The Kessler Syndrome: A Looming Threat in Earth’s Orbit
A scenario first proposed in the 1970s edges closer to reality.
In 1978, NASA scientist Donald Kessler published Collision frequency of artificial satellites: The creation of a debris belt in the Journal of Geophysical Research. (Kessler and Cour-Palais 1978) Dubbed the Kessler Syndrome, the scenario posits the density of objects in low Earth orbit (LEO) becomes high enough that collisions between objects could cause a cascade. Each collision generates space debris that increases the likelihood of further collisions. This self-sustaining cascading effect could severely hinder the use of satellites and other space activities, posing a significant risk to the future of space exploration and communication. The most imaginative scenario has humans trapped on Earth, unable to engage in spaceflight because of an impenetrable debris cloud.
The Kessler Syndrome is an orbital version of a chain reaction. As the number of satellites, spent rocket stages, and other debris in orbit increases, the probability of collisions also rises. When two objects collide, they break into numerous smaller fragments. Traveling at high velocities, these fragments can collide with other objects, creating more debris in a dangerous feedback loop. This phenomenon is particularly concerning in LEO, where most satellites reside, including those critical for communications, weather monitoring, and navigation. The International Space Station (ISS) also orbits within this region, making it vulnerable to increasing space debris.
The stage of the Kessler Syndrome is a subject of debate. Some scientists say it is imminent and we’re already within the beginning stages; others point to the fact that launches are ongoing, and disaster has yet to strike, and thus, the Kessler Syndrome is overstated. Kessler himself, in a 2009 paper (D. J. Kessler 2009), pointed out that the definition of the Syndrome is unclear, though it was popularized by a scenario in the movie Gravity, where space debris crippled a spacecraft and jeopardized the crew’s safety. In the movie, the disaster developed very quickly, as must necessarily happen to move the plot along. As Kessler clarified:
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…although the concept of a critical threshold was an important possibility that was studied in detail more than 10 years later. 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. 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. Based on past accumulation rates, the 1978 publication predicted that random collision would become an important debris source around the year 2000, with the rate of random collisions increasing rapidly after that, if the accumulation rate were not reduced to near zero.
One of Kessler’s points was that collisions would increase exponentially as the amount of debris increased.
Combined with the discovery that 42% of the catalogued objects were the results of only 19 explosions in orbit of U.S. upper stage rockets and that NORAD was not tracking “all man-made objects” as generally believed, NASA took these findings and predictions seriously. (D. J. Kessler 2009)
Kessler’s 2009 paper predated the advent of SpaceX and its Starlink program. SpaceX has created a constellation of internet communication satellites in LEO, around the 300 to 500-kilometer altitude. Begun on May 23, 2019, with the launch of the first sixty Starlink satellites, the program continues to add satellites at the rate of five weekly launches. According to starlinkmap.org, there are, as of this writing, 5,601 orbiting satellites, 4,332 of the Version 1 type and 1,269 of the smaller Version 2 (mini) satellites. Three hundred seventy-six satellites have been deorbited to burn up on reentry due to failure to reach a proper orbit, equipment malfunction, or age. SpaceX intentionally chose the very low orbit to make it easier to deorbit satellites when necessary. SpaceX states satellites will deorbit within five years without propulsion due to atmospheric drag.
It’s not hard to imagine the potential for cascading collisions within such a relatively dense hardware cloud. For this reason, SpaceX has equipped its Gen 2 satellites with propulsive collision avoidance capabilities.
