SpaceX's Million Satellite Plan Could Wreck Ground-Based Astronomy, ESO Study Warns
The core problem is sunlight. Satellites reflect it, which makes them far brighter than the distant galaxies telescopes strain to capture, and every pass leaves a streak that erases whatever sits behind it. The study, authored by ESO astronomer Olivier Hainaut, has been peer reviewed and accepted for publication in Astronomy & Astrophysics, according to ESO. Hainaut simulated the position, motion, and brightness of both existing and proposed fleets to put real numbers on the damage. Those numbers are grim.
For SpaceX's proposed constellation, images taken two hours after dark with the Very Large Telescope at Paranal Observatory in Chile could contain dozens of trails, costing up to 28% of the field of view. That assumes the satellites stay dim enough to remain invisible to the naked eye. Anyone hoping engineers can simply build fainter hardware should temper expectations.
Wide field survey instruments fare even worse. If the satellites run just slightly brighter, a camera like the one inside the Vera C. Rubin Observatory could lose most of its images for hours every night. Reflect Orbital's proposed mirror satellites take things from bad to comical. A single mirror could ruin a Rubin-style observation without ever aiming its beam at an observatory, and with the full fleet of roughly 50,000 mirrors aloft, every image would be lost whenever sunlight hit them. Those mirrors could also make the entire night sky three to four times brighter, washing out faint targets like distant galaxies, Earth-like exoplanets, and asteroids headed our way. ESO says this marks the first time anyone has calculated how constellations raise background sky brightness in this manner.
Hainaut proposes capping the global satellite count at 100,000, provided every spacecraft stays below naked eye visibility from a dark site, though he admits he would prefer 50,000. He describes low Earth orbit as a "celestial seashore," a shared resource that delivers connectivity while preserving humanity's window on the Universe.
The findings have already entered the regulatory arena, where ESO, the International Astronomical Union, and the Royal Astronomical Society used them in responses to US Federal Communications Commission filings from SpaceX and Reflect Orbital, laying out the projected field-of-view losses, the sky brightening, and the case for a hard satellite cap. ESO's Betty Kioko says the agency received over 1,800 comments on Reflect Orbital and nearly 1,500 on SpaceX, and she frames the threat to optical astronomy as existential.
Astronomers stress that the stakes reach well beyond their own telescopes. ESO notes that very bright satellites can disrupt biological clocks and ecosystems here on Earth, while the constant launches and reentry burn-ups required to maintain massive fleets add atmospheric pollution. The decision now sits with regulators, who must weigh global connectivity against a night sky that, once lost, will not easily come back.
