'Beyond the limit': one million satellites and mirrors in space pose grave threat to the night sky, astronomers said

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A new European Southern Observatory (ESO) study has found that current proposals to launch over 1.7 million satellites into orbit, including extremely bright ones, would have “devastating consequences for astronomy.” According to the study, no more than 100 000 faint satellites, below naked eye visibility, should orbit Earth, to safeguard our ability to observe the night sky with modern telescopes. The study is the first to compute the extent to which large and bright satellite constellations — which have also raised concerns about their impacts on health and the environment — would affect astronomical observations by making the night sky brighter.

This image shows satellites crossing the night sky above the northern Atacama Desert in Chile, over a period of just one hour. It is a stack of a time-lapse video taken on 15 October 2025 about two hours after sunset. A few streaks are caused by planes, and can be easily identified by their blinking-coloured lights, but most trails are due to satellites. In the foreground we see the dome of ESO’s Extremely Large Telescope (ELT), the world’s largest optical/infrared telescope, currently under construction atop Cerro Armazones. Behind it we see the lasers of ESO’s Very Large Telescope (VLT) at Paranal Observatory, 22 km away from the ELT. Credit: Credit: F. Kamphues, ESO/M. Kornmesser

Since 2019, the number of satellites orbiting Earth has increased rapidly, to over 14 000 today¹ — dominated by SpaceX’s Starlink telecommunications satellites. Satellite proposals have also escalated, both in number and in potential impact. “Until now we have managed, but it’s getting worse,” stressed Olivier Hainaut, who has been involved in developing recommendations to mitigate the impact of satellite constellations on astronomy. While companies like SpaceX have taken measures to make their satellites less bright, current satellite proposals are going “beyond the limit” of what astronomy can withstand, he said. Hainaut, an astronomer at ESO for over 30 years, is the author of the peer-reviewed study on the impacts of satellite constellations accepted for publication in Astronomy & Astrophysics.

SpaceX plans to send one million more satellites into orbit, for space-based data centres, which would significantly alter the appearance of the sky. The new study shows that, for a large fraction of each night, hundreds of satellites would be visible and, at certain times, up to several thousand, similar to the number of stars seen with the naked eye in good conditions. Other planned satellite constellations such as E-Space’s Cinnamon and China’s CTC-1 and 2 would add hundreds of thousands more satellites into orbit, compounding the problem.

The diagram shows the number of satellites that would be visible above ESO’s Very Large Telescope (VLT) if SpaceX launches their planned constellation of 1 million satellites. The calculations were done about two hours after sunset, well into the truly dark part of the night. The grey dots are satellites in the Earth’s shadow and therefore invisible, whereas the coloured dots are illuminated satellites. The orange dots, almost 2000 of them, correspond to satellites brighter than magnitude 7 –– the faintest brightness visible to the naked eye from extremely dark locations. The red dots, more than 200, are satellites brighter than magnitude 5, which corresponds to the faintest objects visible to the naked eye from a suburban location. Credit: ESO/Hainaut

Reflect Orbital, a US start-up, aims to launch a constellation of very large mirror-like satellites to provide sunlight at night, with reflected beams that span at least five kilometres on Earth’s surface. They intend to start with a prototype satellite in orbit this year and plan to increase their satellite population to 50 000 by 2035. These satellites would be the brightest ever in orbit, with damaging consequences for dark skies on Earth. Hainaut’s calculations show that the full constellation would fill the night sky with hundreds of very brightly visible satellites. Seen from within a reflected beam, the satellite delivering sunlight would appear four times brighter than the full Moon. Even if no satellite points its beam directly at an observer, each would be as bright as the planet Venus, the ‘morning star’. From a light-polluted city, like Munich, Germany, these hundreds of satellites would be the only ‘stars’ visible in the night sky.

These proposals, combined with others considered in the study, would dramatically brighten the night sky, hindering humankind’s ability to observe faint cosmic targets, including far-away galaxies, some Earth-like planets around other stars, and even asteroids potentially dangerous to Earth.

Bright trails and brighter skies
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Hainaut explained that “satellites, illuminated by the Sun, are much brighter than distant galaxies. When a satellite crosses what we observe, it makes a bright streak on our image, zapping whatever is behind it.”

