A study by the European Southern Observatory (ESO) published in Astronomy & Astrophysics says that more than 1.7 million planned satellites in low‑Earth orbit would exceed the amount that can be accommodated without degrading night‑time astronomical observations. The study recommends limiting the total number of operational satellites to 100 000 and requiring them to be below visual magnitude 7 to remain invisible to the naked eye.

Since 2019, the number of satellites in orbit has risen to about 14 000, driven largely by SpaceX’s Starlink network. Proposals now include SpaceX’s plan for an additional one million satellites, as well as constellations such as E‑Space’s Cinnamon, China’s CTC‑1 and CTC‑2, and a proposed mirror‑based system by the U.S. firm Reflect Orbital that aims to launch up to 50 000 satellites by 2035.

Simulations show that a large fraction of each night would have multiple satellites visible, with up to several thousand at certain times, comparable to the number of stars seen under dark skies. Bright satellite trails can reduce usable observation time by up to 28 % in images taken with the Very Large Telescope’s FORS2 instrument, and could render many images from the Vera C. Rubin Observatory unusable for several hours each night.

Light pollution from satellites also raises overall sky brightness; with the full Reflect Orbital constellation the night sky could become three to four times brighter, reducing contrast for faint celestial objects such as distant galaxies, exoplanets, and potentially hazardous asteroids.

ESO Director General Xavier Barcons said the large number of planned satellites challenges the scientific, technical, economic and educational value of astronomy and called for limits on future launches and cooperation among stakeholders.

Olivier Hainaut, lead author of the study, noted that the 100 000 satellite limit is not an exact threshold, but exceeding it could cause losses comparable to equipment failures. He also emphasized that bright satellites can affect biological systems and contribute to atmospheric pollution during re‑entry.

Both SpaceX and Reflect Orbital have filed applications with the U.S. Federal Communications Commission. The FCC has received over 1,800 comments on the Reflect Orbital proposal and nearly 1,500 on the SpaceX application and is reviewing them. ESO, in partnership with the Royal Astronomical Society and the International Astronomical Union, has submitted a response to the FCC.

The study concludes that managing the growth of mega‑constellations is necessary to preserve the night sky for future generations.