As SpaceX stated in its Gen2 application, “[a]ll satellites will have sufficient propellant and capability to perform any avoidance maneuvers required for all phases of the satellites’ mission.” SpaceX has budgeted sufficient propellant to accommodate approximately 5,000 propulsive maneuvers over the life of a satellite, including a budget of approximately 350 collision avoidance maneuvers per satellite over that time period. Using SpaceX’s semi-annual satellite reports for comparison, the average SpaceX Gen1 satellite has conducted fewer than three collision-avoidance maneuvers every six months over the last year, and it conducted these maneuvers predominantly to avoid debris from the November 2021 Russian anti-satellite demonstration. Even under these anomalous conditions, a 350-maneuver budget is extremely conservative. (SpaceX 2022)
SpaceX was referring to an anti-satellite test (ASAT) on November 15, 2021, in which Russia destroyed one of its satellites, Cosmos 1408, which had been in orbit since 1982. The resulting debris field resulted in at least 1,500 trackable pieces of wreckage in LEO, threatening other satellites and the International Space Station. The seven ISS crewmembers were forced to take precautions multiple times as the ISS intersected with the debris field. (Bugos 2021)
Adding to the problem, India conducted a successful test on March 27, 2021, destroying a previously launched target satellite with a ballistic missile interceptor. There are no estimates of the resulting debris field. (Davenport 2019)
Not to be left out, China destroyed an aging weather satellite orbiting at 850 kilometers on January 11, 2007. NASA’s chief scientist for orbital debris at the time, Nicholas Johnson, called the destruction of the satellite “the worst satellite breakup” ever. (Boese 2007)
The US’s hands are not clean either. In the late 1950s, the US Air Force began developing various anti-satellite systems, culminating in the ASM-135, designed to launch from an F-15A in a supersonic zoom climb. After several successful tests, the Reagan Administration terminated the program in 1988 due to cost overruns, technical problems, and testing delays.
In 2022, the United Nations General Assembly approved a non-binding resolution calling for an end to certain types of ASAT tests. The US banned ASAT tests the same year. In 2020, the US was one of the original eight signatories to the Artemis Accords to limit the generation of space debris. Twenty-one nations have now signed the Accords. (Foust 2022)
Current State of Space Debris
According to the European Space Agency:
On average over the last two decades, 12 accidental ‘fragmentations’ have occured (sic) in space every year — and this trend is unfortunately increasing. Fragmentation events describe moments in which debris is created due to collisions, explosions, electrical problems and even just the detachment of objects due to the harsh conditions in space. ( (European Space Agency 2020)
Thousands of active satellites are in orbit, with many more inactive ones. According to the European Space Agency (ESA), there are an estimated 130 million pieces of debris smaller than 1 cm, 1,000,000 between 1 and 10 cm, and around 36,500 pieces larger than 10 cm. (European Space Agency 2023) These objects travel at speeds of up to 28,000 kilometers per hour, making even the smallest pieces of debris potentially catastrophic if they collide with operational spacecraft.
Number of rocket launches since the start of the space age in 1957—6,500 (excluding failures)
Number of satellites placed in orbit—About 16,990
Number of these still in space—About 11,500
Number still functioning—About 9,000
Number of debris object tracked by various space agencies—About 35,150
Estimated number of breakups, explosions, collisions, etc—More than 640
Total mass of space objects in Earth orbit—More than 11,500 tonnes
Potential Consequences
The primary concern of the Kessler Syndrome is the potential for a runaway scenario where space becomes so cluttered that safe and reliable access to orbit becomes impossible. This would have profound implications:
· Communication Disruptions: Satellites that provide global communication services could be damaged or destroyed, disrupting telephone, internet, and television services.
· Navigation Systems: The Global Positioning System (GPS) and other satellite navigation services could be compromised, affecting transportation and military operations.
· Weather Forecasting: Satellites used for monitoring weather patterns and climate would be at risk, impacting weather prediction and disaster preparedness.
· Scientific Research: The loss of observational satellites could hinder scientific research in fields such as climatology, oceanography, and space science.
· Human Spaceflight: Increased debris would significantly threaten the ISS and future manned missions, including those planned for lunar and Mars exploration.
Astronomers are already concerned about the Starlink system, which produces light streaks across photographic exposures made by major telescopes. Interference is also beginning to affect radio telescopes.
Mitigation Strategies
Addressing the Kessler Syndrome requires a multi-faceted approach:
· Debris Removal: Active debris removal (ADR) technologies are being developed to capture and deorbit large objects. Methods include using robotic arms, nets, harpoons, and lasers to nudge debris out of orbit.
· Satellite Design: Designing satellites to minimize debris creation, such as including mechanisms for deorbiting at the end of their operational life, can help reduce the proliferation of space junk.
