Space debris or orbital debris, also called space junk and space waste, are the objects in orbit around Earth created by humans, that no longer serve any useful purpose. They consist of everything from entire spent rocket stages and defunct satellites to explosion fragments, paint flakes, dust, and slag from solid rocket motors, coolant released by RORSAT nuclear powered satellites, deliberate insertion of small needles, and other small particles. Clouds of very small particles may cause erosive damage, like sandblasting.
Space debris has become a growing concern in recent years, since collisions at orbital velocities can be highly damaging to functioning satellites and can also produce even more space debris in the process, called Kessler Syndrome. Some spacecraft, like the International Space Station, are now armored to mitigate damage with this hazard. Astronauts on EVAs are also vulnerable.
According to Edward Tufte's book Envisioning Information, space debris objects have included a glove lost by astronaut Ed White on the first American space-walk, a camera Michael Collins lost near the spacecraft Gemini 10, garbage bags jettisoned by the Soviet Mir Cosmonauts throughout that space station's 15-year life, a wrench and a toothbrush. Sunita Williams of STS-116 also lost a camera during extra-vehicular activity (EVA). During the EVA to reinforce a torn solar panel during STS-120 a pair of pliers was similarly liberated. Most of those unusual objects have re-entered the atmosphere of the Earth within weeks due to the orbits where they were released. Things like these are not major contributors to the space debris environment. On the other side, explosion events are a major contribution to the space debris problem. About 100 tons of fragments generated during approximately 200 such events are still in orbit. Space debris is most concentrated in low Earth orbit, though some extends out past geosynchronous orbit.
In 2006, wreckage from a Russian spy satellite passed dangerously close to a Latin American Airbus carrying 270 passengers, reentering over the Pacific Ocean which is considered among the safest places in the world to bring down satellites due to its unpopulated vastness.
The worst uncontrolled reentry in history occurred in July 1979, when Skylab, America's abandoned, 78-ton space station - which had long since run out of maneuvering fuel - came down earlier than planned, raining debris across the Australian outback.
A hazard analysis conducted for a planned October 2008 mission of the NASA space shuttle Atlantis concluded that its greatest risk was from space debris, with a 1-in-185 chance of catastrophic impact. This level of risk will require a top-level launch decision. A typical space shuttle mission, to the International Space Station at 200 nautical mile altitude, involves a 1-in-300 risk, but the October 2008 mission is to the Hubble Space Telescope, which orbits at 300 nm altitude, where there is more debris. If the mission proceeds, planned mitigating measures include flying the shuttle tail-first, placing the main engines as the first contact with debris.
In order to mitigate the generation of additional space debris, a number of measures have been proposed: The passivation of spent upper stages by the release of residual fuels is aimed at decreasing the risk of on-orbit explosions that could generate thousands of additional debris objects.
Taking satellites out of orbit at the end of their operational life would also be an effective mitigation measure. This could be facilitated with a "terminator tether," an electrodynamic tether that is rolled out, and slows down the spacecraft. In cases when a direct (and controlled) de-orbit would require too much fuel the satellite can also be brought to an orbit where atmospheric drag would cause it to de-orbit after some years. Such a maneuver was successfully performed with the French Spot-1 satellite at the end of 2003. It will re-enter in approximately 15 years.
In orbital altitudes where it would not be economically feasible to de-orbit a satellite, like in the geostationary ring they are brought to a graveyard orbit where no operational satellites are present.
Proposals have been made for ways to "sweep" space debris back into Earth's atmosphere, including automated tugs, laser brooms to vaporize or nudge particles into rapidly-decaying orbits, or huge aerogel blobs to absorb impacting junk and eventually fall out of orbit with them trapped inside. However, currently most effort is being devoted to prevention of collisions by keeping track of larger debris, and prevention of more debris.
The U.S. Strategic Command maintains a catalogue currently containing about 13,000 objects, in part to prevent misinterpretation as hostile missiles. Observation data gathered by a number of ground based radar facilities and telescopes as well as by a space based telescope is used to maintain this catalogue. Nevertheless, the majority of debris objects remain unobserved. There are more than 600,000 objects larger than 1 cm in orbit (according to the ESA Meteoroid and Space Debris Terrestrial Environment Reference, the MASTER-2005 model).
Other sources of knowledge on the actual space debris environment include measurement campaigns by the ESA Space Debris Telescope, TIRA, Goldstone radar, Haystack radar, and the Cobra Dane phased array radar. The data gathered during these campaigns is used to validate models of the debris environment like ESA-MASTER. Such models are the only means of assessing the impact risk caused by space debris as only larger objects can be regularly tracked.
Returned space debris hardware is also a valuable source of information on the (submillimetre) space debris environment. The LDEF satellite deployed by STS-41-C Challenger and retrieved by STS-32 Columbia spent 68 months in orbit. The close examination of its surfaces allowed the analysis of the directional distribution and the composition of debris flux. The EURECA satellite deployed by STS-46 Atlantis in 1992 and retrieved by STS-57 Endeavour in 1993 could provide additional insight.
The solar arrays of the Hubble Space Telescope returned during missions STS-61 Endeavour and STS-109 Columbia are an important source of information on the debris environment. The impact craters found on the surface were counted and classified by ESA to provide another means for validating debris environment models.
The largest space debris incident in history was the Chinese anti-satellite weapon (ASAT) test on January 11, 2007. The event was estimated to have created more than 2300 pieces (updated 12/13/07) of trackable debris (approximately golf ball size or larger), over 35,000 pieces 1cm or larger, and 1 million pieces 1mm or larger. The debris event is more significant than previous ASAT tests in that the debris field is in a higher orbital plane resulting in deorbit times of 35 years and greater. In June 2007, NASA's Terra environmental spacecraft was the first to be moved in order to prevent impacts from this debris.
An event of similar magnitude occurred on February 19, 2007, when a Russian Briz-M booster stage exploded in orbit over Australia. The booster had been launched on February 28, 2006, carrying an Arabsat-4A communication satellite but malfunctioned before it could use all of its fuel. The explosion was captured on film by several astronomers, but due to the path of the orbit the debris cloud has been hard to quantify using radar. Although similar in magnitude, the debris field is at a lower altitude than the Chinese ASAT test and much debris will re-enter the atmosphere in a relatively short time. As of February 21, 2007, over 1,000 fragments had been identified. A third breakup event also occurred on 14 February 2007 as recorded by Celes Trak. This makes three observed events in the first two months of 2007. In 2006, the most breakups occurred since 1993 with eight breakups.
Additionally on February 20th, 2008, the U.S. launched an SM-3 Missile from the USS Lake Erie specially designed to destroy a defective U.S. spy satellite feared to carry 1,000 pounds of toxic hydrazine fuel. The debris created by this event occurring at about 250 km altitude results in all the debris having a perigee of 250 km or lower. Although the apogee of some debris may be higher due to the explosion, the low perigee altitude will cause all debris to re-enter the atmosphere in a relatively short time period.
Only one person has ever been recorded hit by manmade space debris: in 1997 an Oklahoma woman was hit in the shoulder by a 10 x 13 cm piece of blackened, woven metallic material that was later confirmed to be part of the fuel tank of a Delta II rocket which had launched a U.S. Air Force satellite in 1996. She was not injured.