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Dark-sky advocates confront threats from above and below

9 February 2022

Astronomers are working to reduce the impact on the night sky of artificial lighting and swarms of shiny satellites.

Milky Way over Kitt Peak Observatory.
The Milky Way arcs over Kitt Peak National Observatory and the city lights of Tucson, Arizona. Credit: KPNO/NOIRLab/NSF/AURA/B. Tafreshi

In his 1965 book, Starlight Nights, amateur astronomer and renowned comet finder Leslie Peltier recalled looking down at Los Angeles from Mount Wilson Observatory and noting the city lights encroaching on the mountainside. He was also concerned about the reflection of sunlight off growing fields of space debris. He wrote, “I realize that I have been witness to mankind’s latest pollution in the name of progress—the contamination of the skies.”

Today humanity’s ability to enjoy the night sky is increasingly threatened. Sky glow, which is caused by artificial lighting that is directed upward or reflected and scattered by aerosols in the atmosphere, is altering ambient light levels and obscuring once-visible astronomical objects. In 2016 computations with light propagation software showed that more than 80% of the world population, including 99% of people in the US and Europe, lives under light-polluted skies, where anthropogenic sources alter natural light levels. Less than two-thirds of the world’s population can see the Milky Way in the night sky.

The astronomy community has long advocated for controlling light pollution from below, in part to preserve the quality of ground-based observations. But now astronomers also have to worry about new sources of light from above. Over the next several years, companies such as SpaceX plan to launch thousands of communications satellites into low-Earth orbit. Last week SpaceX launched another 49 satellites on the same day the International Astronomical Union (IAU) announced the host sites for its new Centre for the Protection of the Dark and Quiet Sky from Satellite Constellation Interference. With those satellites, along with increasing urbanization across the globe, dark-sky advocates face new challenges that are forcing them to change their decades-old playbook, address different stakeholders, and educate young supporters.

Light from the ground

The dark-sky movement dawned in 1958 in Flagstaff, Arizona, where the city council traditionally welcomed new businesses with searchlights in the sky. Astronomers at nearby Lowell Observatory did not welcome those beacons and successfully petitioned the council to prohibit them.

Following Flagstaff’s restriction, the directors of five Tucson-area observatories documented the burgeoning problem of artificial lighting in urban areas and advocated for its reduction. Their efforts were recognized in 1972, when Tucson adopted an ordinance to keep the night sky dark.

Tucson’s ordinance was adopted partly in recognition of the need to reduce the sky glow that was creating problems for astronomers at Kitt Peak National Observatory, 80 kilometers to the west on a mountaintop in the Sonoran Desert. It also recognized the need “to preserve the relationship of the residents of the City of Tucson and Pima County with their unique desert environment through protection of access to the dark sky.” Later that year, Kitt Peak astronomer David Crawford worked with Tucson and Pima officials to pass an ordinance requiring that outdoor light fixtures be directed downward. Coconino County in northern Arizona, which includes Flagstaff, adopted similar ordinances shortly thereafter.

Map of artificial lighting at night.
The map shows the ratio of artificial to natural night-sky brightness around the world. The ratios span from less than 0.04 (gray) to 1.28–2.56 (yellow) to above 10.24 (pink and white). Credit: F. Falchi et al., Sci. Adv. 2, e1600377 (2016)

As part of his job at the US Naval Observatory Flagstaff Station in the 1980s, astronomer Chris Luginbuhl was tasked with updating Flagstaff’s outdoor lighting code to protect the observatory’s dark skies. He pioneered the use of codes written in terms of allowable light per unit area that have since been adopted and improved. “Right now, I’d say Flagstaff and Coconino County have the best outdoor lighting codes of any jurisdiction in the world,” says Don Davis, an astronomer who cofounded the Planetary Science Institute in Arizona and is former president of the International Dark-Sky Association (IDA). Flagstaff’s efforts were acknowledged in 2001 when IDA named it the world’s first “International Dark Sky City.”

Flagstaff’s code mandates the tightest light-per-acre restriction of any US city and requires that the spectrum of light not be broader than that of old, low-sodium lighting. Other cities have had success replacing streetlights. For example, Tucson recently shifted 18 000 of its streetlights to LED bulbs that allow dimming and data collection on energy use, a move that reduced upward radiance by about 7%.

Now 195 locations hold IDA’s International Dark Sky Place designation, which extends beyond cities to encompass parks, land reserves, and remote wilderness sanctuaries. Luginbuhl, Davis, and their colleagues in the nonprofit Flagstaff Dark Skies Coalition now work with city councils in other cities to implement “the Flagstaff solution” to the light-pollution problem. “For most people, light pollution doesn’t matter for their profession,” says Davis. “But dark skies are part of our quality of life.”

