Space debris, or “space junk,” refers to defunct satellites, spent rocket stages and metallic fragments created by the collision, explosion or disintegration of spacecraft. Every time something is launched into space, more debris gets added to the mix.
What Is Space Debris?
Space debris, sometimes called space junk or orbital debris, is any nonoperational, human-made object left in space. This could be anything from an inactive satellite, the spent upper stage of a rocket or a piece of a metal that broke away due to erosion, collision or explosion.
Space debris is particularly problematic in low-Earth orbit, a heavily populated region with nearly 10,000 satellites. Because space debris travels at 17,500 miles per hour — nearly seven times faster than a bullet — even a small metal shard could cause significant damage to a satellite or an astronaut on a space walk, John Crassidis, a professor of mechanical and aerospace engineering at University at Buffalo, told Built In.
As space debris increases, so too do the odds for collision. Many astrophysicists worry that these collisions will create even more space debris, setting off a chain reaction that clutters outer space to the point where low-Earth orbit is no longer safe for the key infrastructure necessary for telecommunications, weather forecasting and aerospace research.
“We have to be able to keep these orbits clean,” Christopher Johnson, director of legal affairs and space law at the Secure World Foundation, told Built In. “It’s a bit like keeping the highways clear of debris — we need these orbits to be safe, clean and usable.”
What Is Space Debris?
Space debris includes inactive satellites, discarded rocket stages and fragments created by rocket disintegration, satellite collisions and missile strikes on satellites.
Most space junk is in low-Earth orbit, which has an altitude ranging from 180 km to 2,000 km. This orbit is home to the International Space Station, the Hubble Space Telescope and nearly 10,000 other satellites. Objects in the lower regions of low-Earth orbit can fall back to Earth and burn up as they re-enter the atmosphere. But space junk in the higher ranges of low-Earth orbit can remain for hundreds of years.
Space junk can also be found in geostationary orbit, about 36,000 kilometers above the equator. Because it’s so far from Earth’s atmosphere, much of this stuff may never come back down to Earth. While the collision risk in geostationary orbit is lower than the more densely populated low-Earth orbit, it still contributes to the growing problem of space pollution.
How Much Debris Is in Space?
Space debris has been accumulating since 1957, when the first satellite went into orbit. Now, the European Space Agency (ESA) estimates there are about 44,700 pieces of space debris larger than 10 centimeters, including 3,000 inactive satellites. Objects smaller than 10 centimeters (about the size of a softball) are harder to reliably track with telescopes and radar. But the ESA estimates there are 1.2 million pieces between 1 and 10 centimeters, and another 130 million between 1 millimeter and 1 centimeter.
The amount of debris in space is expected to increase as more companies enter the space industry. In 2020, there were roughly 2,000 satellites in orbit. Just five years later, there are more than 11,000 active satellites in orbit, according to astrophysicist Jonathan McDowell. More than 7,000 of those are part of the Starlink mobile broadband network, which eventually plans to release up to 42,000 satellites.
Starlink isn’t the only mega-constellation in space, either. Amazon’s Project Kuiper plans to deploy more than 3,000 satellites for its broadband project, and Chinese companies Qianfan and Guo Wang plan to install 14,000 and 13,000 satellites, respectively. While Starlink satellites can maneuver away from other objects, many other commercial satellites can’t. Because of their abundance and immobility, these new generations of satellites pose an increased risk for creating space debris, Johnson said.
Greg Henning, a project leader in The Aerospace Corporation’s Center for Orbital and Reentry Debris Studies, told Built In that constellation operators in the United States and Europe have been successful at disposing of satellites and preventing debris-generating events so far. But he said it’s impossible to know if other operators will maintain that same level of care.
“If any one of those constellations does not follow good debris mitigation practices, it could cause a serious problem for everyone,” Henning said.
Tracking Space Debris
The U.S. Space Force’s Space Surveillance Network tracks and catalogs space debris the size of a softball or larger using optical telescopes and radar on Earth, as well as in space. Henning explained that data from multiple sensors is combined to estimate the object’s orbit, allowing us to predict its future position and velocity. If there’s a one-in 10,000 chance of an object colliding with a satellite, the Space Force alerts the satellite owner, Crassidis said.
Some companies, like Privateer, use artificial intelligence to spot space debris and predict potential collisions. The Aerospace Corporation’s Center for Orbital and Reentry Debris Studies (CORDS), meanwhile, has developed the Aerospace Debris Environment Projection Tool (ADEPT) to model and simulate how different strategies and policies could affect future debris accumulation, collision frequency and satellite operations.
Notable Space Debris Incidents
China’s 2007 Missile Test
In January 2007, the Chinese government struck one of its defunct weather satellites, Fengyun-1C, with an anti-satellite weapon (ASAT), creating more than 3,000 pieces of trackable debris in low-Earth orbit. Scientists estimate another 32,000 smaller pieces are not able to be tracked. This missile test was condemned by the international community, as it generated the most debris of any event in space history.
