The Kessler Syndrome is a phenomenon in which the amount of junk in orbit around Earth reaches a point where it just creates more and more space debris, causing big problems for satellites, astronauts and mission planners.
Consider this scenario: The destruction of a dead spy satellite spawns a swarm of debris in Earth orbit, which wreaks ever-increasing havoc as it zooms around our planet.
The cloud destroys a number of communications satellites, generating more and more debris with every violent collision. It takes out the iconic Hubble Space Telescope and a NASA space shuttle, killing several crewmembers aboard the winged vehicle. It then lines the International Space Station (ISS) up in its crosshairs, destroying the $100 billion orbiting lab with a hail of fast-flying shrapnel.
This dramatic scene is fictional, of course; it's pulled from the award-winning 2013 sci-fi film "Gravity." But many satellite operators, mission planners and exploration advocates worry that it could be a dark window into a future that's all too real, thanks to the Kessler Syndrome.
Read on to learn more about this feared phenomenon, which describes a snowballing cascade of space junk.
Related: The worst space debris events of all time
Kessler Syndrome: A space-junk visionary's prediction
The Kessler Syndrome is named after former NASA scientist Donald Kessler, who laid out the basic idea in a seminal 1978 paper.
In that study, titled "Collision Frequency of Artificial Satellites: The Creation of a Debris Belt (opens in new tab)," Kessler and co-author Burton Cour-Palais noted that the likelihood of satellite collisions increases as more and more spacecraft are lofted to orbit. And each such smashup would have an outsized impact on the orbital environment.
"Satellite collisions would produce orbiting fragments, each of which would increase the probability of further collisions, leading to the growth of a belt of debris around the Earth," the duo wrote. "The debris flux in such an Earth-orbiting belt could exceed the natural meteoroid flux, affecting future spacecraft designs."
The Kessler Syndrome describes, and warns of, a cascade of orbital debris that could potentially hinder humanity's space ambitions and activities down the road. The original paper predicted that satellite collisions would become a source of space junk by the year 2000, if not sooner, unless humanity changed how it lofted payloads to orbit. But a timeline is not essential to the core idea.
"It was never intended to mean that the cascading would occur over a period of time as short as days or months. Nor was it a prediction that the current environment was above some critical threshold," Kessler wrote in a 2009 paper (opens in new tab) that clarified the definition of the Kessler Syndrome and discussed its implications.
"The 'Kessler Syndrome' was meant to describe the phenomenon that random collisions between objects large enough to catalogue would produce a hazard to spacecraft from small debris that is greater than the natural meteoroid environment," he added. "In addition, because the random collision frequency is non-linear with debris accumulation rates, the phenomenon will eventually become the most important long-term source of debris, unless the accumulation rate of larger, non-operational objects (e.g., non-operational payloads and upper-stage rocket bodies) in Earth orbit were significantly reduced."
And Kessler didn't name this scenario after himself. In that 2009 paper, he explained that "Kessler Syndrome" apparently originated with John Gabbard, a scientist with the North American Aerospace Defense Command (NORAD) who kept an unofficial record of big satellite breakups in orbit.
Gabbard used the term when talking to a reporter shortly after the 1978 study came out. The Kessler Syndrome then worked its way into the public consciousness, "becoming part of the storyline in some science fiction, and a three-word summary describing orbital debris issues," Kessler wrote in the 2009 paper.
To give you another idea of how influential the 1978 study was: A year later, NASA established the Orbital Debris Program Office at Johnson Space Center in Houston and made Kessler its head. (Kessler, who was born in 1940, retired from NASA in 1996 (opens in new tab) with the title of senior scientist for orbital debris research. But he remains active in the debris research community today.)
Kessler syndrome tipping point: How bad are things now?
Earth orbit is getting more and more crowded as the years go by.
Humanity has launched about 12,170 satellites since the dawn of the space age in 1957, according to the European Space Agency (opens in new tab) (ESA), and 7,630 of them remain in orbit today — but only about 4,700 are still operational.
That means there are nearly 3,000 defunct spacecraft zooming around Earth at tremendous speeds, along with other big, dangerous pieces of debris like upper-stage rocket bodies. For example, orbital velocity at 250 miles (400 kilometers) up, the altitude at which the ISS flies, is about 17,100 mph (27,500 kph).
At such speeds, even a tiny shard of debris can do serious damage to a spacecraft — and there are huge numbers of such fragmentary bullets zipping around our planet. ESA estimates that Earth orbit harbors at least 36,500 debris objects that are more than 4 inches (10 centimeters) wide, 1 million between 0.4 inches and 4 inches (1 to 10 cm) across, and a staggering 330 million that are smaller than 0.4 inches (1 cm) but bigger than 0.04 inches (1 millimeter).
