The world got a stark reminder this week that Earth’s orbit is increasingly a crowded and potentially dangerous place. After the Russian military destroyed one of its defunct satellites in a weapons test early Monday morning, the crew of the International Space Station was forced to take shelter as high-speed debris passed uncomfortably close to the outpost. For now, satellites and space stations can simply manoeuvre away from threatening space junk, but we may be approaching a scenario in which snowballing collisions in orbit actually cut us off from space. Disturbingly, we may already be in the early stages of this process, known as Kessler Syndrome.
In 2015, the now-defunct foresight publication MISC Magazine reached out to me for its crisis issue. They asked me a simple question: “What looming potential crisis is nobody talking about?” Without hesitation, I replied Kessler Syndrome, and I said so because I found it weird that such an important issue — the loss of access to Earth orbit — was barely scratching the zeitgeist.
As I reflect on this answer some six years later, and given the appalling news that Russia just blew up one of its own satellites, I firmly stand by that answer. But while “nobody” was talking about the Kessler Syndrome back then, it’s certainly not the case now.
Indeed, awareness is steadily growing about the sorry state of low Earth orbit and the quantity of junk zipping around up there. An estimated 330 million bits of debris currently exist in space, in what is an absolutely eye-watering figure. Now, we obviously didn’t launch 330 million individual pieces of debris up there, but that’s exactly the issue: Objects in space can breakup into many smaller pieces, which can then smash into other objects, and so on and so on, resulting in the exponential creation of debris over time.
Donald Kessler saw this coming. In 1978, the NASA scientist warned that, as “the number of artificial satellites in earth orbit increases, the probability of collisions between satellites also increases. 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.” Because the belt would get increasingly dense over time, Kessler worried about it becoming “a significant problem during the next century.”
This “run-away, self-sustained, cascading collision process,” as the European Space Agency describes Kessler Syndrome, is likely to boost the pace at which satellites get destroyed by fragmentary debris and other satellites, but also the pace at which debris begets more debris. (Only once has a satellite smashed into another satellite, and that happened in 2009 when the functioning Iridium 33 smashed into the defunct Kosmos-2251.) Large swaths of low Earth orbit, particularly the highly useful band between 900 and 1,400 km, would eventually be made inaccessible for protracted periods of time, possibly for decades.
As early as 1991, Kessler said that “it is now necessary to begin limiting the number of expended rocket bodies and payloads in orbit.” Twenty years later, that’s an almost laughable proposition, given the frenetic tempo at which rockets are now being launched to space. Moreover, attempts to thwart the process may already be futile, as the rate at which space debris is being created is now higher than the rate at which debris is falling back into Earth’s atmosphere. Kessler was already aware of this in 2009.
“Modelling results supported by data from [U.S. Air Force] tests, as well as by a number of independent scientists, have concluded that the current debris environment is ‘unstable’, or above a critical threshold, such that any attempt to achieve a growth-free small debris environment by eliminating sources of past debris will likely fail because fragments from future collisions will be generated faster than atmospheric drag will remove them,” as he wrote.
The European Space Agency concurs, saying “generated collision fragments will start to dominate” in useful portions of low Earth orbit, and this “will be true even if all launch activities were to be discontinued now, which is an extremely unlikely development.”
As an aside, geosynchronous orbit, which rises about 36,000 km above Earth and hosts hundreds of satellites, is likewise not immune to Kessler Syndrome.
The phrase “Kessler Syndrome” was coined by John Gabbard, who tracked major satellite breakup events for NORAD, and it came into use without having a strict definition. Kessler would go on to clarify the term, saying it 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 that the “phenomenon will eventually become the most important long-term source of debris” unless we finally do something about it.
Kessler never claimed that a destructive cascade would appear over a short time frame, such as a few days or months, or that a cascade could be sparked by a single trigger event (the 2013 film Gravity portrayed such a scenario, in which the destruction of a Russian satellite led to a cascading ball of space junk that eventually destroyed the International Space Station). Indeed, a single trigger event is unlikely to wipe out satellites en masse, but collisions that create large volumes of debris can speed the process along. Russia, having shot down its Kosmos-1408 satellite earlier this week, has directly contributed to the process; its anti-satellite (ASAT) test produced thousands of new pieces of debris which will threaten spacecraft for years to come. The U.S., India, and China have performed similar tests, so there’s plenty of blame to go around.
Concerns exist that Envisat — a retired, 8,165 kg Earth observation satellite — could also trigger a catastrophic cascade should it somehow be destroyed, but as Kessler himself told Space Safety Magazine in 2012, that’s not likely. But it would accelerate the process by an order of magnitude, Kessler explained:
“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. But since Envisat is so massive, if the collision had occurred it would instantly [produce] a debris environment that, under the most optimistic conditions, we would not expect to have for at least 100 years. That is close to what most might call a ‘trigger’ event.”
Trends suggest we’ll continue to fuel Kessler Syndrome. Rocket launches are now cheaper than they’ve ever been, making space an increasingly viable place for conducting business activities. Miniaturization is allowing for the creation of smaller and cheaper satellites, but that means we can pump more satellites into space with each launch. And then there’s the trend towards satellite megaconstellations, in which fleets of orbiting spacecraft work in tandem to provide services such as broadband internet access. SpaceX, having launched over 1,700 Starlink satellites to orbit, is leading the megaconstellation charge, but other companies, such as OneWeb and Amazon, intend to send up similar systems.
Around 7,630 satellites are currently orbiting Earth, of which roughly 4,700 are functioning, according to ESA. Each object added to orbit results in an increased collision risk, and each collision in turn boosts the chance of future collisions. This “could make prospects for long-term viability of satellites in [low Earth orbit] extremely low,” as Louis de Gouyon Matignon, an expert in space law, writes in Space Legal Issues.
Indeed, the situation could get very grim. The unacceptably high density of debris would make Earth orbit an unsuitable place for satellites, space stations, and astronauts. In a kind of worst-case scenario, a cascading debris field would wipe out swaths of satellites and render portions of Earth orbit unusable to human activities, at least for a while. The debris cloud below 500 km would eventually fall back to Earth, but that would take a decade or more. As for the area above 600 km, that could potentially remain off limits indefinitely, unless we find a way to manually clean things up.
I’ve previously detailed the implications of losing our satellites, but the Coles Notes version is that our ability to communicate would be severely hampered, GPS would be non-existent (along with those systems dependent upon it), space-based synchronisation for timekeeping and navigation would grind to a halt, our financial systems would crash, we’d lose significant military capabilities, and we’d be deprived of our weather satellites, among many other things.
Very serious stuff. Most of us would prefer that we not return to the mid-20th century, and thankfully there are ways for us to reduce the volume of space debris. These include limiting the amount of space debris caused by routine space operations (such as the maintenance and repair of satellites in orbit), preventing in-space collisions (for example, by making all satellites manoeuvrable and prohibiting anti-satellite weapons tests), making satellites more resistant to impacts (such as through shielding), and by responsibly disposing of retired satellites (designing satellites that can deorbit themselves, for example). Finding new and effective ways to remove space junk is also critical.
We also need sensible constraints on the volume and types of objects that can be sent into space. Getting everyone on board, including public and private sectors, won’t be easy, but it’s necessary to maintain a healthy orbital ecology. Earth orbit is a very special place, and it’d be a shame to lose it.