Kessler Syndrome: How runaway space junk could trap humans on Earth

When you’re screaming through the void faster than a speeding bullet, any traffic you encounter might as well be made of, well, speeding bullets. Defensive driving is recommended.

Hazardous space debris has twice threatened the International Space Station recently. On or around November 12, a debris field generated by a Russian military anti-satellite missile, or ASAT test, sent ISS crew members — including Russian cosmonauts — to shelter in the station’s Dragon and Soyuz spacecraft in case they needed to evacuate, while on December 2, the station maneuvered around a chunk of a defunct American Pegasus rocket.

No one was hurt in either case, and the risks were hardly novel — the ISS has maneuvered around space debris more than 30 times since 1999. But it’s also a problem that’s almost guaranteed to worsen given worrying trends in the militarization of space and the fact that all signs point to ever more objects being launched into space every year.

Take into consideration mega-constellations like SpaceX’s Starlink, for which “the plan is to launch 100,000 active satellites in the next few years,” Jonathan McDowell, a Harvard astrophysicist who has been tracking satellites on the side for more than a decade, tells Inverse. “The collision rate grows as the square of the number of satellites. If you have 10 times as many satellites, you will have 100 times as many collisions.”

And the thing is, space debris is not simply the detritus of old space missions. A satellite stricken by orbital debris becomes debris itself, which can then hit another satellite, creating debris that can strike another, and so on. It’s a chain reaction known as Kessler Syndrome, and while it doesn’t take place in the half-hour time frame as dramatized in the movie Gravity, the result may be the same: no more outer space for anybody.

“At least some models suggest that, yeah, it's already underway,” McDowell says, “it's just going to take a century to play out.”

Space Junk basics

It’s a good thing the sky is so big, because humans have flung a lot of things up there. And every bit of it, from large spacecraft to tiny pieces of cloth, are careening around at 17,500 miles-per-hour or faster. At those speeds, even collisions between somewhat small objects can be catastrophic.

“The unit I like to use is a megajoule, which is the kinetic energy of a one-ton truck hitting you at 100 miles an hour,” McDowell says. Collisions between small satellites can generate tens of thousands of megajoules of kinetic energy, while even tiny pieces of debris still pack enough of a punch to drill bullet holes in the ISS and other space assets. The Hubble Space Telescope carries a Whipple shield, for instance, a sort of bulletproof vest to absorb the energy of more minor debris impacts.

It’s an imperfect solution, “smaller” being relative and “big” being game over.

“If you get hit by a big enough piece of debris, [a Whipple shield] is not gonna be enough,” McDowell says. “And if something comes down the telescope aperture and hits the Hubble mirror, that’s also not good.”

The good news is that organizations that track debris, such as the US Space Command, have a pretty good handle on the big stuff in orbit — anything from multi-ton dead satellites to debris 10 centimeters across.

“The trackable debris we follow as individual objects, and we’re tracking about 40,000 objects, of which 5,000 or so are working satellites and the rest is junk,” McDowell says. “If you look at stuff down to just one centimeter, there’s probably a million of those. But we don’t really know because they’re too small.”

There are two main sources of space debris at the moment, the primary being old rocket stages still in orbit decades after the delivery of their payload. “The fuel and the oxidizer get together because the seals fail,” McDowell says, “And they go bang.”

The secondary source is military anti-satellite tests, he says, which generate debris clouds that can persist for decades.

But if a Kessler Syndrome cascade is already underway, and continues apace unmitigated, eventually the most significant source of space debris will be the pulverized remains of satellites, spacecraft, and space stations dashed upon rocks of our own making.

A history of space junking and space punking

Putting aside the occasional meteor shower, space debris is an entirely human creation — satellites don’t launch themselves.

But not all space debris is created equally. Many objects are merely byproducts of early space exploration, while the birth of others was more intentional.

For example, while Russia has drawn international criticism for its ASAT test in November, in the early 1960s, it was the Soviet Union who accused the United States of purposefully polluting the spaceways.

Between 1961 and 1963, the United States launched almost half a billion copper needles into low-Earth orbit, Caltech historian of technology Lisa Ruth Rand tells Inverse. Called Project West Ford, it was an attempt to create an artificial ionosphere for long-range radio communications in case a US-USSR nuclear war disrupted other means.

The Soviets were not amused, and accused the US of “trying to destroy all space so that no one else could use it, out of spite,” Rand says.

Both the Soviet Union and the United States developed and tested anti-satellite missile technologies in the 1970s and 80s, creating orbital debris and leading to a lull in ASAT tests until 2007, when China used an ASAT to destroy an old weather satellite.

The US used an ASAT missile to destroy a spy satellite that failed after its launch in 2008, and India launched a small satellite in January of 2019 only to shoot it down with an ASAT in March 2019.

The next and most recent ASAT test to actually destroy a satellite in space was the Russian test in November, and all four of the tests created debris, some of which will remain in orbit for years to come.

“Most of the debris from the Russian ASAT will be down on a timescale of like five years, and the rest of it will be down on the timescale of 10 to 20 years, which, it’s still not good,” McDowell says. “For the Chinese ASAT, which was up at a higher altitude, more like 900 kilometers, some of that debris is likely going to be up there for many decades.”

Such intentional creation of space debris seems irrational and irresponsible given how problematic incidental space debris already is. A 2009 collision between an Iridium communications satellite and a defunct Russian satellite over Siberia first turned Rand on to studying space debris as a research focus while in grad school, and for every impact, there are many more close calls.

In 2012, for instance, a defunct Soviet Kosmos satellite threatened the Fermi Gamma-ray Space Telescope and presented its operators with a tough decision, Rand says. “Either light up thrusters that had been dormant for years, that were cold and could blow the whole works,” or hope the debris would pass further from Fermi than projected, such predictions always coming as probabilities rather than certainties.

