Space debris

During the 1980s, NASA and other U.S. groups attempted to limit the growth of debris. One trial solution was implemented by McDonnell Douglas for the Delta launch vehicle, by having the booster move away from its payload and vent any propellant remaining in its tanks. This eliminated one source for pressure buildup in the tanks which had previously caused them to explode and create additional orbital debris.[18] Other countries were slower to adopt this measure and, due especially to a number of launches by the Soviet Union, the problem grew throughout the decade.[19]

A new battery of studies followed as NASA, NORAD and others attempted to better understand the orbital environment, with each adjusting the number of pieces of debris in the critical-mass zone upward. Although in 1981 (when Schefter's article was published) the number of objects was estimated at 5,000,[14] new detectors in the Ground-based Electro-Optical Deep Space Surveillance system found new objects. By the late 1990s, it was thought that most of the 28,000 launched objects had already decayed and about 8,500 remained in orbit.[20] By 2005 this was adjusted upward to 13,000 objects,[21] and a 2006 study increased the number to 19,000 as a result of an ASAT test and a satellite collision.[22] In 2011, NASA said that 22,000 objects were being tracked.[23]

A 2006 NASA model suggested that if no new launches took place the environment would retain the then-known population until about 2055, when it would increase on its own.[24][25] Richard Crowther of Britain's Defence Evaluation and Research Agency said in 2002 that he believed the cascade would begin about 2015.[26] The National Academy of Sciences, summarizing the professional view, noted widespread agreement that two bands of LEO space—900 to 1,000 km (620 mi) and 1,500 km (930 mi)—were already past critical density.[27]

In the 2009 European Air and Space Conference, University of Southampton researcher Hugh Lewis predicted that the threat from space debris would rise 50 percent in the next decade and quadruple in the next 50 years. As of 2009, more than 13,000 close calls were tracked weekly.[28]

A 2011 report by the U.S. National Research Council warned NASA that the amount of orbiting space debris was at a critical level. According to some computer models, the amount of space debris "has reached a tipping point, with enough currently in orbit to continually collide and create even more debris, raising the risk of spacecraft failures". The report called for international regulations limiting debris and research of disposal methods.[29]

By the late 2010s, plans by multiple providers to deploy large megaconstellations of broadband internet satellites had been licensed by regulatory authorities, with operational satellites entering production by both OneWeb and SpaceX. The first deployments occurred in 2019 with six from OneWeb, followed by 60 227 kg (500 lb) satellites from SpaceX in May, the first satellites for the project Starlink.[30] While the increased satellite density causes concerns, both licensing authorities and the manufacturers are well aware of debris problems. The vendors must have debris-reduction plans, and are taking measures to actively de-orbit unneeded satellites and/or ensure their orbits will decay naturally.[31]

• As of 2009, 19,000 debris over 5 cm (2 in) were tracked.[11]
• As of July 2013, estimates of more than 170 million debris smaller than 1 cm (0.4 in), about 670,000 debris 1–10 cm, and approximately 29,000 larger pieces of debris are in orbit.[32]
• As of July 2016, nearly 18,000 artificial objects are orbiting above Earth,[33] including 1,419 operational satellites.[34]
• As of October 2019, nearly 20,000 artificial objects in orbit above the Earth,[7] including 2,218 operational satellites.[8]

There are estimated to be over 128 million pieces of debris smaller than 1 cm (0.39 in) as of January 2019. There are approximately 900,000 pieces from one to ten cm. The current count of large debris (defined as 10 cm across or larger[35]) is 34,000.[9] The technical measurement cutoff is c. 3 mm (0.12 in).[36] Over 98 percent of the 1,900 tons of debris in low Earth orbit as of 2002 was accounted for by about 1,500 objects, each over 100 kg (220 lb).[37] Total mass is mostly constant despite addition of many smaller objects, since they reenter the atmosphere sooner. There were "9,000 pieces of orbiting junk" identified in 2008, with an estimated mass of 5,500 t (12,100,000 lb).[38]

Space debris in LEO, with sizes exaggerated

In the orbits nearest to Earth—less than 2,000 km (1,200 mi) orbital altitude, referred to as low-Earth orbit (LEO)— there have traditionally been few "universal orbits" which keep a number of spacecraft in particular rings (in contrast to GEO, a single orbit that is widely used by over 500 satellites). This is beginning to change in 2019, and several companies have begun to deploy the early phases of satellite internet constellations, which will have many universal orbits in LEO with 30 to 50 satellites per orbital plane and altitude. Traditionally, the most populated LEO orbits have been a number of sun-synchronous satellites that keep a constant angle between the Sun and the orbital plane, making Earth observation easier with consistent sun angle and lighting. Sun-synchronous orbits are polar, meaning they cross over the polar regions. LEO satellites orbit in many planes, typically up to 15 times a day, causing frequent approaches between objects. The density of satellites—both active and derelict—is much higher in LEO.[39]

