10.1.2 Space debris

Space debris are man-made objects that are remainders of human spaceflight activities. In early 2008 only 7% of the catalogued orbit population are operational spacecraft, while 41% can be attributed to decommissioned satellites, spent upper stages, and mission related objects (launch adapters, lens covers, etc.). The remainder of 52% is originating from more than 200 on-orbit fragmentations which have been recorded since 1961. These events are collisions (in three cases) and explosions of spacecraft and upper stages. The total population of objects larger than 1 cm is on the order of 500 000 to 700 000.

The major source of space debris, fragmentations of space objects, originates from spare fuel that mostly remains inside pressurized tanks once the rocket stage is discarded into Earth orbit. Over time, and in the harsh environment of space, the mechanical integrity of the booster's internal components breaks down and tanks start to leak. The resulting sudden releases of pressure or even high energetic explosions expel numerous fragments into orbit. The most important non-fragmentation source is solid rocket motor firings during which aluminium oxide (Al₂O₃) in the form μm-sized dust and mm to cm-sized slag particles is exhausted. A second important source was the ejection of reactor cores during the end of operation of the Russian RORSATs (Radar Ocean Reconnaissance Satellites) in the 1980’s, which released droplets of the reactor coolant (sodium potassium alloy (NaK)) 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 1960’s. Finally, under the influence of the harsh space environment (extreme ultra violet radiation, impinging atomic oxygen and micro particle impacts), surfaces of space objects start to erode. This leads to mass losses of surface coatings and to the detachment of flakes of the surface paint, both with μm and mm sizes.