To compute the impact of this and other effects of satellite constellations on astronomical observations, Hainaut simulated the positions, motion and brightness of all present and planned satellite constellations.

For the SpaceX satellite mega-constellation, he found that dozens of trails would appear in each image taken two hours into the night with ESO’s Very Large Telescope (VLT) at Paranal Observatory in Chile, representing field-of-view losses of up to 28%[^2]. This assumes that the satellites would be faint enough not to be seen with the naked eye in good conditions. If they are just a little brighter, some instruments would be even more affected: for instance, a camera like that of the US National Science Foundation’s Vera C. Rubin Observatory could have most of its images rendered unusable for several hours each night[^3]

This image illustrates how sunlight scattered by Reflect Orbital’s space mirrors would increase the overall brightness of the sky above ESO’s Very Large Telescope (VLT). The left image was captured with an all-sky camera on a moonless night on 16 May 2026. North is up, and West is to the right. The domes hosting the four 8-m telescopes of the VLT can be seen to the top-right. The image to the right is a simulation showing how much brighter the sky would be with Reflect Orbital’s full constellation of 50 000 mirrors. Even though the mirrors are not pointing directly at the observatory, they still diffuse light sideways, which is then further scattered by the atmosphere. As a result, the sky would be up to three to four times brighter. Credit: ESO/O. Hainaut

Hainaut’s simulations assumed that no Reflect Orbital satellite would point its beam directly at or near an observatory. Even so, the trail from a single mirror-satellite could spoil an observation with a camera like that of Rubin Observatory. With the full fleet of Reflect Orbital satellites in orbit, every image from such a camera would be lost when the satellites are illuminated by the Sun.

There are currently more than 14 000 satellites in orbit, but new proposals by SpaceX, Reflect Orbital and other companies could increase that number to over 1.7 million satellites. In this video, two ESO experts tell us about the devastating consequences this would have on astronomy, and what are the technical and legal options to limit this damage.
Credit: ESO. Directed by: L. Calçada, M. Kornmesser, B. Ferreira. Hosted by: S. Randall. Written by: E. Elkington, S. Randall. Editing: M. Kornmesser, L. Calçada. Videography: A. Tsaousis. Animations & footage: ESO, L. Calçada, M. Kornmesser, Future/Brett Tingley, ESA, S. Guisard, Torsten Hansen/IAU OAES, S. Brunier, F. Kamphues, B. Häuẞler, SpaceX, Reflect Orbital, @EmericTimelapse, RubinObs/NSF/AURA/H. Stockebrand, C. Malin, B. Tafreshi, G. Lombardi, INAF-VST/OmegaCAM, P. Horálek, satellitemap.space, J. McDowell. Music: Envato. Web and technical support: R. Yumi Shida. Fact-checking: O. Hainaut, B. Kioko. Promotion: J. C. Muñoz Mateos, O. Sandu. Filming Locations: ESO Supernova (supernova.eso.org). Produced by ESO, the European Southern Observatory (eso.org)

However, it’s not just the criss-crossing paths of satellites that limit what we can observe: their light can pollute the entire sky. Satellites too faint to be seen directly produce a veil of ‘diffuse’ light, while light from brighter satellites is ‘scattered’ in all directions as it passes through the atmosphere. Both contributions increase the overall brightness of the night sky. This study is the first to consider the impacts on astronomy due to the contribution of satellite constellations to background sky brightness, revealing the full extent of satellite light pollution.

This time-lapse shows satellites crossing the night sky above the northern Atacama Desert in Chile, over a period of just one hour. The video was taken on 15 October 2025, about two hours after sunset. A few streaks are caused by planes, and can be easily identified by their blinking-coloured lights, but most trails are due to satellites.
In the foreground we see the dome of ESO’s Extremely Large Telescope (ELT), the world’s largest optical/infrared telescope, currently under construction atop Cerro Armazones. Behind it we see the lasers of ESO’s Very Large Telescope (VLT) at Paranal Observatory, 22 km away from the ELT. Credit: F. Kamphues, ESO/M. Kornmesser

Very bright constellations like Reflect Orbital would have a particularly significant effect on background sky brightness. With the full 50 000 Reflect Orbital satellites, the sky would be up to three to four times brighter overall.

