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Space debris introduction
Space debris is man-made and, contrary to the micro-meteoroids, growing in numbers. The lifetime of space debris can be over extended periods of time at higher altitudes, due to the small amount of the perturbing forces, such as atmospheric drag and the gravitational attraction of Earth, Moon and Sun. Other perturbations do not generally affect orbital lifetime. Space debris can be split up in four parts:  
Fragmentation debris
Breakups are destructive events that generate numerous smaller objects with a wide range of orbital parameters. These breakups are typically accidental, but there are occasions where non-functional spacecraft were blown up. Products of deterioration (spacecraft degradation) can be large enough to be detected from Earth. Parts of the spacecraft are detaching from the spacecraft and become space debris. Examples are thermal blankets, protective shields and parts of solar panels. Most of the deterioration is the result of the harsh space environment, such as thermal cycling and atomic oxygen. The fragmentation material is the single largest component of the tracked space debris population, accountable for over 40 percent of the total trackable space debris.
Non-functional spacecraft
These are the spacecraft that are intact structures, which have completed their mission, or satellites which had a non-destructive malfunction that shortened their lifetime. This group is accountable for 25 percent of the total trackable space debris.
Rocket bodies
These rocket bodies are of particular importance for the future evolution of the space debris population, due to their large dimension and the potentially explosive residual propellant. With the deployment of a satellite mission, many parts of the launcher become space debris. An example is presented below:
Space debris resulting from a Proton launch
Rocket bodies are accountable for 19 percent of the tracked orbital debris [Belk et al., 1997].
Mission related debris
Mission related debris are items, related to the functional operation of the satellite itself. Examples are explosive bolts, vehicle shrouds and lids that covered telescopes and other fragile equipment. These satellite parts were necessary to perform the satellite mission, and to operate the instruments on the satellite within the requirements.

This sub-section also comprises all other man-made items which are the result of space flight. Examples are exhaust products from Solid Rocket Motors (SRM), paint flakes, Westford needles and Radar Ocean Reconnaissance SATellite (RORSAT) droplets. During operation, SRM can release droplets, such as Al2O3. These particles are generally small of mass, but the flux rate is high. These droplets can have diameters from 0.1 micron up to 3 centimeter. Paint flakes are the result of the harsh space temperature environment. Small pieces of paint on the satellite will de-attach from the spacecraft and become space debris. Despite the origin of paint flakes, i.e. the harsh space environment, they are positioned in this section.

Westford needles was a project of the Department of Defence of the Unites States of America in 1962. A large number of copper needles were intentionally released in an attempt to lay a radio-reflective ring around the Earth. These dipoles would serve as an artificial scattering medium for radio signals in the centimeter band. The experiment was greatly criticized by astronomers who feared optical and radio pollution. The first experiment did not work as a radio reflector. The second one was successful but now the needles contribute to the space debris problem. Still new needle populations are discovered by radar and optical measurements from Earth.

The RORSAT droplets are coolant droplets from the Russian RORSAT satellites. These inactive satellites released coolant droplets (NaK, liquid metal) from the nuclear reactors, when the reactor was separated from the spacecraft.

Mission related debris is generally small in diameter. Therefore they are hard to detect with the current observation methods, which can trace space debris from a diameter of 10 cm and larger, depending on the debris altitude. The amount of released debris can be quite large. For example, 200 pieces of mission related space debris were linked to the Russian space station Mir during its first eight years of operation. Most of the mission related debris was dumped intentionally, but there are also examples of astronauts who lost items during Extra-Vehicular Activity (EVA).

The sub-section “mission related debris” is accountable for over 14 percent of the total traceable space debris population [Belk et al., 1997].

The 2 percent remaining of the traced space debris has an unknown source [Belk et al., 1997].
Last update: 5 April 2007

Related sites:
Space Environment Information SystemGeant4 for spaceMULASSISSSATGRAS (PDF)ESABASE2/DebrisMASTER-2005Martian Climate DatabaseCOMOVASPISSpacegrid
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