At any given moment, more than 10,000 satellites are whizzing around the planet at roughly 17,000 miles per hour. This constellation of machinery is the technological backbone of modern life, making GPS, weather forecasts, and live television broadcasts possible. 

But space is getting crowded. Ever since the Space Age dawned in the late 1950s, humans have been filling the skies with trash. The accumulation of dead satellites, chunks of old rockets, and other litter numbers in the tens of millions and hurdles along at speeds so fast that even tiny bits can deliver lethal damage to a spacecraft. Dodging this minefield is already a headache for satellite operators, and it’s poised to get a lot worse — and not just because humans are now launching thousands of new crafts each year. 

All the excess carbon dioxide generated by people burning fossil fuels is shrinking the upper atmosphere, exacerbating the problem with space junk. New research, published in Nature Sustainability on Monday, found that if emissions don’t fall, as few as 25 million satellites — about half of the current capacity — would be able to safely operate in orbit by the end of the century. That leaves room for just 148,000 in the orbital range that most satellites use, which isn’t as plentiful as it sounds: A report from the U.S. Government Accountability Office in 2022 estimated that as many as 60,000 new satellites will crowd our skies by 2030. According to reports, Elon Musk’s SpaceX alone wants to deploy 42,000 of its Starlink satellites.

“The environment is very cluttered already. Satellites are constantly dodging right and left,” said William Parker, a PhD researcher in the Massachusetts Institute of Technology’s Department of Aeronautics and Astronautics and the lead author of the study. In a recent six-month period, SpaceX’s Starlink satellites had to steer around obstacles 50,000 times. “Aslong as we are emitting greenhouse gases, we are increasing the probability that we see more collision events between objects in space,” Parker said.

Until recently, the effects of greenhouse gas emissions on the upper atmosphere was so understudied that scientists dubbed it the “ignorosphere.” But research using modern satellite data has revealed that, paradoxically, the carbon dioxide that warms the lower atmosphere is dramatically cooling the upper atmosphere, causing it to shrink like a balloon that’s been left in the cold. That leaves thinner air at the edge of space.

The problem is that atmospheric density is the only thing that naturally pulls space junk out of orbit. Earth’s atmosphere doesn’t suddenly give way to the vacuum of space, but gets dramatically thinner at a point known as the Kármán line, roughly 100 kilometers up. Objects that orbit the planet are dragged down by the lingering air density, spiraling closer to the planet until eventually reentering the atmosphere, often burning up as they do. 

According to the nonprofit Aerospace Corporation, the lowest orbiting debris takes only a few months to get dragged down. But most satellites operate in a zone called “low Earth orbit,” between 200 and 2,000 kilometers up, and can take hundreds to thousands of years to fall. The higher, outermost reaches of Earth’s influence are referred to as a “graveyard” orbit that can hold objects for millions of years. 

“We rely on the atmosphere to clean out everything that we have in space, and it does a worse job at that as it contracts and cools,” Parker said. “There’s no other way for it to come down. If there were no atmosphere, it would stay up there indefinitely.”

Parker’s study found that in a future where emissions remain high, the atmosphere would lose so much density that half as many satellites could feasibly fit around all the debris stuck in space. Nearly all of them would need to squeeze into the bottom of low Earth orbit, where they would regularly need to use their thrusters to avoid getting dragged down. Between 400 and 1,000 kilometers, where the majority of satellites operate, as few as 148,000 would be safe. More than that, and the risk of satellites crashing into debris or each other poses a threat to the space industry.

“The debris from any collision could go on to destroy more satellites,” said Jonathan McDowell, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Massachusetts who was not involved with the Nature study. “And so you can get a chain reaction where all the satellites are hitting each other, breaking up, and creating more and more debris.”

This domino effect, commonly known as Kessler syndrome, could fill the orbit around Earth with so much destructive clutter that launching or operating satellites becomes impossible. It’s the runaway scenario that the paper cautions greenhouse gas emissions will make more likely. “But the chain reaction doesn’t happen overnight,” McDowell said. “You just slowly choke more and more on your own filth.” 

According to the European Space Agency, at least 650 breakups, explosions, or collisions have flung their wreckage into space since space exploration started. Space surveillance networks, like the U.S. Space Force, are currently tracking nearly 40,000 pieces of debris, some as large as a car. At least 130 million objects smaller than 10 centimeters are also estimated to be orbiting Earth but are too tiny to be monitored.

Scientists have recently been researching ways to remove this debris, by, as McDowell metaphorically put it, “sending garbage trucks into space.” In 2022, a Chinese satellite successfully grabbed hold of a defunct one by matching its speed before towing it into graveyard orbit. In 2024, a Japanese company, Astroscale, managed to maneuver a retrieval device within 15 meters of a discarded rocket — close enough to magnetically capture it — before backing away.

“In general, it’s an environmental problem being stored up for future generations,” McDowell said. “Are we going to hit our capacity? I think we’re going to find out the hard way.”