Colliding Satellites Could Create a Belt of Space Junk Around Earth
Illustration: Cathryn Virginia

This Is What Happens When Two Satellites Collide In Space

The first-ever collision between artificial satellites changed how the world looked at space. Now, low-Earth orbit is about to get a lot more crowded.

More than 1,300 active human-made satellites are flying in low orbit right now. They ceaselessly glide, boosting up and down to avoid bits of space junk and occasionally each other in a robotic ballet hundreds of miles above their human controllers.

In a few years, there could be ten times as many artificial satellites in low-Earth orbit alone (the band of space where the International Space Station resides) thanks to private companies that have proposed launching spacecraft to deliver services such as beaming the internet down from space. That huge influx of corporate satellites is going to complicate things in space, where there are effectively no traffic rules—like a highway with no cops and everyone driving blindfolded.


Already, stress fractures have appeared in the global system that keeps satellites from colliding. Last week, the European Space Agency announced that it boosted its Aeolus satellite into a higher orbit to avoid a collision with a recently-launched SpaceX satellite, Starlink 44, just one of a possible 12,000 satellites that could fly under Elon Musk's banner by 2020. SpaceX initially rebuffed the agency's warning of a possible collision, and blamed a software bug for not receiving updated figures from the U.S. military that might have spurred the American company to coordinate with the European agency.

“This example shows that in the absence of traffic rules and communication protocols, collision avoidance depends entirely on the pragmatism of the operators involved,” said Holger Krag, Head of Space Safety at ESA, in a statement.

Although the possibility of two satellites colliding above our heads seems new and rather frightening, it has happened before. The first-ever recorded collision between two human-made satellites occurred 10 years ago, in 2009, and drastically changed how the world approaches space traffic management and debris mitigation.

With even more urgent shifts on the horizon, a writhing rat-king of deeply human issues need to be worked out to avoid an anthropogenic tragedy in the void: a cascading band of debris that renders a swath of space effectively unusable.

What happens when two satellites collide?


On the evening of February 13, 2009, the National Weather Service in Jackson, Kentucky received calls from people reporting possible explosions and earthquakes.

According to a bulletin from the National Oceanic and Atmospheric Administration (NOAA), the feds advised local law enforcement that the loud booms and flashes of light in the sky in Kentucky were likely the result of debris from a collision between two satellites 500 miles above Siberia three days prior, on February 10. It was the first recorded incident involving two artificial satellites slamming into each other in orbit.

The spacecraft in question were an active Iridium 33 satellite—operated by U.S.-based Iridium Communications LLC—and Kosmos 2251, a decommissioned Russian satellite. According to a 2009 article by Brian Weeden, director of program planning for the Secure World Foundation and former instructor in the Air Force's orbital analyst training program, the satellites were travelling at slightly different inclinations but their orbits intersected at a nearly 90-degree angle close to the North Pole. Orbits intersecting isn't uncommon, Weeden said in an interview, but satellites colliding is incredibly rare.

"We tend to think of the probability as astronomically low, and it is pretty low, but given enough rolls of the dice it will eventually happen."

The finer details of what led to the collision are still unclear a decade later, Weeden said, which speaks to the messy realities of observing and controlling objects hundreds of miles away using math (playing "billiards in space," as Weeden put it in his article). The uncertainties and communication breakdown that led to the ESA thrusting out of the way of a Starlink satellite is a reflection of these difficulties.


"On the one hand, I was fully aware that this was a potential problem and people were looking at it, but I was still pretty shocked that it happened." Weeden said in an interview. "We tend to think of the probability as astronomically low, and it is pretty low, but given enough rolls of the dice it will eventually happen."

As for the consequences of the 2009 collision, the world had a dry run two years prior, in 2007, when China destroyed one of its own satellites in low-Earth orbit as part of a test. (The U.S. had conducted its own version of this military experiment in 1985). That 2007 explosion, and the Iridium-Kosmos collision, threw clouds of debris into space that could threaten other satellites or spacecraft. And they did.

"Those two events caused a lot of debris in space and the satellites we had at the time were frequently getting close [approaches] or had alerts with respect to those three satellites," said Michel Doyon, who manages the Canadian Space Agency's satellites from Earth and started the year following the Iridium-Kosmos collision.

