Space mirrors 40 times brighter than the full moon alarm astronomers

Full text

Each of the sun-reflecting mirrors proposed by U.S.-based startup Reflect Orbital will appear 40 times brighter than the full moon to observers standing within its beam on Earth, a study has found. The light pollution produced by each of those mirrors would completely wipe out stars from the sky for those underneath them, turning the starry night into an eerie twilight zone.

"It will be worse than big cities like London," Gaspar Bakos, a professor of astrophysics at the University of Princeton and one of the authors of the study, told Space.com. "You would not see any stars if you were in the beam."

Reflect Orbital recently received a license from the U.S. Federal Communications Commission (FCC) to launch a demonstration satellite called EARENDIL-1 into an orbit 373 miles (600 kilometers) above Earth. The spacecraft will be fitted with a giant mirror 60 by 60 feet (18 by 18 meters) in size to test how sunlight can be reflected to Earth to extend the operations of solar power plants into nighttime hours or even to illuminate disaster zones after dark for free.

Despite the company's intentions, the FCC's decision stirred an outcry among astronomers , environmental campaigners, commercial pilot associations and other stakeholders, who fear the effects that might come to pass if the FCC approves Reflect Orbital's plan to launch 40,000 space mirrors even larger than EARENDIL-1 in the coming years.

Bakos is one of the astronomers who filed objections to the FCC against the plan. He said that the new study provides hard numbers on Reflect Orbital's impact where there previously were only rough estimates.

"There has been some disbelief from Reflect Orbital about our original estimates, so we have taken data on light scattering in Earth's atmosphere which is based on papers that go back for decades to calculate the light pollution," said Bakos. "These are scattering calculations that have been extensively tested in real life situations. So, we now have scientifically accurate renderings based on this paper."

Instead of focusing on EARENDIL-1, the researchers calculated the impact of Reflect Orbital's operational mirrors, which will be 174 by 174 feet (54 by 54 m) in size. They found that to observers on Earth standing at the center of the mirror's 3.4-mile-wide (5.4 kilometers) beam, the satellite would appear 40 times as bright as the full moon and the sky as luminous as that in the after-sunset twilight.

An illustration of Reflect Orbital's satellites in orbit. (Image credit: Reflect Orbital) But thanks to atmospheric light scattering — the bouncing of photons off particles in the air — the area affected by the mirror's passage would be much wider, the study found. Even at a distance of nearly 9 miles (14 kilometers) from the beam, the sky would be brighter than when the moon is full. At a distance of 21 miles (34 kilometers), the light-polluted area above the horizon in the direction of the beam would still appear brighter than the night sky during a full moon.

That would be just a single mirror. But Reflect Orbital wants to fly 40,000 such satellites. Bakos said that data suggests that to trigger energy generation even in the most efficient solar panels of today would require the combined reflected light from at least 400 such mirrors at the same time. Together, the beam from those mirrors would be 10,000 brighter than the full moon , which, however, only makes up about 2.4 percent of the brightness of the sun, the paper states.

The sky glow resulting from these combined beams, the researchers found, would be visible from 50 miles away (80 kilometers), and extend the twilight zone up to 9 miles (14 kilometers) from the beam.

A rendering of the effect of Reflect Orbital's space mirrors on the night sky in a dark sky location. This image depicts a beam of light from one of the company's mirrors from roughly 8.5 miles (14 kilometers) away. (Image credit: Gaspar Bakos/Princeton University) "It would be almost daytime brightness," said Bakos. "It would be just like before sunrise. But in fact, that's too low for solar panels to actually operate. Even the very best ones would barely turn on even with 400 mirrors illuminating the patch."

In renderings created by Bakos and his team based on the calculations, a pristine dark sky at a dark location is shown to completely transform into a star-free twilight zone as the mirror flies overhead.

Reflect Orbital's spokesperson told Space.com in response that the study's findings were based on "inaccurate assumptions" and insisted that "the vast majority of people will never see the light in their lifetime as the vast majority of service locations will be very remote," serving "large construction and solar projects in the middle of nowhere."

The spokesperson further added that "only the satellites required to serve approved locations would be active; inactive satellites would remain below naked-eye visibility."

The researchers, however, criticize the company for failing to provide technical data to back up their claims.

Reflect Orbital, however, insists that they "engaged substantively with legitimate concerns raised by astronomers, environmental researchers and scientists" and that such "feedback has informed our technology and operational plans."

The company says that they are "working on a coordination agreement with the National Science Foundation and commissioning third-party research through independent researchers and federal partners."

An illustration of the brightness of the full moon in a dark sky location. (Image credit: Gaspar Bakos/Princeton University) For comparison, SpaceX's Starlink currently consists of around 11,000 satellites. According to some estimates , around 30 Starlink satellites are visible in the sky from a single spot on Earth at any point in time. Starlink satellites, however, are much smaller and dimmer than the Reflect Orbital mirrors.

Bakos said the overall effect of the mirrors would depend on the reflectiveness of the illuminated surface and the cloudiness of the sky. A forest, for example, might absorb most of the light while snow-covered fields or concrete city streets would scatter it further.

"It's not just the beam," said Bakos. "The glow of the reflected light from the surface will cause the sky to glow. For example the light pollution from a town illuminated by the beam will be ten to 20 times brighter than from the artificial lighting in that same town. Artificial light only illuminates streets and sidewalks, but here, everything would be illuminated — every backyard, park, roof or river."

The FCC received around 1,900 objections against the Reflect Orbital application. In its Order and Justification of Reflect Orbital's mission license, the agency stated that "impacts on optical astronomy fall outside [the FCC's] review and authorization."

In the U.S., satellite projects are exempt from the National Environmental Policy Act (NEPA ), which requires federal agencies to assess environmental impacts of their decisions. The exemption has been in place since the 1980s when only a handful of satellites were being launched in a year.

The study was accepted for publication in the Astrophysical Journal and is currently available on arXiv.