The Kessler syndrome

The Kessler syndrome is a theory proposed by NASA scientist Donald J. Kessler in 1978, used to describe a self-sustaining cascading collision of space debris in LEO. In an article published on June 1, 1978 in the American Journal of Geophysical Research, a peer-reviewed – the evaluation of work by one or more people with similar competences as the producers of the work – scientific journal, containing original research on the physical, chemical, and biological processes that contribute to the understanding of the Earth, Sun, and Solar System, authors Donald J. Kessler and Burton G. Cour-Palais, two NASA experts, identified the risk of an exponential increase in the number of space debris or orbital debris under the effect of mutual collisions. The two authors believed that a belt formed by these objects or fragments of objects around the Earth would soon form. Eventually threatening space activities, this phenomenon will be popularized a few years later under the name of Kessler syndrome.

The Kessler syndrome, also called the Kessler effect, collisional cascading or ablation cascade, is a scenario in which the density of objects in Low Earth Orbit (LEO) is high enough that collisions between objects could cause a cascade where each collision generates space debris that increases the likelihood of further collisions. One implication is that the distribution of debris in orbit could render space activities and the use of satellites in specific orbital ranges impractical for many generations. Every satellite, space probe, and manned mission has the potential to produce space debris. A cascading Kessler syndrome becomes more likely as satellites in orbit increase in number. The most commonly used orbits for both manned and unmanned space vehicles are Low Earth Orbit (LEO). Clearly, the number of space debris that naturally falls back into the atmosphere is less than the number of those generated by the collision of existing space debris. Even if all space activity and launch were halted tomorrow, the debris population would continue to increase exponentially, leading to a situation in which some orbits would become impassable in the long run.

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. This process parallels certain theories concerning the growth of the asteroid belt. The debris flux in such an earth-orbiting belt could exceed the natural meteoroid flux, affecting future spacecraft designs. A mathematical model was used to predict the rate at which such a belt might form. Under certain conditions the belt could begin to form within this century and could be a significant problem during the next century. The possibility that numerous unobserved fragments already exist from spacecraft explosions would decrease this time interval. However, early implementation of specialized launch constraints and operational procedures could significantly delay the formation of the belt” – Collision frequency of artificial satellites: The creation of a debris belt, Journal of Geophysical Research, Volume 83, Issue A6, p. 2637-2646 (1978).

Earth orbits

An orbit is the curved path through which objects in space move around a planet or a star. The 1967 Treaty’s regime and customary law enshrine the principle of non-appropriation and freedom of access to orbital positions. Space Law and International Telecommunication Laws combined to protect this use against any interference. The majority of space-launched objects are satellites that are launched in Earth’s orbit (a very small part of space objects – scientific objects for space exploration – are launched into outer space beyond terrestrial orbits). It is important to precise that an orbit does not exist: satellites describe orbits by obeying the general laws of universal attraction. Depending on the launching techniques and parameters, the orbital trajectory of a satellite may vary. Sun-synchronous satellites fly over a given location constantly at the same time in local civil time: they are used for remote sensing, meteorology or the study of the atmosphere. Geostationary satellites are placed in a very high orbit; they give an impression of immobility because they remain permanently at the same vertical point of a terrestrial point (they are mainly used for telecommunications and television broadcasting). A geocentric orbit or Earth orbit involves any object orbiting Planet Earth, such as the Moon or artificial satellites. Geocentric (having the Earth as its centre) orbits are organised as follow:

1) Low Earth Orbit (LEO): geocentric orbits with altitudes (the height of an object above the average surface of the Earth’s oceans) from 100 to 2 000 kilometres. Satellites in LEO have a small momentary field of view, only able to observe and communicate with a fraction of the Earth at a time, meaning a network or constellation of satellites is required in order to provide continuous coverage. Satellites in lower regions of LEO also suffer from fast orbital decay (in orbital mechanics, decay is a gradual decrease of the distance between two orbiting bodies at their closest approach, the periapsis, over many orbital periods), requiring either periodic reboosting to maintain a stable orbit, or launching replacement satellites when old ones re-enter.

2) Medium Earth Orbit (MEO), also known as an intermediate circular orbit: geocentric orbits ranging in altitude from 2 000 kilometres to just below geosynchronous orbit at 35 786 kilometres. The most common use for satellites in this region is for navigation, communication, and geodetic/space environment science. The most common altitude is approximately 20 000 kilometres which yields an orbital period of twelve hours.

3) Geosynchronous Orbit (GSO) and geostationary orbit (GEO) are orbits around Earth at an altitude of 35 786 kilometres matching Earth’s sidereal rotation period. All geosynchronous and geostationary orbits have a semi-major axis of 42 164 kilometres. A geostationary orbit stays exactly above the equator, whereas a geosynchronous orbit may swing north and south to cover more of the Earth’s surface. Communications satellites and weather satellites are often placed in geostationary orbits, so that the satellite antennae (located on Earth) that communicate with them do not have to rotate to track them, but can be pointed permanently at the position in the sky where the satellites are located.

4) High Earth Orbit (HEO): geocentric orbits above the altitude of 35 786 kilometres.