Iridium-Cosmos Collision of 2009
In February 2009, an inactive Russian communications satellite, Cosmos 2251, collided into Iridium 33, a communications satellite operated by U.S. company Iridium Satellite. Both satellites shattered, creating more than 2,000 pieces of trackable debris and thousands of smaller, untrackable debris. This was the first time two satellites collided in orbit.
Russian Missile Test of 2021
While not as significant as the Chinese missile test or the Iridium-Cosmos Collision, a Russian missile test in 2021 generated more than 1,500 pieces of trackable debris and hundreds of smaller pieces of debris. The test, which destroyed Kosmos 1408 — an old Soviet spy satellite — also forced astronauts in the International Space Station to take cover in detachable lifeboat pods.
The Risks of Space Debris
Damage to Satellites
Space debris poses a danger to all of the satellites in orbit. Satellites are used for commercial, defense and scientific purposes, and they are critical for GPS, telecommunications, weather predictions and military surveillance.
Our most significant space asset, the International Space Station, can maneuver away from trackable debris and is protected by Whipple shields that can withstand impacts from objects up to 3 millimeters in size. However, not all satellites have thrusters and shields, and current tracking systems can’t detect every piece of debris out there.
“We have seen satellites hit by debris that is too small to avoid. They can survive some of these impacts, but sometimes they do not,” Henning said. “Unfortunately, we cannot track all space debris, or even most of it that might pose a mission-ending threat to satellites.”
Even if a satellite owner is given enough notice about a potential collision, each maneuver uses valuable fuel, making space operations more costly.
Injury to Astronauts
Small, untrackable pieces of debris are particularly dangerous to astronauts walking outside of a space station. Astronaut suits are made of resilient materials that may provide some protection against very small bits of debris, but an untrackable piece of debris traveling at 17,500 miles per hour can puncture a space suit and injure an astronaut.
“They are completely exposed when they’re out doing their space walks,” Crassidis said. “That’s what I’m most worried about.”
Objects Falling to Earth
Most space debris burns up as it enters the Earth’s atmosphere, but not all of it. Roughly 1,200 “in-tact objects” reentered the atmosphere in 2024, not including smaller fragments, according to the ESA report.
The odds of a person being hit by fallen debris is estimated at one-in-a trillion. The only known incident occurred in Oklahoma in 1997, and the woman was not injured by the impact. There have been other close calls, though. In March 2024, a 1.6-pound piece of space debris pierced the roof of a Florida home, crashing through two floors of the house. The debris came from a pallet of old batteries, roughly the size of a sport utility vehicle, that was ejected from the International Space Station in 2021. NASA expected the debris would burn up as it re-entered Earth’s atmosphere, but not all of it did.
Kessler Syndrome
Kessler Syndrome, a theory posited in 1978 by NASA scientist Donald Kessler, warns of space debris accumulating to the point where it creates uncontrolled collisions, causing additional space debris that leads to further collisions. In this scenario, low-Earth orbit would become too polluted for space travel, satellites and other space infrastructure.
“If we don't do something to fix this problem in the next 50 years, we’re going to see Kessler Syndrome become a reality,” Crassidis said. “The chances of colliding are going to be so great, it’s not worth putting those satellites in low-Earth orbit. Everything we rely on today is going to be gone, and it’s going to affect our lives.”
Mitigating Space Debris
The most important step we can take to solving the space debris crisis is to mitigate the creation of new debris. New satellites should be “designed for demise,” Johnson said, meaning they should be developed with forethought into how they will be deorbited. In the United States and Europe, satellite operators are required to bring their satellites down within five years after their missions end.
Large satellites with thrusting capabilities can maneuver themselves out of orbit and either perform a controlled re-entry over the Pacific Ocean or disintegrate upon entering the Earth’s atmosphere. That means the propulsion systems, avionics and other technology need to be built to last many years — sometimes decades — until the end of the satellite’s mission, Henning said.
Satellites without thrusters need different solutions. They should either be placed low enough in orbit to allow for their natural descent into the Earth’s atmosphere, or be equipped with devices like balloons or dragsails that slow them down enough to fall out of orbit.
For satellites in higher orbits, like geostationary orbit, the ideal solution is to dispose of them outside the Earth-moon system, Henning said. More commonly, though, these satellites are pushed into farther-out “graveyard orbits,” where they are less likely to collide with active satellites. However, this still leads to a build-up of objects that could “someday lead to collisions and more debris generation,” he added.
Active Debris Removal Technologies
We don’t currently have the technology to remove debris from outer space. It’s a complex, expensive problem, but several companies are developing promising solutions.
