ESO study warns 1.7 million planned satellites pose existential threat to astronomy

A new peer-reviewed study by the European Southern Observatory (ESO) warns that current proposals to launch more than 1.7 million satellites into orbit will have “devastating consequences” for ground-based astronomy

The study, accepted for publication in Astronomy & Astrophysics, is the first to calculate how large, highly reflective satellite mega-constellations will systematically brighten the night sky, creating a veil of light pollution that could obstruct humanity’s view of the cosmos.

Surpassing the celestial carrying capacity

Since 2019, the number of active satellites orbiting Earth has rapidly grown to more than 14,000, primarily driven by SpaceX’s Starlink internet constellation. However, newer commercial applications filed with regulatory agencies represent a massive scale-up.

According to the study’s author, veteran ESO astronomer Olivier Hainaut, the current proposals go “beyond the limit” of what modern astronomy can withstand. To preserve the viability of ground-based telescopes, Hainaut concludes that the absolute ceiling for low-Earth orbit should be capped at 100,000 faint satellites—and only if they remain below the threshold of naked-eye visibility (visual magnitude 7).

The Major Contributors to Light Pollution

The study models the trajectories and reflectivity of several massive planned fleets that threaten to fundamentally alter the appearance of the night sky:

• SpaceX data Centre Fleet:

  • SpaceX plans to launch an additional one million satellites dedicated to space-based data centres. Simulations reveal that for large portions of the night, hundreds to several thousand of these satellites would be simultaneously visible to the naked eye, matching or outnumbering the visible stars in a pristine night sky.

• Global Telecommunications Fleets:

  • Other planned mega-constellations, including E-Space’s Cinnamon and China’s CTC-1 and CTC-2, seek to add hundreds of thousands more assets to low-Earth orbit.

• Reflect Orbital’s Mirror Satellites:

  • A U.S. startup intends to orbit a 50,000-satellite constellation of massive orbital mirrors designed to reflect sunlight down to Earth at night. The study notes these would be the brightest human-made objects in space. If viewed from within their 5-kilometre light beam, they would shine four times brighter than a full Moon. Even from the outside, each mirror satellite would equal the brightness of Venus, making them the only “stars” visible from light-polluted cities.

Direct Impacts on Astronomical Data

The study quantifies how these constellations will physically damage data collection at world-class observatories, identifying two primary mechanisms of interference:

1. Saturation and Ghost Trails

When a bright satellite crosses a telescope’s field of view, it burns a bright streak across the image, destroying the data beneath it. Simulations using ESO’s Very Large Telescope (VLT) in Chile indicate that the SpaceX constellation alone would cause a field-of-view loss of up to 28% for traditional cameras just two hours into the night.

For ultra-sensitive wide-field instruments—such as the U.S. National Science Foundation’s Vera C. Rubin Observatory—the impact is catastrophic. Bright reflections trigger electrical saturation in high-density detectors, creating a grid of “ghost trails” that can render images completely unusable for hours. A single mirror satellite from Reflect Orbital could ruin a Rubin Observatory exposure; its full fleet could wipe out every single image taken while the satellites are illuminated by the Sun.

2. Sky Glow and Scattered Light

Beyond visible streaks, the study highlights a previously unquantified threat: cumulative background sky glow. Millions of satellites too faint to be seen individually create a collective veil of “diffuse” light. Concurrently, light from brighter satellites is scattered in all directions as it passes through Earth’s atmosphere. The full Reflect Orbital constellation alone would increase the overall background brightness of the global night sky by three to four times, severely limiting our ability to observe faint, distant targets like early galaxies, Earth-like exoplanets, and hazardous near-Earth asteroids.

A coordinated regulatory defence

Because these infrastructure projects pose an existential threat to optical astronomy, the ESO has teamed up with the UK’s Royal Astronomical Society and the International Astronomical Union to lodge formal interventions with the U.S. Federal Communications Commission (FCC), where both SpaceX and Reflect Orbital have filed for launch permissions.

The public response has been highly contested, with the FCC receiving over 1,800 formal comments regarding Reflect Orbital and nearly 1,500 concerning SpaceX.

Beyond astronomy, the study underscores that low-Earth orbit must be managed as a shared global ecosystem.

Mass satellite deployments have broader ecological consequences, including light pollution that disrupts biological clocks on Earth, atmospheric pollution generated by the frequent rocket launches required to maintain the fleets, and unknown chemical impacts on air quality as thousands of decommissioned satellites burn up during atmospheric re-entry.