🔭 A million satellites and mirrors in space pose a serious threat

A new study from the European Southern Observatory (ESO) has revealed that current plans to put more than 1.7 million satellites into orbit, some of them extremely bright, would have "devastating consequences for astronomy."

According to this study, to preserve our ability to observe the night sky using modern telescopes, the number of dim satellites, invisible to the naked eye, orbiting Earth should not exceed 100,000. This study is the first to assess the extent to which large, very bright satellite constellations—which also raise concerns about their impacts on health and the environment—would affect astronomical observations by brightening the night sky.


Since 2019, the number of satellites orbiting Earth has increased rapidly, reaching more than 14,000 today [1]—mainly SpaceX Starlink telecommunications satellites. Satellite projects have also multiplied, both in number and in terms of potential impact. "So far we have managed to handle the situation, but it is getting worse," notes Olivier Hainaut, who participated in developing recommendations to mitigate the impact of satellite constellations on astronomy.

While companies like SpaceX have taken steps to reduce the brightness of their satellites, current satellite projects exceed "the limits" of what astronomy can withstand, he explains. Olivier Hainaut, an astronomer at ESO for more than 30 years, is the author of a peer-reviewed study on the impacts of satellite constellations, whose publication has been accepted in the journal Astronomy & Astrophysics.

SpaceX plans to put an additional million satellites into orbit for space-based data centers, which would significantly alter the appearance of the sky. The new study shows that for much of each night, hundreds of satellites would be visible, and at times up to several thousand, a number comparable to the stars visible to the naked eye under good conditions. Other planned satellite constellations, such as E-Space's "Cinnamon" and the Chinese constellations CTC-1 and CTC-2, would add hundreds of thousands more satellites into orbit, worsening the problem.

Reflect Orbital, a US startup, aims to launch a constellation of very large mirror-like satellites to provide sunlight at night, using reflected beams spanning at least 3.1 miles (5 km) on Earth's surface. The company plans to start by putting a prototype satellite into orbit this year and aims to increase its satellite count to 50,000 by 2035. These satellites would be the brightest ever placed in orbit, with detrimental effects on the quality of the night sky on Earth.

Olivier Hainaut's calculations show that the full constellation would fill the night sky with hundreds of very bright satellites. Seen from within a reflected beam, the satellite scattering sunlight would appear four times brighter than the full Moon. Even if no satellite directed its beam directly toward an observer, each would be as bright as the planet Venus, the "morning star." From a city suffering from light pollution, such as Munich, Germany, these hundreds of satellites would be the only "stars" visible in the night sky.

These projects, combined with others examined in the study, would significantly brighten the night sky, hampering humanity's ability to observe faint cosmic targets, including distant galaxies, some Earth-like planets orbiting other stars, and even asteroids potentially hazardous to Earth.

Bright streaks and an even brighter sky
Olivier Hainaut explains that "satellites, illuminated by the Sun, are much brighter than distant galaxies. When a satellite passes in front of what we are observing, it leaves a bright trail on our image, masking everything behind it."

To assess the impact of this phenomenon and other effects related to satellite constellations on astronomical observations, Olivier Hainaut simulated the positions, movements, and brightness of all existing and planned satellite constellations.

Regarding SpaceX's mega-constellation, he found that dozens of streaks appeared on each image taken two hours after the start of the night with ESO's Very Large Telescope (VLT) at the Paranal Observatory in Chile, representing a loss of field of view of up to 28% [2]. This calculation assumes that the satellites would be faint enough not to be visible to the naked eye under good conditions. If they were even slightly brighter, some instruments would be even more affected: for example, a camera like that of the US National Science Foundation's Vera C. Rubin Observatory could see most of its images rendered unusable for several hours each night [3].

Olivier Hainaut's simulations assumed that no Reflect Orbital satellite would direct its beam directly toward or near an observatory. Even so, the trail left by a single mirror satellite could ruin an observation made with a camera such as that of the Rubin Observatory. Once the entire Reflect Orbital satellite fleet is in orbit, all images taken by such a camera would be lost when the satellites are illuminated by the Sun.

However, it is not only the intersecting trajectories 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 phenomena increase the overall brightness of the night sky. This study is the first to examine the astronomical impacts related to the contribution of satellite constellations to sky background brightness, revealing the full extent of light pollution caused by satellites.

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

Limiting satellite numbers to preserve the night sky
Olivier Hainaut concludes that the proposed 1.7 million new satellites would have dramatic consequences for ground-based astronomy. These repercussions can only be avoided by limiting the total number of satellites, existing and future, to 100,000, and these must be faint enough not to be visible to the naked eye from a dark site.

"This is not an absolute number, like 99,999 is fine and 100,001 is bad: obviously, I would prefer 50,000," says Hainaut. "But 100,000 leads to losses of an order of magnitude comparable to other technical losses, such as equipment failures." He adds, however, that satellites must have a visual magnitude below 7 [4]; if some turn out to be too bright—exceeding the threshold for naked-eye visibility—the total number would have to be much lower.

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

"The FCC has received more than 1,800 comments regarding Reflect Orbital and nearly 1,500 comments on SpaceX's application," explains Betty Kioko, ESO's Head of Institutional Affairs, who coordinated ESO's response to these proposals. "The ball is now in the FCC's court, and we are waiting to see what decisions it will make on these two cases. For optical astronomy, this is an existential threat, and we hope regulators will share this view."

"Astronomy brings invaluable value to humanity, both scientifically, technically, economically, and educationally, and helps us understand our place in the Universe," says Xavier Barcons, ESO's Director General. "The large number of planned satellites in low Earth orbit strains this capability, highlighting the need to limit future satellite launches and to encourage astronomers, engineers, satellite operators, and other stakeholders to collaborate in adopting strict mitigation measures."

"The launch of thousands of satellites has economic, ecological, and astronomical implications," adds Olivier Hainaut. The light pollution generated by very bright satellite constellations can have repercussions on health and the functioning of life on Earth, disrupting biological clocks and ecosystems. Large constellations also have direct impacts on air quality, due to the many launches required to place and maintain thousands of satellites in orbit, as well as atmospheric pollution caused by their burning up during reentry at the end of their life cycle. "My job is astronomy, so I quantify the effects on astronomy," explains Olivier Hainaut. "I hope others will assess the other impacts in their area of expertise."

Olivier Hainaut concludes: "Low Earth orbit is a celestial coastline that brings invaluable value to modern life, from global connectivity to our direct access to the Universe. However, we must manage the footprint of mega-constellations—from light pollution affecting astronomy to the atmospheric effects of satellite reentry—to ensure this resource remains pristine and accessible for future generations."

Notes

1 - The number of satellites currently in orbit is 32,000 if inactive satellites and space debris are included.

2 - The instrument considered for the simulation is FORS2, the workhorse of the VLT, which is representative of traditional cameras on large telescopes.

3 - In cameras such as that of the Rubin Observatory, with complex, high-density electronics, a satellite trail bright enough to saturate the detector not only causes a wide trail on an astronomical image but also a series of ghost trails that multiply losses and can potentially contaminate the entire image.

4 - A satellite with a visual magnitude lower than 7 ensures it will not saturate the detector of cameras such as that of the Rubin Observatory. This also coincidentally means these satellites would be too faint to be seen with the naked eye, even under a perfectly dark sky.