Kessler syndrome

Willy Ley predicted in 1960 that "In time, a number of such accidentally too-lucky shots will accumulate in space and will have to be removed when the era of manned space flight arrives". After the launch of Sputnik 1 in 1957, the US North American Aerospace Defense Command (NORAD) began compiling a database (the Space Object Catalog) of all known rocket launches and objects reaching orbit: satellites, protective shields, and upper- and lower-stage booster rockets. NASA later published modified versions of the database in a two-line element set format; and during the early 1980s, the CelesTrak bulletin board system re-published them.

The trackers who fed the database were aware of other objects in orbit, many of which were the result of in-orbit explosions. Some were deliberately caused during the 1960s anti-satellite weapon testing, and others were the result of rocket stages blowing up in orbit as leftover propellant expanded and ruptured their tanks. To improve tracking, NORAD employee John Gabbard kept a separate database. Studying the explosions, Gabbard developed a technique for predicting the orbital paths of their products, and Gabbard diagrams (or plots) are now widely used. These studies were used to improve the modeling of orbital evolution and decay.

When the NORAD database became publicly available during the 1970s, NASA scientist Donald J. Kessler applied to the database of known objects a technique developed to study the asteroid belt. In June 1978, Kessler and Burton Cour-Palais co-authored "Collision Frequency of Artificial Satellites: The Creation of a Debris Belt", demonstrating that the process controlling asteroid evolution would cause a similar collision process in LEO in decades rather than billions of years. They concluded that by about 2000, space debris would outpace micrometeoroids as the primary ablative risk to orbiting spacecraft.

At the time, it was widely thought that drag from the upper atmosphere would de-orbit debris faster than it was created. However, Gabbard was aware that the number and type of objects in space were under-represented in the NORAD data and was familiar with their behavior. …

Every satellite, space probe, and crewed mission has the potential to produce space debris. The theoretical cascading Kessler syndrome becomes more likely as satellites in orbit increase in number. In 2025 the number was estimated at over 11,800, most of which (7,135) belonged to Starlink, the constellation of SpaceX. As of 2021, it was estimated that there were 600,000 pieces of space junk ranging from 1 to 10 cm (1⁄2 to 4 in), and 23,000 larger than that. On average, every year, one satellite is destroyed by collision with other satellites or space junk. As of 2009, there had been four collisions between catalogued objects, including a collision between two satellites in 2009.

Orbital decay is much slower at altitudes where atmospheric drag is insignificant. Slight atmospheric drag, lunar perturbation, and solar wind drag can gradually bring debris down to lower altitudes where fragments finally re-enter, but this process can take millennia at very high altitudes.

The Kessler syndrome is troublesome because of the domino effect and feedback runaway wherein impacts between objects of sizable mass spall off debris from the force of the collision. The fragments can then hit other objects, producing even more space debris: if a large enough collision or explosion were to occur, such as between a space station and a defunct satellite, or as the result of hostile actions in space, then the resulting debris cascade could make prospects for long-term viability of satellites in particular low Earth orbits extremely low. However, even a catastrophic Kessler scenario at LEO would pose minimal risk for launches continuing past LEO, or satellites travelling at medium Earth orbit (MEO) or geosynchronous orbit (GEO). The catastrophic scenarios predict an increase in the number of collisions per year, as opposed to a physically impassable barrier to space exploration that occurs in higher orbits.

Some astronomers have hypothesized Kessler syndrome as a possible or likely solution to the Fermi paradox, the lack of any sign of alien life in the universe. Any intelligent civilization which becomes spacefaring could eventually extinguish any safe orbits via Kessler syndrome, trapping itself within its home planet. Such a result could happen even with robust space pollution controls, as a lone malicious actor on a planet could cause a Kessler syndrome scenario. Humanity could be on the path to a similar fate, soon to trap itself on Earth with no future as a spacefaring civilization. Some exoplanet researchers have attempted to survey other planets for signs of a Kessler syndrome cascade as a sign of intelligent life.

Designers of a new vehicle or satellite are frequently required by the International Telecommunication Union (ITU) to demonstrate that it can be safely disposed of at the end of its life, for example by use of a controlled atmospheric reentry system or a boost into a graveyard orbit. For US launches or satellites that will have broadcast to US territories—in order to obtain a license to provide telecommunications services in the United States—the Federal Communications Commission required all geostationary satellites launched after 18 March 2002 to commit to moving to a graveyard orbit at the end of their operational life. US government regulations similarly require a plan to dispose of satellites after the end of their mission: atmospheric re-entry, movement to a storage orbit, or direct retrieval.

A proposed energy-efficient means of deorbiting a spacecraft from Medium Earth Orbit is to shift it to an orbit in an unstable resonance with the Sun or Moon that speeds up orbital decay.

Several space debris models, designed on catalogues in-orbit fragmentation events, can be used both during the design phase and operative life to estimate the flux of space debris in order to accurately design shielding and collision avoidance manoeuvres.

One technology proposed to help deal with fragments from 1 to 10 cm (1⁄2 to 4 in) in size is the laser broom, a proposed multimegawatt land-based laser that could deorbit debris: the side of the debris hit by the laser would ablate and create a thrust that would change the eccentricity of the remains of the fragment until it would re-enter and be destroyed harmlessly.

ESA and the Swiss startup ClearSpace plans a mission to remove the PROBA-1 satellite from orbit.

An analysis determined that of the 50 "statistically most concerning" debris objects in low Earth orbit, the top 20 were all SL-16 upper stages.

The Envisat satellite is a large, inactive satellite with a mass of 8,211 kg (18,102 lb) that orbits at 785 km (488 mi), an altitude where the debris environment is the greatest—two catalogued objects can be expected to pass within about 200 m (660 ft) of Envisat every year—and likely to increase. Don Kessler predicted in 2012 that it could easily become a major debris contributor from a collision during the next 150 years that it will remain in orbit.

SpaceX's Starlink program raises concerns about significantly worsening the possibility of Kessler syndrome due to the large number of satellites the program is placing in LEO, as the program's goal will more than double the satellites currently in LEO. In response to these concerns, SpaceX said that a large number of Starlink satellites are launched at a lower altitude of 550 km (340 mi) to achieve lower latency (versus 1,150 km (710 mi) as originally planned), and failed satellites or debris are thus expected to deorbit within five years even without propulsion, due to atmospheric drag.