· Regulation and Policy: International cooperation and stringent regulations on satellite launches, orbital management, and debris mitigation are crucial. The United Nations Committee on the Peaceful Uses of Outer Space (COPUOS) and national space agencies are working towards establishing such frameworks.
· Space Traffic Management: Improved tracking and monitoring of space objects to predict potential collisions and maneuver satellites to avoid them can mitigate risks.
Earlier this year, the Committee on Peaceful Uses of Outer Space issued a less optimistic report about the solution. (Dickey, Uvarov and Wang 2024) According to the abstract:
Besides the legal, political and economic hurdles that are hindering timely remediation of orbital debris, four widely-held perspectives are inhibiting progress: “analysis paralysis”, national leadership, blaming others, and industry readiness concerns. Although grounded in fact and law, they are also fostering unintended consequences harmful to achieving results.
The report also noted that:
Of all the man-made debris in space, Massive Derelicts are the most dangerous. Clustered in a few circular orbits high above the International Space Station, intertwined within the most valuable but increasingly over-populated neighborhoods in LEO, and sharing physical characteristics, a few thousand discarded rocket bodies and defunct satellites hurtle around the globe. Collisions involving these huge objects are imminent and inevitable unless we take action to avoid them.
Designing satellites for removal after their useful life has ended is not easy. It adds weight and bulk that must be accommodated by launch vehicles. Volatile fuel must be stored aboard the satellite, which is hazardous both during launch and while in space. These are technical issues that are probably solvable, but at what price?
The Kessler Syndrome represents a significant challenge to the sustainable use of space. While the scenario described by Kessler has not yet fully materialized, the increasing number of satellites and amounts of space debris highlights the urgency of addressing this issue. Collaborative efforts across nations and industries are essential to develop and implement effective strategies to prevent a catastrophic chain reaction in Earth’s orbit. The future of space exploration and the myriad of services that depend on satellite technology hinge on our ability to manage and mitigate the risks posed by space debris.
Works Cited
Boese, Wade. 2007. “Chinese Satellite Destruction Stirs Debate.” Arms Control Today (Arms Control Association) 37.
Bugos, Shannon. 2021. “Russian ASAT Test Creates Massive Debris.” Arms Control Today (Arms Control Association). https://www.armscontrol.org/act/2021-12/news/russian-asat-test-creates-massive-debris.
Davenport, Kelsey. 2019. “Indian ASAT Test Raises Space Risks.” Arms Control Today (Arms Control Association) 49.
Dickey, Chuck, Valentin Uvarov, and Guoyu Wang. 2024. Through a Glass Darkly — How Four Good Ideas are Inhibiting Remediation of Orbital Debris. Legal Subcommittee, Sixty-third session, Vienna: Committee on the Peaceful Uses of Outer Space.
European Space Agency. 2023. Space debris by the numbers. European Space Agency. https://www.esa.int/Space_Safety/Space_Debris/Space_debris_by_the_numbers.
— . 2020. The current state of space debris. December 10. Accessed May 20, 2024. https://www.esa.int/Space_Safety/Space_Debris/The_current_state_of_space_debris.
Foust, Jeff. 2022. United Nations General Assembly approves ASAT test ban resolution. December 13. Accessed May 2024. https://spacenews.com/united-nations-general-assembly-approves-asat-test-ban-resolution/.
Kessler, Donald J, and Burton G. Cour-Palais. 1978. “Collision frequency of artificial satellites: The creation of a debris belt.” Journal of Geophysical Research (American Geophysical Union) 83 (A6): 2637–2646. doi:10.1029/JA083iA06p02637.
Kessler, Donald J. 2009. The Kessler Syndrome as discussed by Donald J. Kessler. Unknown.
Kessler, Douglas J. 2009. Ibid.
SpaceX. 2022. “Electronic Filing.” “Re: IBFS File Nos. SAT-LOA-20200526–00055 and SAT-AMD-20210818–00105. EXHIBIT B. SATELLITE DIMENSIONS AND DAS OUTPUTS”. October 4.
