The term "space debris" in its largest sense includes all naturally occurring remains of solar and planetary processes: interplanetary dust, meteoroids, asteroids, and comets. Human-made space debris in orbit around Earth is commonly called orbital debris. Examples include dead satellites, spent rocket bodies, explosive bolt fragments, telescope lens covers, and the bits and pieces left over from satellite explosions and collisions.
Orbital debris is found wherever there are working satellites. Of the more than 9,000 objects larger than 10 centimeters (3.94 inches) in Earth orbit, 94 percent are debris. The densest regions are low Earth orbit (LEO), an altitude range from 400 to 2,000 kilometers (248.5 to 1,242.7 miles) above Earth; and geosynchronous Earth orbit (GEO) at 35,786 kilometers (22,300 miles), sometimes called the Clarke orbit , where the orbital period of a satellite is one day. More than 100,000 particles between 1 and 10 centimeters (0.39 and 3.94 inches) are thought to exist and probably tens of millions of times smaller than that can be found in space. By mass, there are more than 4 million kilograms (4,409.2 tons) in orbit.
The U.S. Air Force and Navy operate a network of radar sensors all over the world that can observe objects in space. These observations are combined to produce mathematical orbits that are maintained at U.S. Space Command as the Space Surveillance Catalog (SSC). Objects in LEO as small as 10 centimeters (3.94 inches) can be reliably tracked. GEO objects are harder to track because of their high altitude. Telescopes are used to observe GEO objects, and those observations are converted into orbits that are included in the SSC.
From 10 centimeters down to about 3 millimeters (0.12 inches), powerful ground radars like the Massachusetts Institute of Technology Haystack radar in Westford, Massachusetts are used to statistically sample the debris population. Analysis of impact craters on returned spacecraft surfaces, such as those from the Long Duration Exposure Facility, produce data concerning very small particles, those under 0.5 millimeters (0.02 inches) in size.
Orbital debris can severely damage or destroy a spacecraft. Due to the high average speed at impact, about 10 kilometers (6.21 miles) per second, a 3-millimeter (0.12 inch) fragment could penetrate the walls of a pressurized spacecraft. An unmanned satellite could be disabled by debris smaller than 1 millimeter (0.04 inches) if such particles were to disable critical power or data cables. The International Space Station (ISS) carries a variety of shields to protect it against space debris up to 1 centimeter in size. Debris objects larger than 10 centimeters will be avoided using orbital information from U.S. Space Command. Too large to shield against and too small to track with radar, objects of 1 to 10 centimeters pose a risk to the ISS that, while small, cannot be eliminated.
Because satellites can stay in orbit for more than 10,000 years, care must be taken in the world's policies concerning orbital debris. The Inter-Agency Space Debris Coordination Committee, composed of representatives from the world's leading space agencies, has developed agreements that minimize the creation of new debris. Most countries deplete their spent rocket bodies and payloads of stored energy, thereby decreasing their possibility of exploding and minimizing the largest historical source of debris. Other topics, such as forced de-orbiting of satellites, continue to be discussed in an international effort to control space debris.
see also Long Duration Exposure Facility (LDEF) (volume 2).
Jeffrey R. Theall
National Research Council. Orbital Debris, A Technical Assessment. Washington, DC:National Academy Press, 1995.
Orbital Debris Research at JSC. NASA Johnson Space Center. <http://sn-callisto.jsc.nasa.gov/>.