Up to 1.7 million satellites planned for launch in the coming years could have devastating consequences for astronomy, according to new research published on Wednesday by the European Southern Observatory (ESO). The study, led by astronomer Olivier Hainaut, is the first to calculate how large, bright satellite constellations would brighten the night sky, threatening the ability of ground-based telescopes to explore the universe.
An Existential Threat to Telescopes
The ESO describes the planned swarms of satellites as an “existential threat” to optical astronomy. Currently, about 14,000 satellites orbit Earth, many of them part of Elon Musk’s Starlink internet constellation. But that number is set to skyrocket: SpaceX has announced plans to launch more than 1 million satellites by 2028 to serve as data centres for artificial intelligence. Other projects, such as E-Space’s “Cinnamon” constellation and Chinese initiatives CTC-1 and CTC-2, would add hundreds of thousands more. US startup Reflect Orbital aims to launch 50,000 satellites equipped with giant mirrors to reflect sunlight back to Earth, providing nighttime illumination.
“When a satellite crosses what we observe, it makes a bright streak on our image, zapping whatever is behind it,” Hainaut told AFP. “For the past few years, this has been happening but it is still manageable. But if we go from 14,000 to 1.7 million, we are really going to have problems.”
Reflect Orbital Satellites: A Particular Threat
The Reflect Orbital satellites pose an especially severe risk. Even when their mirrors are not pointed directly at an observer, scattered light will make each satellite as bright as Venus, the “morning star,” Hainaut said. The researchers determined that almost all images captured by the Vera C. Rubin Observatory in Chile—home to the largest camera ever built—would be rendered unusable. The sky, whether observed from France, the Sahara Desert, or Chile, “would no longer be clear, resembling instead the sky seen in the suburbs of a city,” Hainaut warned. In light-polluted urban areas, the satellites “would be the only ‘stars’ visible in the night sky,” according to the ESO. All 50,000 Reflect Orbital satellites could make the entire night sky up to four times brighter.
A Reflect Orbital spokesperson told AFP that the company is commissioning independent research on the impact of its technology and is “committed to ongoing dialogue with astronomers.” The satellites’ default position will be “off,” and the company says it will “systematically avoid redirecting light near observatories.”
Call for a Limit of 100,000 Satellites
Hainaut and his team call for a maximum of 100,000 satellites in orbit, and that they be made dim enough to be invisible to the naked eye. The ESO has submitted its findings to the US Federal Communications Commission (FCC), which is considering applications from SpaceX and Reflect Orbital to launch their constellations. “The ball is now in the FCC’s court,” said Betty Kioko, the ESO’s institutional affairs officer. “For optical astronomy, this is an existential threat and we hope that the regulators will share that view.”
The light pollution from bright satellite constellations is not just a problem for astronomers. Loss of dark skies disrupts the biological clocks of humans and animals and interferes with ecosystems. There are also concerns about the energy and environmental impact of launching nearly 2 million satellites, as well as the risk of space debris collisions triggering a dangerous chain reaction known as Kessler syndrome.
As Europe grapples with the consequences of rapid technological change, the stakes for preserving dark skies are high. The ESO’s research underscores the need for international regulation to balance innovation with the protection of scientific observation and natural environments.