These objects pose more than just a hypothetical threat. From 1999 to May 2021, for example, the ISS conducted 29 debris-avoiding maneuvers, including three in 2020 alone, according to NASA officials (opens in new tab). And that number continues to grow; the station performed another such move in November 2021, for example.
Many of the smaller pieces of space junk were spawned by the explosion of spent rocket bodies in orbit, but others were more actively emplaced. In January 2007, for instance, China intentionally destroyed one of its defunct weather satellites in a much-criticized test of anti-satellite technology that generated more than 3,000 tracked debris objects (opens in new tab) and perhaps 32,000 others too small to be detected. The vast majority of that junk remains in orbit today, experts say.
Spacecraft have also collided with each other on orbit. The most famous such incident occurred in February 2009, when Russia's defunct Kosmos 2251 satellite slammed into the operational communications craft Iridium 33, producing nearly 2,000 pieces of debris (opens in new tab) bigger than a softball.
That 2009 smashup might be evidence that the Kessler Syndrome is already upon us, though a cataclysm of "Gravity" proportions is still a long way off.
"The cascade process can be more accurately thought of as continuous and as already started, where each collision or explosion in orbit slowly results in an increase in the frequency of future collisions," Kessler told Space Safety Magazine in 2012 (opens in new tab).
Related: Who's going to fix the space junk problem?
What can we do to avoid Kessler Syndrome?
The space community is taking the orbital-debris threat increasingly seriously these days, and not just because of the jolts provided by the Chinese ASAT test and the Iridium-Kosmos crash. Multiple satellite "megaconstellations" are in the works, making space traffic management and space-junk mitigation more pressing issues than they've ever been. (Such networks could also transform the night sky for professional and amateur astronomers, a separate but also important issue.)
For instance, SpaceX has already launched more than 1,700 satellites for its Starlink broadband constellation, which could eventually consist of more than 40,000 craft. OneWeb has lofted more than half of the satellites for its planned 648-member constellation, which may also grow beyond that initial number as time goes on.
Amazon aims to assemble its own internet-satellite network, which will consist of more than 3,200 spacecraft. And in November 2021, Bay Area launch startup Astra filed an application with the U.S. Federal Communications Commission for its own 13,600-satellite broadband constellation.
In addition, launch and satellite construction costs continue to fall, allowing more and more people to get satellites up and operate them — including folks with very little experience in the field. This opening of the final frontier is generally a good thing, most experts say, but it further highlights the need for forethought and responsible action when it comes to satellite operation.
In 2019, for example, the Space Safety Coalition (SSC) laid out a set of proposed voluntary guidelines designed to keep the Kessler Syndrome, and space junk in general, at bay over the coming years.
One recommendation is that all satellites operating above 250 miles (400 km) be equipped with propulsion systems allowing them to maneuver away from possible collisions. Drawing the line there makes sense for multiple reasons, according to the SSC: It's the altitude at which the ISS flies, and satellites that circle below this boundary tend to encounter enough atmospheric drag to fall out of orbit relatively soon after their operational lives come to an end.
The SSC also recommends that satellite designers consider building encryption systems into the command systems of their craft, so they'll be harder for chaos-seeking hackers to hijack. And operators who control satellites in low Earth orbit should include in their launch contracts a requirement that rocket upper stages be disposed of in the atmosphere shortly after liftoff.
Related: 7 wild ways to destroy orbital debris
More active debris-fighting strategies could also be part of the solution. Removing just a handful of rocket bodies or big, dead satellites every year could help us keep our space-junk problem under control, according to some studies. And researchers around the world are developing and testing ways to do just that, using nets, harpoons and other methods.
Such activities would have to be carefully coordinated and thought out. Space objects, including junk such as spent rocket bodies, belong to the nation that launched them, so the U.S. government or a U.S. company couldn't just unilaterally de-orbit a bunch of spent Russian rocket bodies (or vice versa). Such action might spark an international incident, not least because debris-removing tech might also be viewed as a potential space weapon that could take out operational satellites.
But the space-junk issue is a global one, so governments around the world should already be having meaty conversations about how to deal with it. Let's hope the talks, the decisions and the tech end up outpacing the problem, for all of our sakes.
- Space Debris and Human Spacecraft (NASA) (opens in new tab)
- NASA Orbital Debris Program Office (opens in new tab)
- Space Debris research and updates from European Space Agency
Mike Wall is the author of "Out There (opens in new tab)" (Grand Central Publishing, 2018; illustrated by Karl Tate), a book about the search for alien life. Follow him on Twitter @michaeldwall (opens in new tab). Follow us on Twitter @Spacedotcom (opens in new tab) or on Facebook (opens in new tab).