The “Gravity” Scenario: How Dangerous is Space Debris?

We live in an era that, in the future, might well be called the beginning of humanity’s full-scale expansion into space. In 2014, there were just over 1,000 active satellites in Earth’s orbit, but by the end of 2023, that number exceeded 9,000.

This explosive growth has been driven by the deployment of satellite mega-constellations like Starlink and OneWeb and it is unlikely that this trend will significantly slow down anytime soon. The International Telecommunication Union has received applications to create over 300 satellite constellations, totaling one million (!) devices. Of course, most of these systems will likely remain on paper, but even the fraction that will eventually be launched will further congest Earth’s orbit.

All of this leads us to a logical question: how much does this increased density increase the risk of dangerous situations like collisions between satellites or space debris?

Let’s take a look.

A brief history of space debris

Most space enthusiasts know that the first piece of space debris in history was the same legendary star that appeared in the night sky after October 4, 1957. In truth, the first Sputnik was too small to be seen with the naked eye: what observers actually saw was the upper stage of the rocket that had launched it, which remained in orbit.

In subsequent decades, few paid attention to the problem of space debris, which at the time made sense because near-Earth space was still practically empty. Meanwhile, the superpowers were actively developing numerous devices and conducting experiments that are impossible to imagine now. Take, for example, the series of high-altitude nuclear explosions conducted in the late 1950s and early 1960s. Or consider Project West Ford, which entailed nearly half a billion copper needles being released into orbit to create an artificial ionosphere around the Earth. Both superpowers also tested various anti-satellite weapons, with little regard for the eventual consequences of such actions.

The first concerns about space debris were publicly voiced in the late 1970s. In particular,  the scientist Donald Kessler described a scenario originally known as a “cascading effect” but later named the Kessler Effect (or Kessler Syndrome) in his honor. Its starting point is the collision of two sufficiently large objects in orbit or some other event that leads to the creation of a large debris field. This debris then collides with other satellites, leading to the creation of new debris, which in turn collides with other objects. This process continues until an orbit becomes completely unusable. A similar scenario, albeit in a highly dramatized form, is depicted in the movie “Gravity.”

After Kessler’s report, some measures began to be taken to limit the increase in space debris. Such measures included moving old devices to “disposal orbits” and passivating, or draining remaining fuel from, spent rocket stages and boosters. This helps to prevent decommissioned space equipment from exploding due to the formation of fuel vapors, though batteries can also be a source of explosions. Exploded rocket stages are responsible for a significant portion of space debris.

Such measures, however, can only slow the rate of space debris accumulation, particularly since it has taken many years for them to become generally accepted standards adhered to by most countries.

The situation was further exacerbated by a series of anti-satellite weapons tests in the 21st century, beginning with a Chinese test in 2007 and continuing through the infamous Russian test that destroyed the Kosmos-1408 satellite. These tests created thousands of debris fragments, many of which remain in orbit to this day.

Given all this, a truly dangerous incident involving space debris was only a matter of time. As it happened, this “historic” event occurred in 2009, when the Kosmos-2251 and Iridium 33 satellites collided in the sky over Antarctica. Both satellites disintegrated, generating several thousand debris fragments, giving the world a stark example of what will continue to happen if the problem of space debris is not addressed.

The current situation in orbit

So how are things now? As of early 2024, there were 12,597 satellites in orbit. Of these, 3,356 are inactive, meaning that they are essentially space debris.

But dead satellites are just the tip of the iceberg. In total, tracking devices have detected over 25,000 objects larger than 10 cm in low-Earth orbit. These objects include debris from anti-satellite weapons tests, exploded rocket stages, pieces of rockets and upper stages, and even tools lost by astronauts.

For each fragment that we have been able to track, there are likely dozens of fragments ranging in size from 1 to 10 cm that are too small to be seen by radar. Once we take into account objects smaller than 1 cm, then we can assume that there are millions of pieces of potentially hazardous space debris in orbit.

Space, of course, is vast. But the problem is that this debris populates near-Earth space unevenly. It tends to be concentrated on several main orbital echelons. The first is the region between 400 and 600 km, where most satellites are located. Here, you can find almost everything from Starlink satellites to long-decommissioned Soviet and Russian military satellites.