Space debris is the real risk of satellite collisions in orbit, other than a possible loss of service planetside. To be clear: these are robots, essentially, fancy dishwashers. When they slam into each other, nobody gets killed in space or on Earth. But that doesn't mean there's no risk. According to NASA, space debris "is the No. 1 threat to spacecraft, satellites, and astronauts." In space, a 10-centimetre projectile packs the same punch as 7 pounds of TNT on Earth. This debris moves fast, and unlike satellites it cannot be maneuvered—all we can do is track it and try to get out of its way if it appears catastrophic.


The worst-case scenario for space debris is known as Kessler Syndrome, named for astrophysicist Donald Kessler. The idea is that eventually low-Earth orbit may reach a "critical mass" of debris. After that, collisions will begin to cascade—more collisions create more debris, which means more collisions with debris, and so on—even without new spacecraft joining the orbital party. Eventually, a "belt" of debris will form and the junk-choked orbit will become very dangerous for new satellites or space stations, and possibly unusable.

For a future space station, Kessler and co-author Burton G. Cour-Palais wrote, this situation would mean more shielding, accepting a much higher risk of impact damage, or being restricted to a lower orbit.

According to NASA's summation of Kessler Syndrome, some experts believe we're already at the point of critical mass in low-Earth orbit, and thousands more satellites are expected to launch into space in coming years.

"Kessler Syndrome is global warming; it’s not a nuclear reaction—it’s this relatively slow accumulation that has accelerating negative effects in the future," Weeden said.

How do we stop satellites from colliding?

The 2009 Iridium-Kosmos collision was a wake-up call for the world and kicked off important shifts in how humanity collectively manages spacecraft and debris.

"That triggered something very major in the world," Doyon said. "As you may know, space has no borders, it’s everyone’s property, and debris is a global problem that affects operators all over the world."


Before the collision, Weeden said, the U.S. military tracked a priority list of objects in space and screened them against known space debris. If a collision seemed possible, the military reached out to the satellite operator and warned them. The military also had two lists: a public catalog of object positions, and a more accurate internal catalog. "They were concerned that publicizing a better catalog would reveal their capabilities to adversaries," Weeden said.

"[The U.S.] was not sharing as much information as they are now, so we had to make decisions with fairly limited information," Doyon said.

This meant that operators would receive some imperfect information about objects in orbit—for example, a measurement from one direction but not others—crunch the numbers manually, and make a decision about whether the possibility of a collision warrants thrusting out of the way. Operators have a roughly 10-minute window every 90 minutes to "talk" to their spacecraft.

After the Iridium-Kosmos collision the U.S.military started doing collision assessment for every satellite in the world, Weeden said, and bolstered its space situational awareness data-sharing program. By the end of 2010, the U.S.had signed 17 data sharing agreements with commercial partners. By 2012, the military had signed similar agreements with other nations, including a multi-year deal with Canada.

"We now have a lot more information and we can make more sound decisions," Doyon said, "or decisions supported by a lot more observations and calculations." Besides better data, satellite operators now also have software to help do the math, he said.


Ideally, satellite operators will share information and coordinate their actions for the functional life of their craft, before moving them into disposal orbits where they will eventually burn up in the atmosphere. Debris mitigation plans are part of the approval process for satellites in the U.S.—for satellites delivering broadband, for example, such plans are reviewed by the FCC.

To prep spacecraft for inevitable debris collisions in space, NASA (sometimes for itself, sometimes under contract to commercial operators) literally shoots projectiles at satellite shielding using light-gas guns. This is called hypervelocity impact testing and it's carried out at NASA's White Sands Test Facility in New Mexico.

According to Eric Christiansen, NASA's lead for Micrometeoroid and Orbital Debris (MMOD) protection, a shielded satellite returning from orbit may have 30,000 small debris impacts visible to the naked eye. To mimic these impacts, the scientists at the White Sands facility use pressurized hydrogen to shoot small projectiles down a firing range at nearly 10 times the speed of a bullet.

"If we didn’t have something at the far end of the gun, and we didn’t have atmosphere on Earth, it’d almost go all the way around the world; it’s going that fast," Christiansen said.

"They have propellant tanks. They need shielding, and it requires more testing"

According to Christiansen, NASA and commercial operators are not all that interested in testing for collisions with large objects such as other satellites. "It doesn’t happen very often, so NASA chooses to spend their money on things that happen more often than not," Christiansen said.


