loading . . . Have We Reached a Space-Junk Tipping Point?
**Low Earth orbit****,** where most satellites operate, has become a whirlwind of metal shards and dead, tumbling debris.
Anyone with hardware or human crew in orbit knows the drill. Orbital collision warnings can be unremitting. Whether the object is a defunct satellite or a stray hunk of glass from a solar panel that shattered long ago, every item circling Earth is also a potential projectile. And nearly all of this junk, traveling at least eight times as fast as a rifle bullet, can be damaging in a collision. SpaceX’s Starlink satellites maneuvered around possible debris impacts 144,404 times over the first half of 2025. That’s a collision warning every couple of minutes, night and day, for six months straight—three times the rate of the previous six months. Looming on the horizon, too, is the threat of orbital junk overwhelming satellites’ ability to dodge disaster. Each collision then creates more fragments, in a runaway cascade that turns low Earth orbit into a hazard zone.
This article is part of our special report The Scale Issue.
For satellite operators, sudden silences could be the first warning signs. Ground station crews that today coordinate elegant sequences of thruster burns will face more chaotic obstacle courses and bigger debris fields blooming across their display monitors. Communication lines and data traffic may drop from time to time, too, sowing chaos on the ground and menacing flights across the globe. And as the slow catastrophe builds, fuel reserves for satellite constellations will bleed down into the red from so many extensive orbital maneuvers. Spacecraft that’ve run dry today will be the seedbed for tumbling, hypervelocity shrapnel tomorrow.
This doomsday scenario is known as the Kessler syndrome, named after the American astrophysicist Donald Kessler, who in 1976 began circulating his first notices at NASA about possible runaway orbital debris. Now, as the magnitude of the space junk problem rapidly scales up, technological responses are ramping up as well. Solutions in the offing include high-resolution orbital tracking, AI-powered constellation management, and an emerging robotic tech called “active debris removal.” This last item involves lofting a specialized spacecraft into orbit, armed with grippers or other satellite-wrangling tech that can target and grab orbiting stuff. The removal craft then guides the space junk through reentry and the ultimate splashdown of whatever survives reentry into the middle of the ocean.
But tech alone may not be enough for the magnitude of the task ahead. The debris problem could simply be growing too fast. International treaties and government regulations may be needed to classify orbits as globally managed resources, like radio spectrum. Because as Kessler himself has pointed out, space is complicated—sometimes frustratingly so.
## What Is the Kessler Syndrome?
In the early days, those frustrations were related to simply getting the space community to realize the problem that lay ahead. Back in the early 1970s, when low Earth orbit was all but pristine, Kessler was a midcareer NASA scientist, having already notched important contributions to the Apollo and Skylab programs. As his colleague, the late NASA administrator Burton Cour-Palais, noted in a 2004 oral history, Kessler “was bringing up this orbital debris thing, and the higher-ups did not want to know about it at all.”
Cour-Palais also recalled being told to urge Kessler to “come up with solutions rather than problems.” Fortunately, neither took the overly cautious route.
In June 1978, the __Journal of Geophysical Research__ __ published a paper by Kessler and Cour-Palais in which they argued that a rapidly growing belt of defunct satellites, collision fragments, and other detritus could “be a significant problem during the next century.” It’s a prediction that has come to pass. In April of this year, Kessler and Hugh Lewis, professor of astronautics at the University of Birmingham, in England, presented their latest models, concluding that space junk orbiting between 400 and 1,000 kilometers—where most low Earth satellites operate—is already unstable. And between 520 and 1,000 km, the researchers found, debris concentrations are at or near levels that might sustain runaway growth.
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A recent internal report shared with __IEEE Spectrum_ _,__ written by analysts at the Menlo Park, Calif.–based LeoLabs, has divided the problem into what it calls “four waves of the Kessler syndrome.” The first three waves, it says, may have already begun. They are: nontrackable stuff like tiny steel fragments and glass splinters colliding with non-operational trackable objects; nontrackable stuff impacting functioning satellites and causing malfunctions; and trackable objects hitting other trackable objects and creating a clouds of fragments. The fourth wave, in which two large pieces of debris incite a chain reaction of other collisions, has yet to occur. In LeoLabs’ observations and models, satellites and operational spacecraft including the International Space Station, and China’s Tiangong space station continue to face manageable levels of collision avoidance maneuvers—for now.
“It is assumed these operational satellites will avoid catastrophic collisions with trackable objects,” the report concludes.
But according to Luc Piguet, CEO and cofounder of the Lausanne, Switzerland–based startup ClearSpace, challenges for operational satellites are real and mounting. “The Kessler syndrome is a slow, crawling effect—that when it starts accelerating, it’s already too late,” he says. “The Kessler syndrome is happening.”
