The night sky has been the canvas of humanity’s wonder for millennia. The stars guided sailors, inspired poets, and offered astronomers the first glimpse into an infinite universe. But today, the ancient map is being redrawn, not by nature but by technology.

The number of satellites in orbit around Earth has exploded from fewer than 8,000 satellites before 2019 to over 14,000 now. Many are from SpaceX’s own Starlink network, which aims to provide worldwide Internet coverage. But this is just the start. A total of over 1.7 million satellites (many of which are exceptionally bright) are from proposals currently awaiting approval, with Hubble being the most optimistic one.

According to a new study from the European Southern Observatory (ESO), the consequences would be catastrophic.

“Until now we have managed, but it’s getting worse,” stresses Olivier Hainaut, an ESO astronomer who has studied the issue for decades.

Study Evaluates Impact of Satellite Constellations on Astronomical Observations

His peer-reviewed analysis, accepted for publication in Astronomy & Astrophysics, concludes that astronomy could only survive if the total number of satellites is limited to 100,000 faint ones, below naked-eye visibility. Anything more, he warns, would have “devastating consequences for astronomy.

SpaceX alone plans to send one million satellites into orbit, including space-based data centers. Hainaut’s simulations show that on clear nights, hundreds of satellites would be visible at once, and at certain times, thousands, matching the number of stars seen with the naked eye under pristine conditions.

Other companies are joining the race. E-Space’s Cinnamon constellation and China’s CTC-1 and 2 projects could add hundreds of thousands more satellites. And then there is Reflect Orbital, a U.S. start-up with perhaps the most audacious plan: launching mirror-like satellites to reflect sunlight at night. Their beams would cover five kilometers on Earth’s surface.

Hainaut’s calculations are stark. A single Reflect Orbital satellite, seen from within its beam, would appear four times brighter than the full Moon. Outside the beam, each satellite would still shine as brightly in the night sky as Venus does as a “morning star.” In light-polluted cities like Munich, these satellites may be the only “stars” we see in the sky.

Astronomers already struggle with satellite streaks across their images.

“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,” explains Hainaut.

His simulations show that with SpaceX’s mega-constellation, dozens of trails would appear in each image taken just two hours into the night with ESO’s Very Large Telescope in Chile.

That represents field-of-view losses of up to 28%. For sensitive instruments like the Vera C. Rubin Observatory’s wide-field camera, the damage could be worse: entire nights of data rendered unusable.

Reflect Orbital’s satellites would be even more destructive. Even if none pointed directly at an observatory, their trails could spoil every image when illuminated by the Sun.

But the problem isn’t only streaks. Faint satellites add a diffuse glow, while brighter ones scatter light through the atmosphere. Together, they raise the background brightness of the night sky.

Hainaut’s study is the first to quantify this effect, showing that very bright constellations could make the sky three to four times brighter overall.

The impacts extend beyond telescopes. “Sending thousands of satellites has implications: economic, ecological, and astronomical,” says Hainaut. He notes that light pollution from bright constellations can disrupt ecosystems and biological clocks. Launching and maintaining thousands of satellites also affects air quality, while their fiery re-entry pollutes the atmosphere.

“My job is astronomy, so I quantify the effects on astronomy,” he adds. “I hope others will evaluate the other impacts in their field of expertise.”

Hainaut concludes that only by capping satellites at 100,000 faint ones can astronomy survive. “This is not a hard number, like 99,999 is good and 100,001 is bad: clearly I’d prefer 50,000,” he says. “But 100,000 causes losses at about the level of other technical losses, such as equipment failure.” Crucially, he adds, satellites must be fainter than visual magnitude 7, invisible to the naked eye.

The study has already influenced policy. ESO, together with the UK’s Royal Astronomical Society and the International Astronomical Union, submitted it to the U.S. Federal Communications Commission (FCC) in response to filings by SpaceX and Reflect Orbital.

“The FCC received over 1800 comments regarding Reflect Orbital and nearly 1500 comments on the application by SpaceX,” explains Betty Kioko, ESO’s Institutional Affairs Officer. “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.”

ESO Director General Xavier Barcons adds: “Astronomy generates huge value for humankind, including scientific, technical, economic, and educational, and helps us understand our place in the Universe. 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.”

In the end, Hainaut offers a metaphor: “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.”

The night sky has always been humanity’s shared heritage. Whether it remains a window to the cosmos or becomes a crowded billboard of satellites now depends on choices made here on Earth.

Reference:

O. R. Hainaut. Large or bright satellite constellations Effects on observations, including background sky brightness.