Light from the sky

For decades, dark-sky advocates focused their messaging on greater responsibility with lights on the ground. But over the past few years, their attention has turned upward. Although governments now launch fewer than 50 satellites per year, commercial companies are launching thousands because of lower launch costs. Private companies including SpaceX, OneWeb, and Amazon plan to launch more than 100 000 new satellites by 2025. “This shift in the geopolitical landscape of space policy and the associated impact on light pollution from above has awoken concern internationally,” says cosmologist Sarah Marie Bruno of Johns Hopkins University.

SpaceX launched the first and currently largest satellite constellation, known as the Starlink project, in 2019. Since then, reflective properties of the roughly 2000 satellites in low-Earth orbit have resulted in wide-field sky surveys reporting significantly more streaks across astronomical images than had been counted before. The streaks pose a special problem for the large-aperture telescopes used in wide-field surveys. “For optical telescopes, there’s pressure to gather as much data as possible before thousands of satellites launch,” says Bruno. “We’re preparing for a future that looks bright—but not in a good way.”

The IAU’s new program coordinates efforts across the global astronomy community to take steps that protect the dark and quiet sky from satellite constellation interference. One of the leading astronomers doing that work is Anthony Tyson, an experimental physicist at the University of California, Davis, and the chief scientist of the Vera C. Rubin Observatory now under construction in Chile. He’s working with SpaceX to define the problems caused by the satellites and to seek improvements. “They’ve been forthcoming with trying to offer technical solutions,” says Tyson.

One problem involves the satellites’ solar panels, each of which acts like a giant reflector. “The satellites are just very bright. You can even see them in the city,” Tyson says. To combat that brightness, SpaceX is testing an approach of angling the solar panels at a “knife-edge” orientation relative to the Sun as the satellites ascend to their operational altitude. Once the satellites reach their nearly 600 km altitude, company engineers will examine approaches to divert light from ground-based observatories.

Streaks of light from satellites.
Streaks from sunlight reflected off Starlink satellites dominate this image from the Lowell Observatory in May 2019. The satellites had not yet reached their operational altitude. Credit: Victoria Girgis/Lowell Observatory

When SpaceX tests its models and maneuvers, “it’s up to us astronomers to measure the resulting sky brightness,” says Tyson. With those data, he and his colleagues develop tools that let observational astronomers “deal with the satellite trail in a sensible way.” One solution is software that identifies satellite trails and creates masks that erase them. The challenge is to optimize masks that are neither so large that they eliminate valuable data nor so small that the satellite trails bleed out of the edges.

Diplomacy could also help. Since 1970 the US National Environmental Policy Act (NEPA) has required federal agencies to file environmental impact statements that detail environmental effects of any proposed project, so that those factors can be considered in government decision making. Tyson believes that any future international policies designed to mitigate the impact of satellites on the night sky should honor the regulations established by NEPA.

But international regulations, such as those that could be established by the International Telecommunication Union, can take decades. Meanwhile, investor dollars continue to pour into satellite constellations. “We have our work cut out for us,” says Tyson. At least for now, he says, working with individual satellite companies is a better bet than policy change.

Training a new generation

The latest step in the dark-sky movement involves educating new researchers and advocates beyond the astronomy community. In 2019 the University of Utah launched the first academic program to train future urban planners how to quantify and reduce light pollution to keep the skies dark. The Dark Sky Studies minor, run by the university’s city and metropolitan planning department, provides undergraduates with knowledge and tools to assess the impacts of artificial light in the context of community and collaboration.

Daniel Mendoza, program coordinator and an atmospheric scientist, says students learn how to use sensors on the ground and on drones to quantify the artificial light at different altitudes. The students perform measurements in neighborhoods across Utah and compare the features that lead to detrimental effects.

Crucially, the students also learn that tackling light pollution requires building consensus among disparate groups. Kate Magargal, an anthropologist who teaches a course on light pollution and policymaking through a community engagement project, develops curricula that challenge students to weigh the demands of modern space-based communication systems with astronomers’ needs for dark skies. A key goal is to “help our students develop skills to recognize these as issues that require planning and an equitable approach,” she says.

Mendoza says regulations regarding light pollution are 60 years behind those for air pollution. Like air pollution, he says, light pollution is a form of environmental exposure that has negative public health outcomes. Nighttime exposure to blue light, for example, can disrupt the circadian rhythm and suppress melatonin production, leading to poor sleep and associated health issues.

Paths to darkness

Undergraduates who have completed the Dark Sky program are now working with state parks to develop lighting ordinances. Course instructor and physicist Anil Seth would like to see graduates work with the Federal Aviation Administration on regulating the light from satellites. He adds that the value people attach to seeing the Milky Way from rural dark skies is going to lead the public to put dollars and votes behind large-scale implementation of dark-sky policies.

Mendoza says that young people must lead the charge, noting that local school outreach is an important component of Utah’s Dark Sky coursework. “Kids tell their parents to recycle and not to idle their cars,” he says. “We want kids to get with the program on light pollution too, and to involve their friends and families.”

Davis, who is on the board of the Flagstaff Dark Skies Coalition, maintains that “we have to be hopeful.” With the environmental movement taking on more urgency, he hopes increased public awareness will be directed to dark skies.

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