The problem can be further segmented into specific problematic orbits, according to Darren McKnight, senior technical fellow at LeoLabs, which performs high-resolution debris tracking for private clients and government agencies.
“There are certain altitudes where we’ve already passed the threshold for the Kessler syndrome,” McKnight says. For instance, at 775 km altitude, as well as at 840 km and 975 km, the collision risk is scaling up rapidly. (See graph, “Low Earth Orbit’s Most High-Risk Places.”)
“We will hit a point where particular popular orbits are so risky to operate in that the benefits of operating there are outweighed by the cost and risk,” says Danielle Wood, head of MIT Media Lab’s Space Enabled Research Group.
## Why Is the Kessler Syndrome Complicated?
According to the European Space Agency, 14.5 million kilograms of man-made stuff circles the planet today. Compare that to 11 million kg two years ago and 8.9 million kg in 2020—a 63 percent increase over the past five years.
McKnight says the Kessler problem comes into sharper focus when dividing mass in any given orbit by the volume of space that orbit occupies. The __mass density__ in orbit, also known as the mass per cubic kilometer, provides a clue not only to the chance of orbital collisions but also to those collisions’ consequences. Two small orbiting items colliding won’t create nearly as much new debris as will two big ones. The more densely packed an orbit is, in other words, the more treacherous it is to keep a satellite at that orbit. “Mass per cubic kilometer is debris-generating potential,” McKnight says, which would be a great thing to know with confidence in all the different regions of low Earth orbit.
In 2002, Space Shuttle astronauts retrieved these solar panels from the Hubble Space Telescope—revealing how destructive even small projectiles are when traveling at low Earth orbit speeds. ESA
However, says Katherine Courtney, chair of the Global Network on Sustainability in Space, knowing where all orbiting stuff is today has become a tall order. “A substantial portion of smaller space junk can only be extrapolated using data collected from returned spacecraft and historical records. The vast majority can’t be tracked from the ground,” Courtney adds.
Moreover, says Jonathan McDowell, astrophysicist and space historian at the Harvard & Smithsonian Center for Astrophysics, in Cambridge, Mass., once stuff in orbit goes missing, further complications emerge. Collisions between the missing matter and other debris can completely knock the collisions’ by-products into different orbits.
“The operating satellites are in nice circular orbits,” McDowell says, “whereas the collision debris is crossing many orbits and affecting many more.”
What’s now needed as the problem grows larger is a complete rethink, says Moriba Jah, professor of aerospace engineering and engineering mechanics at the University of Texas at Austin.
“I don’t subscribe to the Kessler syndrome,” Jah says. “It’s not that cascading collisions can’t happen. It’s that the framework oversimplifies the problem and doesn’t give us a way to manage or evolve the system.”
Consider instead, Jah says, a parameter he calls “orbital carrying capacity.” “If we start from the end, we can say that carrying capacity is consumed when our ability to make decisions to avert harm no longer work,” he continues. “So to me, that doesn’t necessarily look like you’re bumping into stuff. It also looks like you’re spending fuel moving around stuff so much that you can’t do the things that you wanted to do to begin with.”
## How to Avoid Satellite Collisions
As SpaceX proved 144,404 times from December 2024 through May of this year, the Starlink constellation’s capacity to maneuver its hardware around space junk is impressive.
“Starlink is a brilliant constellation,” McKnight says. “They’re like a granny driving on the highway. They pump their brakes. They avoid everything.”
However, Starlink’s own public record also showcases how rapidly the collision hazards in orbit are evolving. The company’s publicly disclosed data reveals a 22-fold increase since 2020 in the amount of ducking and dodging the constellation has needed to perform to avoid collisions with other stuff in orbit.
Everyone’s ducking and dodging these days, too.
“Collision avoidance is a standard practice now for every operator,” says Tim Flohrer, head of the European Space Agency’s Space Debris Office.
“You want to keep your operations making sense, communicating with everybody else,” says Marlon Sorge, technical fellow at the Chantilly, Va.–based Aerospace Corp., “and not making more of the stuff that you can’t communicate with.”
Yet, space junk isn’t the only class of noncommunicative stuff up there. “More than half of the unidentified objects are Chinese satellites,” says Courtney of the Global Network on Sustainability in Space. “So they’re active satellites, but they’re just not registered as identifiable objects.”
The tracked debris, the untrackable tiny debris, the bigger things that are also incommunicado—all of it combines to make for an increasingly massive headache.
“Every collision-avoidance maneuver is a nuisance,” Holger Krag, head of ESA’s Space Safety office, has said. “Not only because of fuel consumption but also because of the preparation that goes into it. We have to book ground-station passes, which costs money. Sometimes we even have to switch off the acquisition of scientific data. We have to have an expert team available round the clock.”
