Explosions of satellites & 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.

Anti-satellite test: 25 percent more debris

The resulting explosion can destroy the source object and spread its mass across numerous fragments with a wide spectrum of masses and imparted velocities. 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 percent.

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 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, droplets of reactor coolant liquid (a low-melting sodium potassium alloy) were released into space.

Distribution of catalogued objects in space - close-up of the LEO region

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 ultra violet radiation, impinging atomic oxygen and impacting micro particles, surfaces of space objects start to erode. This leads to mass losses of surface coatings and to the detachment of paint flakes with sizes from micrometre to mm sizes.

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 a large amount of debris generated.

Distribution of catalogued objects in space - global view

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 re-enters into the atmosphere. At higher altitudes, i.e. above 800 km, air drag becomes less effective and objects will generally remain in orbit for many decades.

At any given altitude shell, debris generation-processes by normal launch operations, break-ups and other release events are counter-acted by natural cleansing mechanisms, such as air drag and luni-solar 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

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

As a consequence of the rising 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-fold).

As the debris population grows, first 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 ground 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.