Cosmic Space

How astronomers aim to cope with satellites’ glare

If the vision of putting thousands of satellites into low Earth orbit to provide global broadband internet access comes anywhere close to fruition, there’s no way to avoid a negative impact on astronomy.

But the magnitude of the damage depends on what satellite operators, policymakers, professional researchers and amateur astronomers do.

That’s the bottom line from a report issued today by the American Astronomical Society and the National Science Foundation’s National Optical-Infrared Astronomy Research Laboratory, or NOIRLab.

The report recaps findings from a virtual workshop conducted from June 29 to July 2. The workshop brought together more than 250 stakeholders in the satellite-vs.-sky controversy, including representatives from Amazon, OneWeb and SpaceX.

“Our team at the AAS was enthusiastic to partner with NOIRLab and bring representatives of the astronomical and satellite communities together for a very fruitful exchange of ideas,” AAS President Paula Szkody, an astronomer at the University of Washington, said in a news release. “Even though we’re still at an early stage of understanding and addressing the threats posed to astronomy by large satellite constellations, we have made good progress and have plenty of reasons to hope for a positive outcome.”

SpaceX has already launched more than 650 of its Starlink satellites, with the aim of providing broadband access for the billions of people who are currently underserved. OneWeb, which was recently acquired by an Indian-British consortium after going through bankruptcy proceedings, has launched 74 satellites. And Amazon’s Project Kuiper is making plans for its own 3,236-satellite constellation.

If those three ventures follow through on their regulatory filings, tens of thousands of satellites could be in low Earth orbit within a decade — boosting the current tally by an order of magnitude.

SpaceX’s Starlink spacecraft are built at the company’s facility in Redmond, Wash., which is also where Amazon is putting its research and development center for Project Kuiper.

Most astronomers paid little attention to the potential threat to the night sky until SpaceX launched its first full batch of 60 Starlink satellites in May 2019. They were shocked to see how the trains of passing satellites disrupted astronomical observations.

Since then, SpaceX’s representatives have been talking with astronomers about how to mitigate the effect of the satellites’ reflective glare, which is at its worst just after sunset and just before sunrise.

“SpaceX has set an excellent example of a collaborative effort between astronomy and industry to manage this problem, and we certainly hope to see other operators following suit,” Jeff Hall, an astronomer from Arizona’s Lowell Observatory who heads AAS’ committee on light pollution, told reporters today during a teleconference.

As an experiment, SpaceX reduced the reflectivity of one of the Starlink satellites launched in January. Lori Allen of NSF’s NOIRLab said that “DarkSat” was about half as bright as the standard satellites, representing a reduction in visual magnitude from 5.2 to 6.2. (The higher the magnitude, the dimmer the object on a logarithmic scale.)

SpaceX is continuing to work on the reflectivity issue, and its most recent Starlink launch sent up 57 satellites equipped with specially designed sun visors. Allen said it was too early to determine how much the visors cut down on the glare from those satellites — in part because the coronavirus pandemic has forced astronomers to cut back on their observing time.

Based on observations as well as simulations, the report says mega-constellation ventures should reduce the reflectivity of their satellites to make them fainter than a 7th-magnitude star. (Such stars are the faintest objects that can be seen with the naked eye in dark skies.)

The report also calls for satellites in low Earth orbit to fly no higher than 600 kilometers (373 miles).

At altitudes higher than that, the satellites would catch the glint of the sun throughout the night in summer skies. “There’s no place to hide in the middle of the night from such a satellite constellation,” said Tony Tyson, an astronomer at the University of California at Davis who is also chief scientist for the Vera Rubin Observatory in Chile.

That’s a big problem for OneWeb, which is seeking to put tens of thousands of satellites in 1,200-kilometer-high orbits. It’s less of a problem for SpaceX (which is reserving orbital spots at altitudes between 328 and 614 kilometers) and for Amazon’s Project Kuiper (which is targeting heights ranging from 590 to 630 kilometers).

Even at lower altitudes with lower brightness, there’s no avoiding some disruption to astronomical observations. To minimize interference, the report calls on astronomers and satellite operators to develop software for tracking satellites and planning observations more precisely.

“There was this idea — well, actually it came from SpaceX — of having an app that we could all use, amateurs and professionals alike, to tell us precisely when something’s going to come across your field of view, and that’s beautiful,” Tyson said. “I think we can see the light at the end of that tunnel. … Any solutions that we come up with can be adopted by anybody, anytime.”

Tyson said software can also be used to remove some of the artifacts left behind in astronomical imagery by satellite trails. “The jury is out on that,” he said. “It’s quite challenging. These satellite trails are actually quite broad, so a fair fraction of pixels are impacted.”

As contentious as the satellite glare issue is today, it’s sure to get even stickier when the number of satellites rises from hundreds to thousands, to tens of thousands — and when the Vera Rubin Observatory hits prime time in the mid-2020s.

The Rubin Observatory’s wide-field Simonyi Survey Telescope — named after a billionaire software executive and space traveler who lives in the Seattle area — will be optimized to catalog thousands of near-Earth asteroids, study exoplanets in fine detail and shed light on the secrets of dark matter. But for all those efforts to be successful, the observatory will have to find ways to dodge all those satellites.

“It’s making a visual color motion picture of the universe,” Tyson said. “So it’s the perfect machine, unfortunately, to run into these things.”

A follow-up workshop, known as Satellite Constellations 2 or SATCON2, is tentatively planned for early to mid-2021.

Update for 7 p.m. PT Aug. 25: Meredith Rawls is a member of the team at the UW’s DIRAC Institute who also works on the data management team for the Rubin Observatory. What’s more, she contributed to a research paper that’s heavily referenced in today’s SATCON1 report. Here’s what she told me about the state of the satellite-vs.-sky controversy in an email:

“It’s heartening that SpaceX is working to darken their satellites, but they are just one operator. Dimmer satellites is not the same as fewer satellites. While there are lots of technical things that scientists can do to minimize scientific impacts, this is a significant amount of work that nobody has planned or budgeted for, and doesn’t begin to address wider-reaching environmental or cultural effects of a drastically changed night sky.”

Check out her talk about Starlink and the mega-constellation challenge, presented last month as part of the Astronomy at Home lecture series:

Wondering whether Starlink satellites are visible tonight from your locale? Plug your coordinates into and find out.

By Alan Boyle

Mastermind of Cosmic Log, contributor to GeekWire and Universe Today, author of "The Case for Pluto: How a Little Planet Made a Big Difference," past president of the Council for the Advancement of Science Writing.

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