The race to fill low-Earth orbit with satellites is accelerating—and astronomers are warning that humanity could soon pay a scientific price for the increasingly crowded sky.
A new study by the European Southern Observatory (ESO) finds that current proposals for more than 1.7 million additional satellites could have severe consequences for ground-based astronomy, particularly if large numbers of satellites are bright enough to reflect sunlight into telescope observations.
The warning comes as the number of satellites around Earth has surged since 2019, driven largely by the expansion of commercial broadband constellations such as SpaceX’s Starlink. Reuters reported on Thursday that roughly 15,000 satellites are currently in orbit, while proposals for vastly larger constellations are moving forward.
Why astronomers are worried
Satellites do not simply occupy space above Earth. When illuminated by the Sun, they can appear as moving streaks across telescope images.
Those streaks can obscure stars, galaxies and other objects scientists are trying to study. A single satellite crossing a telescope’s field of view may affect only part of an image, but the problem becomes much more serious when thousands—or potentially hundreds of thousands—are passing overhead.
ESO astronomer Olivier Hainaut, who led the study, said the impact depends on both the number and brightness of satellites. Simulations conducted around ESO’s Paranal Observatory in Chile’s Atacama Desert found that thousands of illuminated satellites could be visible above the Very Large Telescope shortly after sunset.
The study estimates that a satellite population of around 100,000 faint satellites could represent a manageable level of interference if those spacecraft remain below naked-eye visibility. But much larger and brighter populations could push some astronomical observations beyond practical limits.
From 100,000 to 1.7 million: the scale changes dramatically
The most alarming figure is not the number of satellites currently orbiting Earth, but the scale of what has been proposed.
ESO says more than 1.7 million satellites are represented in current proposals. The agency warns that allowing such a large population—particularly satellites with high reflectivity—could dramatically increase artificial sky brightness and interfere with astronomical observations.
Reuters reported that ESO simulations indicate a constellation of around 300,000 satellites could already severely impair data collection for some telescopes.
That does not mean 300,000 satellites would automatically destroy astronomy worldwide. The actual impact depends on orbital altitude, satellite design, brightness, orientation, observing time and the telescope itself.
But the direction of travel is clear: more satellites mean more opportunities for observations to be contaminated.
The problem is already affecting major observatories
This is not simply a future scenario.
The Vera C. Rubin Observatory, which is conducting an enormous survey of the southern sky, says low-Earth-orbit satellites already pose a significant challenge to its Legacy Survey of Space and Time.
Rubin’s scientists explain that satellites can appear as bright streaks across its images. During a typical 30-second exposure, a low-Earth-orbit satellite can move roughly 15 degrees across the sky—enough to streak through a substantial portion of the observatory’s wide field of view.
The consequences could be particularly important for searches for potentially hazardous asteroids, precision cosmology and other research that depends on extremely sensitive measurements.
Scientists are developing techniques to identify and mitigate satellite trails, but Rubin notes that the rapidly changing satellite population makes the ultimate scale of the problem difficult to predict.
Even space telescopes are not completely safe
The concern extends beyond telescopes on Earth.
A separate NASA-led study published in Nature in December 2025 examined the effects of satellite megaconstellations on space-based observatories. Researchers estimated that if the number of satellites in low-Earth orbit reached around 560,000, satellite trails could contaminate a substantial share of observations from several space telescopes.
The Nature study found that the Hubble Space Telescope could be affected by satellite trails, while some other observatories could face substantially higher levels of contamination depending on their orbital characteristics and observing programs.
In other words, moving astronomy into space does not automatically solve the satellite problem.
It is not only visible light
Satellite interference is also becoming a concern for radio astronomy.
Researchers have reported unintended and out-of-band emissions from low-Earth-orbit satellite systems at frequencies relevant to radio astronomy. A recent observational program involving Starlink, OneWeb, Amazon Leo and China’s Guowang satellites detected thousands of instances of intended, unwanted or unintended radiation across hundreds of frequencies. Some detections occurred within protected radio-astronomy bands.
That creates a different kind of challenge: while optical astronomers are fighting satellite streaks and reflected sunlight, radio astronomers must contend with electromagnetic interference.
Why satellite operators still matter
The issue is not as simple as satellites versus science.
Satellite networks provide broadband connectivity and other services, including communications in places where conventional infrastructure can be difficult or expensive to deploy. Reuters noted that satellite internet has brought significant benefits, including improved connectivity in remote areas.
The scientific community is therefore increasingly focused on mitigation and regulation, rather than simply opposing satellite technology.
Measures can include reducing satellite reflectivity, changing spacecraft orientation, coordinating satellite operations with observatories, improving orbital planning and protecting particularly sensitive astronomical observations.
The American Astronomical Society has likewise called for continued research and cooperation among astronomers, satellite operators, regulators and governments to understand and reduce the effects of large satellite constellations.
The race against time
The biggest concern may be the speed at which the satellite industry is expanding.
Scientific research takes years to establish how a new technology affects the environment or scientific observations. Satellite constellations, meanwhile, can be deployed comparatively quickly.
That creates a difficult policy question: Should regulators wait for the full scientific impact to become clear, or establish limits before the orbital environment becomes substantially harder to manage?
ESO’s Hainaut argues that the current trajectory is pushing astronomy toward a threshold it may not be able to tolerate indefinitely.
For astronomers, the issue ultimately goes beyond protecting a few telescope images. The night sky is humanity’s laboratory for studying everything from nearby asteroids to distant galaxies and the evolution of the Universe.
The technology filling Earth’s orbit promises faster communications and new commercial opportunities.
But if satellite numbers continue to multiply unchecked, scientists warn that the same sky used to connect the world could become increasingly difficult to use for discovering what lies beyond it.
And with more than 1.7 million satellites already represented in proposals, the question is no longer whether the sky is getting crowded—it is how crowded humanity is willing to let it become.
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