Christiansen only knows of two hypervelocity impact tests that NASA has done involving larger projectiles. "We had to use the biggest gun that we know of in the United States to do those two tests," he said. The firing range for these tests is 1,000 feet long, he said, and scientists use bicycles to get from one end to the other. "It’s a big operation, and it’s very expensive to do those tests."

The smaller light-gas guns that fire projectiles satellites are more likely to come into contact with get fired several times a day at White Sands. "It’s just a lot cheaper to operate those guns," Christiansen said.

Although direct collisions contribute to the spread of space debris, the real risk to satellites lies in these smaller objects. With thousands more satellites expected to go into orbit in the coming years, more testing is needed.

"There’s so many of them that if you look at the total surface area of the satellites, it’s like putting a space station or more above the [existing] space station—that’s what these satellites represent," Christiansen said. "They have propellant tanks. They need shielding, and it requires more testing."

The military-industrial space complex

More satellites isn't the only big change coming to low-Earth orbit. In June, NASA announced that the International Space Station will be open for commercial activities including manufacturing and tourism.

"If a satellite blows up, that’s bad but nobody dies," said Glenn Reynolds, a law professor at the University of Tennessee and an expert in space law. "Everybody expects government astronauts to run a certain amount of risk. But if tourists get killed because somebody’s dead satellite crashes into their spacecraft, people will be a lot more upset."


International cooperation around mitigating space debris has ticked up in recent years and continues under the auspices of organizations such as the Inter-Agency Space Debris Coordination Committee and the UN's Committee on the Peaceful Uses of Outer Space, the latter of which sets international guidelines.

Big and complex changes are already coming in the U.S., however—a critical faucet of space data for the world. The U.S.military will surely play a smaller role in tracking satellites and space debris, and alerting operators to possible collisions.

Per Space Policy Directive 3 (SPD-3), issued last year by the Trump administration, that responsibility will largely fall to the Commerce Department, reflecting the highly commercial nature of future activity in low-Earth orbit. That shift has already started, and the Commerce Department announced in June that it was expecting the "imminent" transfer of a trove of space data from the Air Force.

According to the directive, this shift in responsibilities will leave the Department of Defense open to focusing on "protecting and defending U.S. space assets and interests" in the future. The Air Force spelled out how debris tracking aligns with military goals in a 2018 blog post that states, "[Space situational awareness] enables the continuous preparation of the battlespace in order to fight and win a war in space."

At the same time, the private sector is expected to bolster its debris-tracking and mitigation capabilities to complement the government's efforts. SPD-3 states that one of its goals is to "encourage and facilitate U.S. commercial leadership" in space traffic management and situational awareness. This trend has already started to take off. In 2009, satellite communications companies formed the Space Data Association to share information and best practices. Since 2014, Analytical Graphics Inc. has operated the Commercial Space Operations Center, which uses measurements from commercial sensors to deliver a space catalog.

Junk removal could also be part of the picture going forward, and the private sector could take the lead there, too. A Japanese company called Astroscale has been developing a system for several years and was recently given a shout-out by Commerce Secretary Wilbur Ross for establishing a U.S. footprint.

"Space situational awareness is an amalgam of so many issues," said Joanne Gabrynowicz, director of the independent International Institute of Space Law. "Most of that is evolving, but far from settled."

The question of who's to blame if something goes wrong in corporatized low-Earth orbit is also a concern. "In space the liability is based on fault, and this raises some very difficult questions," Reynolds said. "The standard for what counts as negligence for controlling spacecraft is not very well established yet and proving fault is difficult…if it's a paint chip, or a bolt, or fragment of satellite that’s disintegrated, it’s often difficult to know who’s to blame."

Still, Reynolds said, the mere possibility of financial penalties on Earth—and the fact that an unusable band of space isn't good for anybody—will be enough to force private actors to behave well in orbit.

However, once safety is part of a profit calculus, it's obvious how incentives can be perverted. To return to Weeden's comparison of the Kessler Syndrome to global warming (a slow, but foreseeable creep of negative effects), consider the climate: corporations have royally screwed that up for themselves and the rest of us in the name of profits. Will space be any different?

Regardless, one thing is sure: The math is not on our side when it comes to satellites damaging each other, either directly or with debris.

"It’s bound to happen sooner or later. I can tell you, at this point, it is absolutely bound to happen sooner or later," Reynolds said. "It’s good news for lawyers, of course."