Orbits are affected by gravitational perturbations (which in LEO include unevenness of the Earth's gravitational field due to variations in the density of the planet), and collisions can occur from any direction. Impacts between orbiting satellites can occur at up to 16 km/s for a theoretical head-on impact; the closing speed could be twice the orbital speed. The 2009 satellite collision occurred at a closing speed of 11.7 km/s,[40] creating over 2000 large debris fragments.[41] These debris cross many other orbits and increase debris collision risk.

It is theorized that a sufficiently large collision of spacecraft could potentially lead to a cascade effect, or even make some particular low Earth orbits effectively unusable for long term use by orbiting satellites, a phenomenon known as the Kessler syndrome.[42] The theoretical effect is projected to be a theoretical runaway chain reaction of collisions that could occur, exponentially increasing the number and density of space debris in low-Earth orbit, and has been hypothesized to ensue beyond some critical density.[43]

Crewed space missions are mostly at 400 km (250 mi) altitude and below, where air drag helps clear zones of fragments. The upper atmosphere is not a fixed density at any particular orbital altitude; it varies as a result of atmospheric tides and expands or contracts over longer time periods as a result of space weather. These longer-term effects can increase drag at lower altitudes; the 1990s expansion was a factor in reduced debris density.[44] Another factor was fewer launches by Russia; the Soviet Union made most of their launches in the 1970s and 1980s.[45]^:7

At higher altitudes, where air drag is less significant, orbital decay takes longer. Slight atmospheric drag, lunar perturbations, Earth's gravity perturbations, solar wind and solar radiation pressure can gradually bring debris down to lower altitudes (where it decays), but at very high altitudes this may take millennia.[46] Although high-altitude orbits are less commonly used than LEO and the onset of the problem is slower, the numbers progress toward the critical threshold more quickly.[47]

Many communications satellites are in geostationary orbits (GEO), clustering over specific targets and sharing the same orbital path. Although velocities are low between GEO objects, when a satellite becomes derelict (such as Telstar 401) it assumes a geosynchronous orbit; its orbital inclination increases about .8° and its speed increases about 100 miles per hour (160 km/h) per year. Impact velocity peaks at about 1.5 km/s (0.93 mi/s). Orbital perturbations cause longitude drift of the inoperable spacecraft and precession of the orbital plane. Close approaches (within 50 meters) are estimated at one per year.[48] The collision debris pose less short-term risk than from an LEO collision, but the satellite would likely become inoperable. Large objects, such as solar-power satellites, are especially vulnerable to collisions.[49]

Although the ITU now requires proof a satellite can be moved out of its orbital slot at the end of its lifespan, studies suggest this is insufficient.[50] Since GEO orbit is too distant to accurately measure objects under 1 m (3 ft 3 in), the nature of the problem is not well known.[51] Satellites could be moved to empty spots in GEO, requiring less maneuvering and making it easier to predict future motion.[52] Satellites or boosters in other orbits, especially stranded in geostationary transfer orbit, are an additional concern due to their typically high crossing velocity.

Despite efforts to reduce risk, spacecraft collisions have occurred. The European Space Agency telecom satellite Olympus-1 was struck by a meteoroid on 11 August 1993 and eventually moved to a graveyard orbit.[53] On 29 March 2006, the Russian Express-AM11 communications satellite was struck by an unknown object and rendered inoperable;[54] its engineers had enough contact time with the satellite to send it into a graveyard orbit.

Vanguard 1 is expected to remain in orbit for 240 years.[55][56]

In 1958, the United States launched Vanguard I into a medium Earth orbit (MEO). As of October 2009, it, and the upper stage of its launch rocket, are the oldest surviving human-made space objects still in orbit.[57][58] In a catalog of known launches until July 2009, the Union of Concerned Scientists listed 902 operational satellites[59] from a known population of 19,000 large objects and about 30,000 objects launched.