Next is the region at altitudes of 740 to 780 km, which is home to many old communication satellites. After that is a debris belt between 940 and 1000 km. This region also contains many decommissioned satellites, including Soviet and American satellites equipped with nuclear reactors. Yet another graveyard of Soviet space equipment lies at altitudes of 1400 to 1500 km.

It is important to remember that the higher the orbit, the longer it takes for an object to re-enter Earth’s atmosphere. For the most “contaminated” orbits, this means timeframes measured in years (for altitudes around 400 km) or several decades (for altitudes of 600 km). Solar activity can also play a crucial role in the natural “cleaning” process since it can significantly accelerate the timeframe within which objects re-enter the atmosphere.

However, above 700 km, this natural mechanism is virtually ineffective. Debris in high orbits will remain in space for hundreds or thousands of years. It is also important to mention objects in geostationary orbit. Debris in this orbit will exist almost indefinitely – on the order of millions of years. Therefore, decommissioned satellites in this range are usually moved to a so-called “disposal orbit,” located several hundred kilometers above the geostationary orbit. Currently, there are over 500 decommissioned satellites in disposal orbit.

However, although international regulations require decommissioned geostationary satellites to be moved to a disposal orbit, this sometimes does not happen (usually due to technical reasons). Over time, such satellites begin to drift, creating interference for their neighbors. The situation is further aggravated by the launch of new satellites. According to Slingshot Aerospace, by 2023, the average distance between satellites in geostationary orbit has decreased by 33% compared to 2010. This trend will only continue as new satellites are deployed.

The fight against space debris

All of this leads us to the question posed at the very beginning: how likely is it that one day the situation will spiral out of control? There is a non-zero probability of this scenario, but thus far we have been fortunate. The full-scale collision between Kosmos-2251 and Iridium 33 remains a singular event, although incidents in which debris has collided with operational satellites have occurred more than once.

But our luck is unlikely to last forever. The explosive growth in the number of satellites in near-Earth orbit almost inevitably guarantees a repeat scenario. According to LeoLabs, a company monitoring near-Earth orbit, there were 836 dangerous approaches in 2022, during which objects in space came within less than 100 meters of one another. A third of these incidents occurred in orbits of 800 to 900 km, which are currently considered the most dangerous. This is where, for example, the 8-ton Envisat satellite is located, which some experts refer to as the most dangerous piece of space debris. Its destruction would create a huge debris cloud, which, under certain circumstances, could trigger the cascade effect predicted by Kessler.

Of course, it’s important to understand that this particular scenario would affect just one orbit: the hypothetical destruction of Envisat would not, for example, affect geostationary satellites and would not pose a threat to the International Space Station. That said, it could easily result in humanity being unable to safely use certain regions of near-Earth space for a very long time.

The situation in lower orbits is somewhat better. If there is a large collision there, most of the debris will likely burn up in the atmosphere after a while.

Nevertheless, the overall situation remains genuinely challenging and it is unlikely to improve in the coming decades. The fact is that nearly all current measures to combat space debris are passive. This means they are aimed at preventing the further accumulation of space debris. Key measures include passivation of rocket stages, moving satellites to disposal orbits, or simply deorbiting them at the end of their service. As understanding of the threat from debris increases, such initiatives are shifting from voluntary to mandatory. For example, in 2022, the U.S. Federal Communications Commission required satellite operators to remove their devices from orbit within 5 years after mission completion.

Another interesting innovation is equipping space devices with collision avoidance systems like those on Starlink satellites. Still, such measures remain primarily aimed at preventing the creation of new space debris, rather than addressing the problem of existing debris.

Of course, there are also more “active” projects aimed at removing existing debris from orbit. Thus far, however, these projects have largely amounted to individual demonstration missions. Moreover, they will target only the largest and most dangerous objects. Meanwhile, removing a small, but still dangerous, object like a bolt from orbit remains a significant challenge. Some experts suggest using lasers for this purpose. Proponents of such initiatives, however, overlook the fact that a laser capable of destroying space debris could also destroy ordinary satellites. Such a device, however, would almost certainly be regarded first and foremost as a weapon. In theory, it could be placed under the control of an independent international organization, but given current geopolitical realities, this solution seems extremely unlikely.

Satellite operators and space agencies must therefore begin developing protocols and contingency plans to prepare for a large collision in orbit. There is virtually no doubt that such a collision is a question of “when” and not “if.” 

Given the pace of new satellite launches, it will likely happen much faster than many expect.