ESO study recommends capping Earth satellites at 100,000 to protect astronomy

A study by the European Southern Observatory (ESO) has suggested that the total number of satellites orbiting Earth should not exceed 100,000 if they are faint enough to remain invisible to the naked eye from dark sites. The recommendation follows simulations that evaluated the impact of existing and planned satellite constellations on astronomical observations.

ESO astronomer Olivier Hainaut, lead author of the study, said the figure is not a strict cutoff but a practical limit, noting that “100 000 causes losses at about the level of other technical losses, such as equipment failure.”

Current satellite counts stand at over 14,000, but companies such as SpaceX and Reflect Orbital have filed plans that could raise the total to 1.7 million. Simulations indicate that SpaceX’s proposed megaconstellation could obscure up to 28% of images taken by the Very Large Telescope during the early night, even if the satellites are dim enough to avoid naked-eye detection. Brighter satellites would render many images from the Vera C. Rubin Observatory unusable for several hours each night.

Reflect Orbital’s concept of deploying up to 50,000 mirror-satellites could, according to the study, spoil every image captured by the Rubin Observatory when illuminated by the Sun.

The researchers propose a brightness threshold of magnitude 7 for the 100,000-satellite limit; any satellites brighter than this would require a lower overall count. Both SpaceX and Reflect Orbital are awaiting regulatory approval from the Federal Communications Commission. ESO Institutional Affairs Officer Betty Kioko said the decision now rests with the FCC and emphasized that “for optical astronomy, this is an existential threat, and we hope that the regulators will share that view.”