Astronomers Suggest Strategies to Reduce Interference from Satellite Megaconstellations

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Publication date: 
1 September 2020

A new report warns that planned constellations of low Earth orbit satellites could severely interfere with ground-based astronomical observations. It suggests measures to reduce the impact of light pollution from the satellites, while noting that no combination of mitigation measures can completely eliminate the problem.


Bright trails left by SpaceX Starlink satellites are visible in a wide-field image captured by the Cerro Tololo InterAmerican Observatory in Chile.

Bright trails left by SpaceX Starlink satellites are visible in a wide-field image captured by the Cerro Tololo InterAmerican Observatory in Chile.

 (Image credit – CTIO/NOIRLab/NSF/ AURA/DECam DELVE Survey, CC BY 4.0)

The American Astronomical Society and the National Science Foundation’s NOIRLab released a report last week warning that large constellations of low Earth orbit satellites “threaten the scientific viability” of some current and future ground-based optical observatories. The report recommends various strategies to reduce the impact of light pollution from the satellites, though it finds that no combination of actions — other than greatly reducing the number of satellites — will fully eliminate the problem.

The conclusions are drawn from a virtual workshop held earlier this summer by AAS and NOIRLab with more than 250 participants, including astronomers, satellite operators, and other stakeholders. (AAS is an AIP Member Society.) The workshop focused exclusively on scientific and technical aspects of satellite constellations and their impact on optical and infrared astronomy, leaving policy and regulatory matters to a second workshop to be held early next year.

At an Aug. 25 press conference, Phil Puxley, vice president for special projects at the Association of Universities for Research in Astronomy, remarked, “The reality of this newly emerging challenge of sunlight reflecting off constellations of satellites down to Earth is that there are no remote locations that are immune. There is no place to hide.”

‘Extreme’ impacts expected

Satellite constellations have been an urgent concern for the astronomy community since SpaceX launched its first train of small-scale communication satellites in May 2019, which proved to be unexpectedly bright. While that first launch included only 60 satellites, the company has outlined plans to deploy 30,000 as part of a decade-long project called Starlink to provide global broadband internet in even very remote locations. OneWeb and Amazon’s Project Kuiper are also planning such projects, often referred to as megaconstellations. According to public filings, more than 100,000 low Earth orbit satellites have been proposed for launch over the next decade.

The report estimates impacts on nine representative astronomical observation types, finding they can range from “negligible” to “extreme” depending on the science goals of the observations and the constellation's characteristics. For instance, it points out that satellite trails could stymie time-sensitive observations of gravitational wave sources and other rare transient events, which are critical to the emerging specialty of multi-messenger astronomy. Wide-field surveys will also be unable to avoid contamination from satellite trails, introducing systematic errors that require new data analysis techniques.

The report closely considers the potential impacts on the Vera C. Rubin Observatory, which is currently under construction in Chile and was the top priority for ground-based optical astronomy identified in the National Academies’ 2010 astronomy and astrophysics decadal survey. Due to its wide field of view and the vast volume of data it will collect, it will be particularly vulnerable to the effects of satellite trails.

“It's a perfect machine, unfortunately, to run into these things,” the observatory’s chief scientist Tony Tyson said at the press conference.

Continued collaboration sought with constellation operators

Workshop co-chair Connie Walker of NOIRLab stressed that “no combination of mitigations will eliminate the impact of satellite constellations on optical astronomy.” While observing that the report recognizes the option of not launching constellations at all, she said that route is “not viable for industry.” Accordingly, the report focuses more on actions that astronomers and satellite operators can take in concert and independently to avoid the most serious forms of interference.

One key recommendation is that operators deploy constellations below 600 kilometers to reduce the number of visible satellites. Simulations show that constellations orbiting below that altitude fall entirely into the Earth’s shadow for much of the night, while some satellites in higher constellations remain visible all night long in the summer.

Both Starlink and Kuiper have sufficiently low orbits, but OneWeb’s constellation has a planned altitude of 1,200 kilometers, which would leave hundreds of satellites illuminated. (OneWeb paused its deployment after filing for bankruptcy in March 2020 but soon after filed a proposal to increase its constellation to 48,000 satellites. In July, the company announced the UK government and the Indian mobile network operator Bharti Global were acquiring the company, providing it with $1 billion in capital.)

The reflectivity of the satellites is also a critical factor, as very bright satellites can oversaturate detector pixels, making instruments difficult to calibrate and leaving non-correctable artefacts in the data. Drawing on measurements and simulations, the report recommends darkening satellites to at least apparent magnitude seven, just dim enough to be invisible to the naked eye. It also suggests that operators position the satellites to avoid reflecting sunlight toward major observatory sites.

While the brightness of the Starlink satellites already in orbit varies considerably, the report states the average apparent magnitude of 281 satellites measured in late May and early June was 5.5. (The scale is logarithmic and lower numbers indicate higher brightness.) SpaceX has already begun testing strategies for reducing their brightness. In January, it launched an experimental satellite called “DarkSat,” which uses a coating to reduce reflectivity, followed by “VisorSat” in June, which uses a deployable sunshield to block light from reaching reflective surfaces. NOIRLab’s Lori Allen said last week that more data is necessary to assess the effectiveness of these measures and highlighted the report’s recommendation to pursue a coordinated observation campaign to evaluate the mitigation strategies over time.

Even with such measures in place, satellite trails will still show up in astronomical data, so the report recommends that researchers develop software applications that can mask or remove them from images. Detailed simulations will also be necessary to understand systematic uncertainties and residual signal-to-noise effects caused by masked satellite trails. The report further suggests that satellite operators provide detailed orbital position information for their constellations so that researchers can account for their presence in planned observations.

Although the future of astronomy in the era of megaconstellations remains cloudy, the workshop organizers expressed appreciation for SpaceX’s efforts to address their concerns thus far. “SpaceX has set an excellent example of a collaborative effort between astronomy and industry to manage this problem,” said workshop co-chair Jeff Hall of Lowell Observatory. “We certainly hope to see other operators following suit as we go forward.”

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