So who or what, then, could possibly keep up with the rapidly scaling nature of the Kessler problem? Artificial intelligence is the almost unanimous answer.
Many of the world’s major players in low Earth orbit, including small satellite startups and big national space programs, are currently testing and developing AI constellation-management systems. Machine-learning algorithms are proving increasingly adept at making more accurate collision warnings and performing automated decision-making—as well as sharpening the resolution of small object detection to find smaller orbiting stuff than what non-AI-powered tracking tech can see. Some companies and research teams are also developing AI tools to go beyond just keeping pace with the problem, using AI to optimize fuel usage and maintain ideal satellite configurations for low battery usage and simplified signal traffic as well.
However, for all its smarts, AI still can’t make the most difficult orbital hazards go away. That’s why some companies are approaching the Kessler problem as one of disposing, rather than dodging.
A number of startups are actively pursuing ways to extract the most dangerous orbital objects—defunct rocket stages, dead satellites, space collision fragment clouds, and space-race relics.
“The technology available to remove debris today is really toward larger pieces of debris,” says Andrew Faiola, commercial director of the Tokyo-based company Astroscale. “We’re just maturing that capability to be able to effectively, safely, and securely remove large pieces of debris.”
Astroscale and ClearSpace aim to launch spacecraft over the next few years that will each target an aging satellite (a Eutelsat OneWeb satellite and ESA’s PROBA-1, respectively) for a prototype removal mission.
The European radar imaging satellite Sentinel-1A caught a millimeter-sized particle impacting one of its solar panels, leaving behind a 40-centimer wide zone of damage. ESA
“You need to do controlled entry,” ClearSpace’s Piguet says. “This means you need to push this satellite into Point Nemo over the South Pacific, where there’s no airlines, ground traffic, and no inhabited island.”
Ideally, then, between smart constellation management, active collision avoidance, and active cleanup, low Earth orbit will become something closer to a regulated and moderated space—much like airspace around major metro areas today.
“It’s much the same as air-traffic control,” Faiola says. “As the technology gets better, you start to see aircraft being stacked more closely together. You have the same amount of real estate, but you can put more objects in there more safely when you have better visibility and situational awareness of where everything is. It’s the same in space.”
## What Are the Solutions to the Kessler Problem?
Space tech and space tech alone may one day resolve the Kessler syndrome.
But as a complement to the technological innovation, international space agreements and law are also being reconsidered, because much of the existing space law standards were agreed on decades ago, during an entirely different era in low Earth orbit.
For instance, between the Outer Space Treaty of 1967 and the 1972 Space Liability Convention, even an untraceable fragment of metal in space is effectively owned by the nation that launched it. This arguably means that that nation may need to give permission for anyone else to remove the fragment from orbit.
“There’s no national borders up there,” says Faiola. “But every object that is cataloged is also owned by someone, a state. And you’re not allowed to touch someone else’s stuff without their permission.”
In August, Japan announced it would be developing its own legal frameworks for removing space junk from orbit. And this November, in Vienna, the United Nations Office for Outer Space Affairs will be hosting a space law conference to tackle these issues as well.
International agreements need reconsidering in other ways, too. Some space experts __Spectrum__ spoke with argue for additional regulations to prevent orbits from further clogging up.
“There will have to be internationally coordinated agreements on who gets what orbit and how many satellites you can have in that orbit,” says Smithsonian’s McDowell.
Courtney envisions something like a worldwide space command network. “We need to be designing solutions that allow the growth to continue,” she says. “What we need is a global space traffic control solution like we have for air traffic today.”
Jah of the University of Texas at Austin argues for ultimately bringing orbital space closer to its original state of being, as he puts it, “a viable commons.” When a new player—whether a company or a national space agency—wants to put something into a given orbit, he says, that new orbiting asset should be added to a master spreadsheet somewhere.
“If another country wants to be able to be in that orbit, there should be an equitable way to share the carrying capacity of that orbit,” he says.
Rockets, satellites, and launch systems today still follow the space race–era legacy designs that treat orbital space like an infinite junkyard, he adds. “Right now, every single object that we launch into orbit is the equivalent of a single-use plastic,” Jah says. “We need to invest in reusable and recyclable satellites.”
Even if the Kessler problem on the home planet can be solved, says Courtney, the same thing could happen on other planets and moons. “We’re very worried about low Earth orbit, but [there’s also] all the commercial activity and all of the great-power competition for landing things on the moon and Mars,” she says.
“We have no space-traffic coordination solutions for cislunar space, and yet that’s the race that’s just starting now,” she says. “We’re expanding outward into the solar system, and we’re just taking these problems with us.” https://spectrum.ieee.org/kessler-syndrome-space-debris
