Study predicts space mirrors could outshine full moon and create extensive light pollution
Researchers say each proposed solar-reflecting satellite would appear up to 40 times brighter than the full moon, potentially erasing stars from the night sky, while the company behind the project disputes the assumptions used in the analysis.
A new study estimates that sun-reflecting mirrors planned by U.S. startup Reflect Orbital would be up to 40 times brighter than the full moon for observers directly beneath the reflected beam. The researchers calculated that a single 54-meter mirror would produce a 5.4-kilometre-wide beam whose central brightness would turn the sky into a twilight-like condition, and that scattered light would keep the sky brighter than a full moon even 14 kilometres from the beam’s centre.
The analysis, led by Princeton astrophysicist Gaspar Bakos, also examined a scenario in which 400 mirrors operate simultaneously, a level the authors say would be needed to generate usable power for the most efficient solar panels. In that case, the combined illumination would be about 10,000 times brighter than the full moon, extending a noticeable glow up to 80 kilometres away and creating a twilight zone extending 14 kilometres from each beam.
Reflect Orbital has received a Federal Communications Commission license to launch a demonstration satellite, EARENDIL-1, equipped with a 60-by-60-foot mirror to test nighttime solar-power delivery and emergency illumination. The company plans to eventually deploy up to 40,000 similar satellites.
The firm responded to the study by claiming the calculations rely on “inaccurate assumptions” and that most mirrors would serve remote construction and solar projects, limiting public exposure. A company spokesperson said only satellites needed for approved locations would be active and that inactive units would remain below naked-eye visibility.
Astronomers and environmental groups have filed nearly 2,000 objections to the FCC application, warning that the mirrors could significantly increase light pollution and interfere with optical astronomy. The FCC’s order noted that impacts on optical astronomy fall outside its review jurisdiction.
The researchers emphasized that surface reflectivity and atmospheric conditions would affect the extent of sky glow, noting that bright surfaces such as snow or concrete would scatter more light than darker terrain.
Reflect Orbital says it is engaging with scientific stakeholders and pursuing third-party research to address concerns, while the FCC’s decision proceeds amid broader debates over satellite-related environmental impacts.