She says that the operators ultimately opted to risk using the thrusters, and the space telescope moved, and all was fine, but it was still a risky situation.

And such situations are not always improved when all satellites involved are still live and operational. In September 2019, Rand says, a Starlink satellite and an ESA satellite almost collided when operators at SpaceX failed to check their email and missed some urgent missives from their counterparts at ESA.

And over the summer of 2021, McDowell recently tweeted, the Chinese space station twice dodged Starlink satellites that may have passed within 1 kilometer of the station.

“These are the kinds of close calls are happening a lot and increasingly more as the number of objects in outer space increases,” Rand says. “The number of functioning satellites in space is just exploding. It’s huge. It’s getting bigger and bigger every day.”

What is Kessler Syndrome and why does it matter?

In some ways, the Kessler Syndrome is like a slow-moving zombie apocalypse. (In George Romero’s classic Night of the Living Dead, Rand notes, it’s theorized a contaminated satellite returning from Venus triggers the zombie rise.) The space debris chain reaction converts otherwise operational space assets into further navigational hazards. “It’s an unwanted weaponizing of a valuable object into something that becomes dangerous,” Rand says. Had the Fermi telescope thrusters failed, “that would have been the loss of a major scientific instrument and cultural heritage artifact that had become a series of projectiles.”

And the threat of losing space assets means the Kessler Syndrome has a costly impact long before the chain reaction has progressed enough to prevent access to space.

More satellites in space mean more potential collisions, which means more satellites — and space stations — making more frequent evasive maneuvers to avoid further collisions, all of which can interrupt operations and cost operators money by decreasing the lifespan of their satellites. Every Starlink satellite has a limited amount of krypton propellant onboard for maneuvering around debris and Chinese space stations, for instance, and when the tank is near empty, it’s time for that satellite’s long fiery goodbye bow in the upper atmosphere — failure to do so just increases the problem by adding another dead satellite.

“Space is big,” Rand says, “but once things start to collide, it becomes rapidly small.”

Rapidly, but not linearly. Unfortunately, if the Kessler cascade is already underway, it will take years to reach a point where it’s happening in what humans perceive as “real-time.” By then, it may be too late.

“On a timescale of decades, you’ll have to dodge more and more often, and eventually you won’t be able to dodge anymore because the traffic is so bad,” McDowell says. Adding the caveat that it’s just his back-of-the-envelope math, he says, “we are probably exceeding the carrying capacity of low Earth orbit right now.”

The consequence of full bore Kessler syndrome allowed to run its course would be to negate the sci-fi ambitions of people like Elon Musk. To proscribe humans from the cosmos and limit our future to that of a one-planet species for many lifetimes — physically and intellectually. The space debris could ruin ground-based astronomy, too, hemming in our minds as well as our rockets.

“It would mean to basically close ourselves off from the rest of the cosmos,” Rand says. “That the endgame of the space age is not so much humanity becoming cosmopolitan, becoming multi-planetary, becoming part of the universe, but instead making it so that we just can’t leave anymore.”

What can we do about space debris?

There is some good news when it comes to space debris, beginning with the fact that there are some natural processes that help to clear the space lanes of the dead and dying detritus of human space fairing.

The Sun goes through an 11-year cycle of solar storms that, at its peak, makes the Earth’s atmosphere a little denser, generating more drag on any objects orbiting in lower orbits.

“So there’s sort of a cleaning cycle every 11 years of the lower atmosphere, the lower part of [low-Earth orbit] goes through,” McDowell says. “But in the upper part of [low-Earth orbit] that change is not enough to make a difference, and the stuff keeps orbiting.”

On average, objects orbiting around 200 kilometers altitude will re-enter Earth’s atmosphere within a week or two without active boosting, he says. In comparison, objects orbiting at around 400 kilometers will re-enter within a year or two. By 550 kilometers altitude, things take 25 years or more to re-enter the atmosphere, McDowell says, and “above about 600 kilometers, because the air density falls off so quickly, stuff up there will stay up for centuries, for millennia.”

These physical realities of orbiting objects considered, there are two major paths to dealing with space debris, and we have to walk both of them. First, we need to safely rid space of the most problematic pieces of space debris. Second, we need to stop making more of it.

As to the first, the technical challenges are not as awesome as you might assume, once you accept that you’ll never clear space of all small debris and focus only on the worst offenders.

“The leading term in growing the Kessler cascade is the big things hitting each other because they create so much extra debris,” McDowell says. Get rid of the 100 biggest objects — late dead satellites, old rocket stages — and it would reduce the rate at which the runaway is happening.

But it’s not yet clear who will do such work, who will pay for it, and if it’s even legal.

Unlike ocean salvage, where the Law of the Sea allows third parties to extract resources from abandoned ships or wrecks, spacefaring nations retain responsibility and authority over all objects they have placed in space under the Outer Space Treaty of 1967. “If you want to remove an old Soviet abandoned rocket stage, without causing an international incident, you have to get Russia’s permission,” McDowell says. Since the technology you would use to remove an old satellite could be used to take out a new one, “it’s a bit of a sensitive issue.”

So removal of old space debris may require a lot of international negotiation and new agreements, which is just as well, given the same is necessary to tamp down on the addition of new space debris. The United Nations may soon discuss new proposals to ban the test or use of ASAT weapons, and the proposal may have more lift under its wings given the wind condemnation of Russia’s missile test in November.

“The bright side is that we are starting to restart discussions about what are the right rules of the road in space,” McDowell says. “Maybe if people get alarmed enough that something will actually be done.”