Limiting satellites to safeguard the night sky
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Hainaut concluded that the proposed 1.7 million new satellites would have drastic consequences for ground-based astronomy. These impacts can only be avoided by limiting the total, of both existing and future satellites, to 100 000 satellites faint enough not to be seen with the naked eye from a dark site. “This is not a hard number, like 99 999 is good and 100 001 is bad: clearly I’d prefer 50 000,” Hainaut said. “But 100 000 causes losses at about the level of other technical losses, such as equipment failure.” However, he added, the satellites must be fainter than visual magnitude 7²; should some of them be too bright — above the minimum threshold for naked-eye visibility — the total number would need to be much lower.

This time-lapse shows satellites above ESO’s Paranal Observatory in Chile. It was taken on 24 October 2025, and it spans four hours after sunset. The bright source in the sky is the Moon, which was illuminated at 7% when this time-lapse was captured. The video shows the domes of the huge Unit Telescopes and the smaller Auxiliary Telescopes that comprise ESO’s Very Large Telescope (VLT). The yellow beams are adaptive optics lasers used to measure and correct atmospheric turbulence. Credit: F. Kamphues/ESO

SpaceX and Reflect Orbital, responsible for the most extreme new proposals, have each filed with the US Federal Communications Commission (FCC) for permission to launch. This new study served as the basis for a response to the FCC on these proposals by ESO, in collaboration with the UK’s Royal Astronomical Society and the International Astronomical Union.

“The FCC received over 1800 comments regarding Reflect Orbital and nearly 1500 comments on the application by SpaceX,” explained ESO Institutional Affairs Officer Betty Kioko, responsible for coordinating ESO’s response to the proposals. “The ball is now in the FCC’s court, and we wait to see the determinations they make on both filings. For optical astronomy, this is an existential threat, and we hope that the regulators will share that view.”

This time-lapse shows satellites above ESO’s Paranal Observatory in Chile. It was taken on 19 May 2026, and it spans 5 hours. The video shows the domes of a Unit Telescopes and a smaller Auxiliary Telescope, part of ESO’s Very Large Telescope (VLT). Credit: B. Häußler/ESO

“Astronomy generates huge value for humankind, including scientific, technical, economical, and educational, and helps us understand our place in the Universe,” said ESO Director General Xavier Barcons. “The large number of planned satellites in low-Earth orbit challenges that capacity, underscoring the need to limit future satellite launches and for astronomers, engineers, satellite operators and other stakeholders to work together to adopt strict mitigation measures.”

“Sending thousands of satellites has implications: economical, ecological, and astronomical,” Hainaut added. Light pollution from very bright satellite constellations can impact the health and functioning of life on Earth, by disrupting biological clocks and ecosystems. Large constellations also have direct impacts on air quality from the numerous launches required to send and maintain thousands of satellites, as well as from the atmospheric pollution caused as they burn up on re-entry at the end of life. “My job is astronomy, so I quantify the effects on astronomy,” explained Hainaut, “I hope others will evaluate the other impacts in their field of expertise.”

The Sun sets down at ESO’s Paranal Observatory and the sky is transformed from pale blue to red to black. The four 1.8-metre Auxiliary Telescopes from ESO’s Very Large Telescope open their domes to observe the night sky, as if they were welcoming its new visitor: Comet C/2024 G3. This feathered comet soared the skies of Chile’s Atacama Desert in early 2025, leaving incredible images behind. As the night advances, the comet travels to the horizon together with the stars, but not all objects in the night sky follow this path. Like rain drops, the fast white strikes that inundate the night sky remind us of an increasing source of light pollution: satellite constellations. Credit: B. Häußler/ESO

Hainaut concluded: “Low Earth orbit is a celestial seashore that provides immense value to modern life, from global connectivity to our clear access to the Universe. However, we must manage the footprint of mega-constellations — from the light pollution affecting astronomy to the atmospheric effects of satellite re-entry — to ensure this resource remains pristine and accessible for future generations.”

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The number of satellites currently in orbit rises to 32 000 if dead satellites and debris are included. ↩︎
A satellite below visual magnitude 7 ensures it does not saturate the detector of cameras like that of the Rubin Observatory. It also means, coincidentally, that satellites would be too faint to be viewed to the naked eye, even under pristine dark skies. ↩︎