The following essay was contributed by space systems engineer Kostas Konstantinidis

A reading of this essay is featured on the Metaculus Journal Podcast here.

The space debris population

Since the start of the space age with the launch of Sputnik in 1957, the number of spacecraft orbiting the Earth has grown steadily. These satellites have been performing services that are essential for modern society, including communication, navigation, Earth observation, and scientific research. These satellites have, however, made Earth orbit a crowded place, with roughly 4700 active spacecraft currently in orbit. Adding to the congestion, many decommissioned satellites remain in orbit—as do the spent rocket stages that put them there. Numerous smaller objects like nuts, bolts, and paint specks contribute to this population of objects in orbit, which is commonly known as space debris. These objects travel at velocities that can reach tens of km/s and often collide with each other, fragmenting into many smaller pieces and further adding to the debris population. Even a small piece of debris carries enough kinetic energy to destroy an operating spacecraft on impact (something that currently happens about once a year). The population of space debris is higher in the most used orbits: mainly low-Earth orbit, but also geostationary.

The Kessler syndrome

In 1978, inspired by studying the collisions that created the asteroid belt in the early solar system, a NASA scientist named Donald Kessler proposed the scenario where the density of space debris in low Earth orbit becomes so high that collisions between objects cause a cascade in which each collision generates space debris that in turn increases the likelihood of further collisions. This effect was named after him and is known as the Kessler syndrome. Such an outcome would render the affected orbits increasingly difficult to use, denying several future generations the many benefits of space use.

Mitigating risk from space debris 

Even at its current population, space debris poses significant challenges to spacecraft operators. A first step to mitigating these risks is to monitor them, done by ground- and space-based radar and optical sensors. These sensors currently monitor approximately 30,000 pieces of debris larger than 10 cm, albeit with varying degrees of precision and with a much larger number of smaller pieces remaining undetected. Based on debris databases created by such observations, spacecraft operators are alerted when a piece of debris is flying too near an operating spacecraft, allowing them to schedule an avoidance maneuver to place the spacecraft on a safe orbit. The number of such maneuvers has increased along with the debris population in the last few years, making it necessary to allocate more spacecraft fuel and expensive operational time. As space usage increases—such as by megaconstellations like Starlink—the risks are expected to grow. To alleviate this additional operational burden, efforts to increase autonomy in space traffic management are currently underway.

To reduce the likelihood that LEO and other orbits will become increasingly inhospitable to spacecraft, measures must be taken to decrease the growth of the space debris population and to actively remove debris from these orbits.  

Post-mission disposal guidelines have been put in place in the last decade to mitigate debris population growth. These state that spacecraft should be appropriately disposed of after the end of their operational lifetime. This means that they are either to re-enter the atmosphere and burn there within 25 years or be put in a “graveyard orbit” away from the most frequented areas in space. Satellites in low orbits will usually re-enter the atmosphere within set timelines on their own, due to natural drag from the upper layers of the atmosphere. Spacecraft in higher orbits must carry additional propellant and be reliable enough near the end of their designed lifetimes if they are to perform the required maneuvers. Constraints like these mean that end-of-life disposal rates sit at only around 50%.

Designers of future spacecraft, and of planned space constellations in particular, are taking the issue more seriously, adding built-in post-mission removal capabilities and developing post-mission disposal kits.

But what percentage of spacecraft and payloads will satisfy post-mission disposal guidelines for 2020 - 2029?

And how much space debris will be removed by 2040?

A more direct approach to mitigation is the active removal of space debris from orbit. Doing so reliably is a challenging prospect considering the target will be, for example, a non-operational, non-cooperative, spinning spacecraft or launcher upper stage. A piece of debris must first be captured and then removed from orbit. Candidate concepts for capturing include stiff robotic arms and more nautically inspired ideas such as tentacles, nets and harpoons. Once a piece of debris is captured it must be removed from orbit by the capturing spacecraft. This can be done by using rocket thrusters on the removing spacecraft, or with other methods designed to minimize the need for propellant and thus decrease the launch mass. Such methods include large deployable surfaces such as balloons or sails to increase the atmospheric drag acting on the spacecraft, and electrodynamic tethers using the Earth’s magnetic field to change its orbit. In most concepts the capturing spacecraft is removed from orbit along with the target debris, but more complex concepts exist that allow the capturing spacecraft to remove multiple pieces. There exist also Earth-based methods for the removal of (usually smaller) debris, such as laser or ion-beams.

No space debris removal mission has yet taken place, but several are in the works: ClearSpace-1 is an ESA-commercial cooperative mission that is scheduled to launch in 2025 and will target a Vega launcher upper stage for removal. Space X’s upcoming Starship has also been put forward as an option for debris removal.

When will such a space debris removal mission occur?

Conclusion

Although there is a consensus that space debris and an impending Kessler syndrome pose a significant risk to infrastructure in space that is currently providing vital services, legal issues and disagreements about who should pay the high costs stymie efforts to resolve the issue. Don Kessler, revisiting his seminal work on space debris 30 years later, observes that progress on that front has not been adequate. He further suggests that to avoid reaching a critical point in the debris population, a 100% post-mission removal success rate must be achieved, and moreover a number of key debris objects must be actively retrieved. It remains in the hands of policymakers and space service providers to rise to the occasion.

For this essay we ask: If Kessler syndrome is avoided, when will peak space debris be reached?

And, ending on a grim note, will space debris cause a fatality before 2035?