Evolution of the tracked space debris population
Evolution of the tracked space debris population

About space debris

Satellites in orbit around Earth are used in many areas and disciplines, including space science, Earth observation, meteorology, climate research, telecommunication, navigation and human space exploration. They offer a unique resource for collecting scientific data, which lead to unrivalled possibilities for research and exploitation, both scientific and commercial.

However, in the past decades, with increasing space activities, a new and unexpected hazard has started to emerge: space debris.

50 years of space activity

In almost 50 years of space activities, more than 4900 launches have placed some 6600 satellites into orbit, of which about 3600 remain in space; only a small fraction - about 1000 - are still operational today.

This large amount of space hardware has a total mass of more than 6 300 tonnes. Not all objects are still intact. More than 23 000 space objects Earth orbits (as of September 2012) in total are regularly tracked by the US Space Surveillance Network and maintained in their catalogue, which covers objects larger than approximately 5 to 10 cm in low Earth orbit (LEO) and 30cm to 1m at geostationary altitudes (GEO).

Objects in orbit include spent upper stages

Debris objects in orbit include spent upper stages
Debris objects in orbit include spent upper stages

Only 6% of the catalogued orbit population are operational spacecraft, while about 30 percent can be attributed to decommissioned satellites, spent upper stages and mission-related objects (launch adapters, lens covers, etc.).

More than 250 in-orbit fragmentation events have been recorded since 1961. Only a few were collisions (less than 10 accidental and intentional events); the majority of the events were explosions of spacecraft and upper stages.

Explosions of satellites and rocket bodies

Sources of space debris include explosions of rocket bodies
Sources of space debris include explosions of rocket bodies

These are assumed to have generated a population of objects larger than 1 cm on the order of 600 000. Only near sizes of 0.1 mm to 1mm may the sporadic flux from meteoroids prevail over man-made debris.

The main cause of in-orbit explosions is related to residual fuel that remains in tanks or fuel lines once a rocket stage or satellite is discarded in Earth orbit. 

Over time, the harsh space environment can deteriorate the mechanical integrity of external and internal parts, leading to leaks and/or mixing of fuel components, which could trigger self-ignition. The resulting explosion can destroy the object and spread its mass across numerous fragments with a wide spectrum of masses and imparted velocities.

Anti-satellite test: 25% more debris

Besides such accidental break-ups, spacecraft interceptions by surface-launched missiles have been a major contributor in the recent past.

The Chinese Feng-Yun 1C engagement in January 2007 alone increased the trackable space object population by 25%.

Other sources of debris fragments

The most important non-fragmentation debris source have been more than 1000 solid rocket motor firings, which released aluminium oxide (Al2O3) in the form of micrometre-sized dust and mm- to cm-sized slag particles.

A second important source was the ejection of reactor cores from Buk reactors after the end of operation of Russian RORSATs (Radar Ocean Reconnaissance Satellites) in the 1980s. In 16 such ejection events, numerous droplets of reactor coolant liquid (a low-melting sodium potassium alloy) were released into space.

Another historic source was the release of thin copper wires as part of a radio communication experiment during the MIDAS missions in the 1960s.

Finally, under the influence of extreme ultraviolet radiation, impinging atomic oxygen and impacting micro particles, the surfaces of space objects, such as satellites, start to erode. This leads to mass loss of surface coatings and to the detachment of paint flakes with sizes from micrometre to mm sizes.

Observations with ESA’s 1m telescope at Tenerife have found a population of objects with extremely high area-to-mass ratios. The origin and nature of those objects is not fully understood. It is generally agreed now that these objects have been created in the geostationary region, possibly from thermal covering material of disposed satellites.

First-ever in-orbit collision

The first-ever accidental in-orbit collision between two satellites occurred at 16:56 UTC, 10 February 2009, at 776 km altitude above Siberia. An American privately owned communication satellite, Iridium 33, and a Russian military satellite, Kosmos-2251, collided at a relative speed of 11.7 km/second. Both were destroyed, and more than 2200 trackable fragments were generated.

Distribution of catalogued objects in space - global view

Distribution of debris objects larger than 10 centimetres in space
Distribution of debris

Satellites launched into LEO are continuously exposed to aerodynamic forces from the tenuous upper reaches of the Earth's atmosphere.

Depending on the altitude, after a few weeks, years or even centuries, this resistance will have decelerated the satellite sufficiently so that it reenters into the atmosphere. At higher altitudes, above 800 km, air drag becomes less effective and objects will generally remain in orbit for many decades.

At any given altitude, the generation of debris through normal launch operations, break-ups and other release events is counter-acted by natural cleansing mechanisms, such as air drag and luni-solar gravitational attraction. The result of these balancing effects is an altitude-and-latitude-dependent concentration (spatial density) of space debris objects.

Maximum debris concentrations can be noted at altitudes of 800 to 1000 km, and near 1400 km. Spatial densities in GEO and near the orbits of navigation satellite constellations are smaller by two to three orders of magnitude.

Forecast if 'business as usual': debris growth

'Business-as-usual' activities will lead to a progressive, uncontrolled increase in debris objects, with collisions becoming the primary debris source in less than 50 years.

With today's annual launch rates of 60 to 70 new satellites per year, and with future break-ups continuing to occur at average historic rates of four to five per year, the number of debris objects in space will steadily increase.

As a consequence of the rising debris object count, the probability for catastrophic collisions will also grow in a progressive manner; doubling the number of objects will increase the collision risk approximately four times.

As the debris population grows, more collisions will occur.

In a 'business-as-usual' scenario, such collisions will start prevailing over the now-dominating explosions within a few decades from now. Ultimately, collision fragments will collide with collision fragments, until the entire population is reduced to sub-critical sizes.

This self-sustained process, which is particularly critical for the LEO region, is known as the 'Kessler syndrome'. It is a scenario that must be avoided by the timely application of space debris mitigation and remediation measures on an international scale. 

Last update: 18 April 2013
