference. “They’re trying again to create a mitigation strategy to lower the brightness of their satellites.”

Besides technical solutions, astronomers are also considering policy approaches. Julie Davis, an AAS public policy fellow, said that includes potential language in a wide-ranging competitiveness bill currently being negotiated by a House-Senate conference committee that would fund studies to measure the impact of satellite constellations on astronomy.

She said there was not a lot of awareness of the issue among policymakers, and that astronomers had to balance their concerns with the demand for broadband access that satellites can offer. “We need to be explicit in explaining what our problem is here. We are not against the internet, we just want it to not be super-reflective.”

Seitzer suggested potential solutions could come from the national security space community. “Previously, outside of the classified defense industry, satellite brightness has not been a design criterion,” he said. “I suspect that knowledge exists on the dark side, pun intended.”

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