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By Alexandra Witze, Nature - 26 AUGUST 2020
SpaceX launched 60 Starlink satellites (shown here in stacks, waiting to deploy) in May of last year.Credit: SpaceX
‘Megaconstellations’ of satellites increasingly launching into orbit around Earth will contaminate the data astronomers collect — and profoundly shift humanity’s view of the night skies. That’s the conclusion of the most detailed assessment yet of how these satellite networks, launched by companies including Amazon and SpaceX, might affect astronomical observations from Earth.
From the perspective of a telescope gazing at the sky, satellites can reflect sunlight and appear as bright streaks. These streaks interfere with astronomical observations, including studies of fundamental physics and cosmology, planets beyond the Solar System and asteroids that threaten Earth. “Some phenomena will surely go undiscovered as a result of significant interference,” says a 25 August report from the American Astronomical Society and NOIRLab, an umbrella organization for several US observatories that is funded by the National Science Foundation (NSF).
Astronomers can take some steps to reduce the impact, such as scheduling observations to avoid times when satellites whiz by, or working with satellite operators to develop less-reflective versions. But high-flying satellites that catch the sunlight are a particular problem, according to the report.
“There’s no place to hide in the middle of the night from such a satellite constellation,” says Tony Tyson, a physicist at the University of California, Davis.
Unavoidable damage
Tens of thousands of satellites, which will provide broadband Internet to people around the globe, are expected to soar into space in the coming years. SpaceX, an aerospace company in Hawthorne, California, has already launched more than 650 of a planned 12,000 Starlink satellites. Other operators include the London-based OneWeb, which has launched 74 of what it hopes will be a gigantic fleet of 48,000 satellites, and Amazon, which last month received US-government approval to launch 3,236 satellites for its planned project called Kuiper.
SpaceX, OneWeb and Amazon have all been in talks with astronomers about how to mitigate the satellites’ brightness. Scientists realized the problem in June 2019, only after SpaceX launched its first set of Starlinks. Astronomers have been working since to determine how bad the problem will be and what they can do about it. The report is the most technical assessment to date of the extent of the problem. It describes the results of a June workshop that brought together astronomers, satellite operators, dark-sky enthusiasts and others.
The only way to avoid any impact on ground-based visible-light and infrared telescopes would be to launch zero satellites, says Connie Walker, an astronomer at NOIRLab and co-chair of the team that wrote the report.
Given that many of the megaconstellations have already been approved and some have started launching, astronomers are trying to determine how to work around the bright streaks. Most affected are wide-field sky surveys — especially those set to be conducted by the Vera C. Rubin Observatory, an 8.4-metre telescope that the NSF is building atop the Chilean mountain of Cerro Pachón and for which Tyson is the chief scientist. Starting in 2022, the facility will take wide-field images that cover the entire visible sky every few days for ten years to build a picture of how the Universe changes over time.
The streaks are worst for ground observations in the hours around sunset and sunrise, when satellites catch the gleam of the Sun and are not yet in Earth’s shadow. This harms the research that is done in these hours, such as searches for near-Earth asteroids. Simulations published earlier this year1 fed into the new report’s conclusions, detailing how 30–40% of exposures made at the Rubin observatory around twilight and dawn could be affected.
The problem is particularly acute for satellites that orbit at high elevations — more than 600 kilometres above ground — where they remain out of Earth's shadow and visible all night in some locations, according to the report. Orbiting at 1,200 kilometres above Earth, OneWeb’s planned megaconstellation would fall into this category.
Streak reduction
OneWeb representatives have met with the American Astronomical Society to begin discussing ways to mitigate the problem. The UK space agency, which is part of a consortium that took over OneWeb after the company declared bankruptcy earlier this year, is in similar discussions with the Royal Astronomical Society. Amazon has also been talking with representatives from the astronomical community about what it might do.
There are no national or international regulations on how bright or dim satellites should be. The report recommends that astronomers and satellite operators continue to work together to develop methods for assessing and reducing satellite brightness.
SpaceX has been at the forefront of this, mostly because it was the first company to start launching so many of these satellites. The most recent set of Starlinks launched on 18 August; 5 more launches, of roughly 60 satellites each, are planned for the rest of the year. Around twilight, once the full constellation of around 12,000 Starlinks is launched, hundreds of them might be visible at once to people looking up from dark-sky sites2.
One of the first steps SpaceX took to reduce the satellites’ visibility was to orient them in a different direction soon after launch so that they reflect less sunlight to the ground. The company also painted one Starlink black and launched it in January to see whether that would help. But the black paint made the satellite thermally ‘hot’, harming its Internet operations, said Patricia Cooper, SpaceX’s vice-president of satellite government affairs, at a recent webinar. So in April, SpaceX instead began launching Starlinks with a sunshade to block sunlight that would otherwise reflect off the antennas, which are the main source of brightness once the satellites have reached their final positions.
So far, only one of these ‘VisorSat’ Starlinks has finished the journey to its final orbit of 550 kilometres above Earth. Amateur astronomers have reported that the sunshade appears to make the Starlink fainter than it would otherwise be. But astronomers are still awaiting further confirmation that the modifications have a significant effect, says Meredith Rawls, an astronomer at the University of Washington in Seattle. Regardless, SpaceX is now launching all of its Starlinks with the sun-shielding visor.
Astronomers might be able to avoid some of the satellite streaks by scheduling their observations around the time when a satellite is passing overhead, or using image-cleaning software to strip out the contamination. But the former requires very precise information from satellite operators about where a satellite will be at any particular time, and the latter cannot restore the fundamental science lost in the pixels that were stripped out3, Tyson says.
Satellite megaconstellations can interfere with observations by radio telescopes on the ground and contribute to congestion in space. NOIRLab and the American Astronomical Society are planning a workshop for next year to assess the policy and regulation issues surrounding these megaconstellations.
The JASON advisory group, an influential and secretive set of US scientists and engineers, is also working on a report on satellite megaconstellations and astronomy for the NSF.
References
1. Hainaut, O. R. & Williams, A. P. Astron. Astrophys. 636, A121 (2020).
2. McDowell, J. C. Astrophys. J. Lett. 892, L36 (2020).
3. Tyson, J. A. et al. Preprint at https://arxiv.org/abs/2006.12417 (2020).
Original Article https://www.nature.com/articles/d41586-020-02480-5
