ESO warns 100,000 satellites could corrupt telescope data 100% for astronomy

A new study from the European Southern Observatory (ESO) is putting hard math behind a worry astronomers have carried for years: if satellite counts in low Earth orbit climb past 100,000, some of the world’s most important telescopes may effectively stop working. Olivier Hainaut, ESO’s director of operations and lead author of the study, tells Space.com that in the worst scenario “basically, there is no point in operating the telescopes anymore because all the data will be corrupted. All. 100 percent.”

The study’s modeling hinges on two knobs: how many satellites are in orbit and how bright they are in the night sky. If there are 100,000 satellites but they are barely visible to the naked eye, astronomy could cope. Change the brightness, though, and the problem escalates fast. Hainaut says satellites around magnitude 7 or below (astronomical terms) would make research more difficult and more expensive.

Why brightness matters so much comes down to how satellites mess with images. Hainaut frames it as two effects. First, reflected sunlight increases the overall brightness of the sky, creating light pollution that reduces how many natural stars telescopes can see. Second, brighter satellites produce streaks in telescope images that mar observations. The exposure-time math is brutally direct: Hainaut explains that if you increase light pollution by 10 percent, telescopes have to increase all exposure times by 10 percent, because the relationship scales linearly. For a 100 percent increase in light pollution, exposure times need to rise by 100 percent too. That means fewer observations in a given window of telescope time, and each observation costs more.

This is not just an abstract sky aesthetic issue. The International Astronomical Union says that a light pollution increase of more than 10 percent compared to natural dark-sky conditions is an “astronomy killer.” And because light pollution on Earth has spread with urban development over the past two centuries, astronomers have increasingly retreated to remote, dark sites. Many top telescopes are in Chile’s Atacama Desert, including the Vera C. Rubin Observatory as well as ESO’s Very Large Telescope and Extremely Large Telescope, where the night sky is still nearly perfectly dark.

But Hainaut warns there is no version of “remote enough” when the light source is in orbit. Satellite light pollution would brighten the sky above telescopes even if researchers are far from cities, whether they are in Chile, camping in the Australian outback, or on expeditions to Antarctica or the Amazon rainforest. In other words, the industry cannot out-run the physics. And the study goes one step further: it tests what happens if plans to deploy not just satellites, but reflective orbital structures, reach their full scale.

That part of the concern is aimed at Reflect Orbital, a U.S. company headquartered in Hawthorne, California, which wants to deliver light on demand to solar power plants at night and to illuminate warzones and areas struck by natural disasters. Reflect Orbital has applied to the Federal Communications Commission (FCC) to launch a demonstration space mirror into orbit later this year. The satellite, called Eärendil-1, is 59 by 59 feet (18 by 18 meters), and should be the first in a constellation of 50,000 if things proceed according to the plan. Hainaut calls these proposed mirrors “really bad,” saying they would make observations “close to impossible,” describing them as “super bright satellites.”

The modeling-backed imagery isn’t subtle. Astronomers calculate that each Reflect Orbital space mirror would be brighter than the full moon when observed from the area where its beam is aiming. And unlike a traditional spotlight that only affects viewers in the beam, Hainaut says the satellites would be visible regardless of where their beams are aiming. Even outside the beam, he says the satellite would appear brighter than Venus, the brightest object in the night sky after the moon. If 50,000 of these mirrors were launched, Hainaut estimates there could be many hundreds, or even a few thousand, of these super bright objects visible to observers anywhere on Earth. In his calculations, the full Reflect Orbital constellation could brighten the night sky by up to 300 percent.

Hainaut also brings SpaceX into the comparison, specifically in the context of the company’s planned “orbital data centers.” These would feature 230-foot-wide (70 m) solar panels, but Hainaut says they would be much dimmer and about as visible as Starlink satellites, based on available information. He attributes that to design choices: reflective surfaces tilted away from Earth, and a narrow satellite body that points to Earth with its small end. Still, the direction of the conclusion stays consistent. The researchers caution that satellite operations across all operators should remain below 100,000 combined to avoid severe impacts on astronomy.

Right now, that number is already a live policy question. SpaceX is awaiting an FCC decision on its application to launch one million orbital data centers, while today there are about 14,000 satellites in orbit. In that backdrop, Robert Massey, Deputy Executive Director of the Royal Astronomical Society, tells Space.com that Hainaut’s findings are “not hugely surprising” and repeats the core warning: “For astronomy, this would obviously be catastrophic.” Massey adds an additional stakeholder point: the public has “not signed up” for an entirely transformed sky.

Regulation is where boardrooms should pay attention, because the story is as much about process as it is about planets. Massey points out that, under international law governing space activities, it is perfectly legal for a U.S. organization to decide on something that impacts the entire world, and that if the FCC approves the plan it would be “deeply regrettable.” He argues that the outcome would signal a world where large corporations can determine the view of the sky above people’s heads, similar to how they can transform the environment on Earth, but without the same tight regulatory discipline. The Trump administration has been taking steps to reduce the burden on satellite operators to prove projects will have no negative environmental impacts. Currently, the FCC or private companies do not require an environmental review before satellite applications go for approval.

ESO’s Betty Kioko, an institutional affairs adviser, adds the legal framework layer: the United Nations Outer Space Treaty, signed in 1967, assigns responsibility for space launches to the nation states where those space objects are registered. She says the treaty requires states to use space “for the common good of humankind.” For executives, the strategic stakes are obvious: if the FCC’s decisions enable scale, the operational reality for observatories is not incremental. It is a potential step-change where the cost of seeing declines in science output, or worse, becomes functionally impossible.

For peers in adjacent industries, the second-order question is whether “optics as a service” will be treated like other environmental and infrastructure externalities. If light pollution becomes a line-item outcome of licensing, then every operator, investor, and regulator will face the same board-level test: what does it mean to optimize for performance when the sky itself is the shared asset?