Space Law

Let’s have a look at the Kessler syndrome and Space Law. Near-Earth space is formed of different orbital layers. Earth orbits are limited common resources and inherently repugnant to any appropriation: they are not property in the sense of law. Orbits and frequencies are res communis (a Latin term derived from Roman law that preceded today’s concepts of the commons and common heritage of mankind; it has relevance in international law and common law). It’s the first-come, first-served principle that applies to orbital positioning, which without any formal acquisition of sovereignty, records a promptness behaviour to which it grants an exclusive grabbing effect of the space concerned. Geostationary orbit is a limited but permanent resource: this de facto appropriation by the first-comers – the developed countries – of the orbit and the frequencies is protected by Space Law and the International Telecommunications Law. The challenge by developing countries of grabbing these resources is therefore unjustified on the basis of existing law. Denying new entrants geostationary-access or making access more difficult does not constitute appropriation; it simply results from the traditional system of distribution of access rights. The practice of developed States is based on free access and priority given to the first satellites placed in geostationary orbit.

The geostationary orbit is part of outer space and, as such, the customary principle of non-appropriation and the 1967 Space Treaty apply to it. The equatorial countries have claimed sovereignty, then preferential rights over this space. These claims are contrary to the 1967 Treaty and customary law. However, they testify to the concern of the equatorial countries, shared by developing countries, in the face of saturation and seizure of geostationary positions by developed countries. The regime of res communis of outer space in Space Law (free access and non-appropriation) does not meet the demand of the developing countries that their possibilities of future access to the geostationary orbit and associated radio frequencies are guaranteed. New rules appear necessary and have been envisaged to ensure the access of all States to these positions and frequencies.

The Kessler syndrome

Although national space agencies are working to make outer space cleaner, the space debris population is still growing. For some, the situation is so serious that the Earth has hung an exponential. Others believe that the problem was taken in time and that solutions will be found to guarantee a clean space for future generations. Since 1957, the beginning of space exploitation, many objects accumulate in outer space. In LEO, this population of objects, called the orbital population, increases from year to year. Less than ten percent of the orbital population are active satellites. The rest is what we call space debris, or orbital debris, that is to say, artificial objects in orbit but no longer used for anything. These space debris pose a threat to ground populations as they fall back randomly to the surface of the Earth. They also generate a risk of orbital collisions, which can disrupt an active satellite or produce more debris as a result of these high-energy shocks. The long-term crowding of the most useful orbits could disrupt the exploitation of outer space. A set of international rules have been put in place to limit this pollution but they are poorly respected, and last-resort orbital cleaning scenarios are emerging.

In Low Earth Orbit, the Kessler syndrome is troublesome because of the domino effect (a cascading failure is a process in a system of interconnected parts in which the failure of one or few parts can trigger the failure of other parts and so on) and feedback runaway (a process that occurs in a feedback loop in which the effects of a small disturbance on a system include an increase in the magnitude of the perturbation) wherein impacts between objects of sizable mass spall off (flakes of a material that are broken off a larger solid body and can be produced by a variety of mechanisms, including as a result of projectile impact, corrosion, weathering, cavitation, or excessive rolling pressure, as in a ball bearing) debris from the force of the collision. The shrapnel 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 outer space, then the resulting debris cascade could make prospects for long-term viability of satellites in Low Earth Orbit (LEO) extremely low.

Since the development of the Kessler syndrome, we still do not know formally whether this chain reaction has started, because the modelling of orbital collision phenomena and the fate of associated debris is extremely complex. Even though a large part of the debris eventually disintegrates and destroys itself by entering the upper atmosphere, it must be noted that the mass of the orbital population has grown continuously since 1957, just as the number of debris. More than three hundred objects on average each year increase the orbital population, which corresponds to the low limit predicted by Kessler.

To limit the number of space debris in orbit, preventive measures have been issued by the Inter-Agency Space Debris Coordination Committee. The Inter-Agency Space Debris Coordination Committee (IADC) is an international governmental forum for the worldwide coordination of activities related to the issues of man-made and natural debris in space. The primary purposes of the IADC are to exchange information on space debris research activities between member space agencies, to facilitate opportunities for cooperation in space debris research, to review the progress of ongoing cooperative activities, and to identify debris mitigation options. The first limit is twenty-five years, the time that a satellite can stay in outer space after the end of its mission. The second provides for the passivation of the upper stages after use of the residual fuel, to limit the risk of an explosion of unburnt which would generate thousands of new debris. Space agencies, for their part, fund research and development programs aimed at developing satellites and technologies that can deorbit larger and more dangerous debris.

The first rule of good conduct was NASA’s 1995, prepared by Donald J. Kessler, who became the first director of the debris bureau at the U.S. Space Agency. The NASA Orbital Debris Program Office, administratively located at the Johnson Space Center, is recognized world-wide for its initiative in addressing orbital debris issues. The NASA Orbital Debris Program Office has taken the international lead in conducting measurements of the environment and in developing the technical consensus for adopting mitigation measures to protect users of the orbital environment. Work at the Center continues with developing an improved understanding of the orbital debris environment and measures that can be taken to control debris growth. Followed the Japanese regulations in 1997 and French in 1999, before reaching the first international text, published in 2002 by the Inter-Agency Space Debris Coordination Committee and approved unanimously by the major space agencies. This text was extended in May 2011 to all space activities, institutional or private, via an International Organization for Standardization (ISO) standard covering all aspects of space debris. That is what we can say about the Kessler syndrome.