The U.K.’s Surrey Satellite Technology led one of the first successful experiments with its RemoveDEBRIS project, which tested out the feasibility of capturing space debris with a net and harpoon. More recently, Japan-based Astroscale developed a satellite that uses a magnetic docking plate to collect debris and guide it into Earth’s atmosphere to burn up. The company is currently working on technology that can bring down multiple pieces of debris in one trip.
Meanwhile, Swiss startup ClearSpace is building a spacecraft that can autonomously gather debris with its robotic arms, similar to a claw machine. The European Space Agency has contracted ClearSpace to lead the ClearSpace-1 mission, which will capture a defunct satellite and transport it through the Earth’s atmosphere, burning it up along the way.
While Astroscale and ClearSpace may someday remove larger objects from outer space, it will be more challenging to pick up the thousands of smaller fragments caused by collisions, explosions and disintegration. NASA has analyzed the feasibility of using either ground- or space-based lasers to remove debris between 1 and 10 cm — either by nudging it down into Earth’s atmosphere or up into a less-crowded graveyard orbit. These smaller bits of debris will be very difficult to remove, though, as they cannot be reliably tracked.
“Given how difficult this problem is to solve, focusing on mitigation and remediation of large debris objects still appears to be the most effective approach we have right now,” Henning said. “If you can prevent the creation of small debris in the first place, we may never have to figure out how to clean them up.”
Policy and Regulation Efforts
There are currently no international laws or treaties requiring nations to mitigate space debris, but several countries have created their own rules. The U.S. government, for example, adopted orbital debris mitigation standard practices in 2001. In 2022, the Federal Communications Commission required satellite operators to dispose of their satellites within five years of completing their mission — a substantial reduction from the previous 25-year limit.
Many countries have also agreed to follow non-binding international guidelines. Space agencies around the world established the Inter-Agency Space Debris Coordination Committee (IADC) in 1993 to coordinate space debris mitigation efforts. The organization has published guidelines and policy proposals since 2002, with its most recent set of guidelines published in 2025.
The original IADC guidelines provided the foundation for the United Nations’ Space Debris Mitigation Guidelines, which were adopted in 2007. The UN guidelines offer seven non-binding provisions, such as minimizing the potential for accidental break-ups, limiting inactive satellites’ presence in low-Earth orbit and avoiding the intentional destruction of spacecraft.
The Future of Space Sustainability
Space is a shared resource that affects everyone in the world, regardless of their national identity. But it’s also a competitive industry, and debris mitigation too often takes a back seat to short-term economic or military interests.
While the UN adopted non-binding debris mitigation guidelines in 2007, not all nations and companies abide by them. To bring greater attention to the issue, scientists have called for legally binding treaties and for the inclusion of space sustainability in the UN’s list of 17 Sustainable Development Goals.
In some cases, space companies may be unaware of best practices. The Secure World Foundation educates space operators about the dangers of space debris and the best ways to mitigate it. The organization also facilitates dialogue between stakeholders to improve space sustainability efforts.
We can also mitigate space debris by increasing space situational awareness. To manage the growing commercial presence in space, the U.S. called for the creation of the first national space traffic management organization in 2018. This would transition oversight of civilian aerospace traffic to the Department of Commerce, leaving defense aerospace traffic under the purview of the Department of Defense. Meanwhile, aerospace companies are developing increasingly sophisticated AI and machine learning tools to better predict the probability of collisions — a crucial advancement considering all the new commercial satellites that will be flooding low-Earth orbit in the near future.
In the end, addressing space debris will take technological innovation, international cooperation and a sense of urgency. If we don’t take action now, we risk losing the critical space infrastructure that humanity depends on.
“They say space is big, but to me, it’s getting smaller and smaller,” Crassidis said. “You can’t think in terms of five years. You have to think in terms of 20, 30, 50 or 100 years. What’s going to happen when you don’t do something? When you look that far out from what I’m seeing right now, it’s pretty scary.”
Frequently Asked Questions
What is space debris?
Space debris is any human-made object in space that no longer serves a purpose. This includes defunct satellites, discarded rocket stages and smaller chunks of metal that break away due to a technical design flaw, collision with another object or explosion from a missile test.
Has space debris hit anyone?
Only one person has ever reported being hit by space debris. That woman, Oklahoma resident Lottie Williams, was struck in the shoulder by a 5-inch piece of fiberglass from a rocket’s fuel tank in 1997. She was not injured by the debris.
Does space junk eventually fall to Earth?
Space junk in low-Earth orbit will fall to Earth, but objects in the farther reaches of lower-Earth orbit may take centuries to return. Objects in geostationary orbit and beyond may never come down.
How much debris is left in space?
As of August 2024, there are nearly 45,000 pieces of space debris larger than a softball, according to the European Space Agency. Millions of smaller fragments are too small to be reliably tracked.