An example of additional derelict satellite debris is the remains of the 1970s/80s Soviet RORSAT naval surveillance satellite program. The satellites' BES-5 nuclear reactors were cooled with a coolant loop of sodium-potassium alloy, creating a potential problem when the satellite reached end of life. While many satellites were nominally boosted into medium-altitude graveyard orbits, not all were. Even satellites which had been properly moved to a higher orbit had an eight-percent probability of puncture and coolant release over a 50-year period. The coolant freezes into droplets of solid sodium-potassium alloy,[60] forming additional debris.[61]

These events continue to occur. For example, in February 2015, the USAF Defense Meteorological Satellite Program Flight 13 (DMSP-F13) exploded on orbit, creating at least 149 debris objects, which were expected to remain in orbit for decades.[62]

Orbiting satellites have been deliberately destroyed. United States and USSR/Russia have conducted over 30 and 27 ASAT tests, respectively, followed by 10 from China and one from India. The most recent ASATs were Chinese interception of FY-1C, trials of Russian PL-19 Nudol, American interception of USA-193 and Indian interception of unstated live satellite.

A drifting thermal blanket photographed in 1998 during STS-88.

According to Edward Tufte's book Envisioning Information, space debris includes a glove lost by astronaut Ed White on the first American space-walk (EVA); a camera lost by Michael Collins near Gemini 10; a thermal blanket lost during STS-88; garbage bags jettisoned by Soviet cosmonauts during Mir's 15-year life,[57] a wrench and a toothbrush. Sunita Williams of STS-116 lost a camera during an EVA. During an STS-120 EVA to reinforce a torn solar panel, a pair of pliers was lost, and in an STS-126 EVA, Heidemarie Stefanyshyn-Piper lost a briefcase-sized tool bag.[63]

Spent upper stage of a Delta II rocket, photographed by the XSS 10 satellite

In characterizing the problem of space debris, it was learned that much debris was due to rocket upper stages (e.g. the Inertial Upper Stage) which end up in orbit, and break up due to decomposition of unvented unburned fuel.[64] However, a major known impact event involved an (intact) Ariane booster.[45]^:2 Although NASA and the United States Air Force now require upper-stage passivation, other launchers do not. Lower stages, like the Space Shuttle's solid rocket boosters or Apollo program's Saturn IB launch vehicles, do not reach orbit.[65]

On 11 March 2000 a Chinese Long March 4 CBERS-1 upper stage exploded in orbit, creating a debris cloud.[66][67] A Russian Briz-M booster stage exploded in orbit over South Australia on 19 February 2007. Launched on 28 February 2006 carrying an Arabsat-4A communications satellite, it malfunctioned before it could use up its propellant. Although the explosion was captured on film by astronomers, due to the orbit path the debris cloud has been difficult to measure with radar. By 21 February 2007, over 1,000 fragments were identified.[68][69] A 14 February 2007 breakup was recorded by Celestrak.[70] Eight breakups occurred in 2006, the most since 1993.[71] Another Briz-M broke up on 16 October 2012 after a failed 6 August Proton-M launch. The amount and size of the debris was unknown.[72] A Long March 7 rocket booster created a fireball visible from portions of Utah, Nevada, Colorado, Idaho and California on the evening of 27 July 2016; its disintegration was widely reported on social media.[73] In 2018-2019, three different Atlas V Centaur second stages have broken up.[74][75][76]

A past debris source was the testing of anti-satellite weapons (ASATs) by the U.S. and Soviet Union during the 1960s and 1970s. North American Aerospace Defense Command (NORAD) files only contained data for Soviet tests, and debris from U.S. tests were only identified later.[77] By the time the debris problem was understood, widespread ASAT testing had ended; the U.S. Program 437 was shut down in 1975.[78]

The U.S. restarted their ASAT programs in the 1980s with the Vought ASM-135 ASAT. A 1985 test destroyed a 1-tonne (2,200 lb) satellite orbiting at 525 km (326 mi), creating thousands of debris larger than 1 cm (0.39 in). Due to the altitude, atmospheric drag decayed the orbit of most debris within a decade. A de facto moratorium followed the test.[79]

Known orbit planes of Fengyun-1C debris one month after the weather satellite's disintegration by the Chinese ASAT

China's government was condemned for the military implications and the amount of debris from the 2007 anti-satellite missile test,[80] the largest single space debris incident in history (creating over 2,300 pieces golf-ball size or larger, over 35,000 1 cm (0.4 in) or larger, and one million pieces 1 mm (0.04 in) or larger). The target satellite orbited between 850 km (530 mi) and 882 km (548 mi), the portion of near-Earth space most densely populated with satellites.[81] Since atmospheric drag is low at that altitude the debris is slow to return to Earth, and in June 2007 NASA's Terra environmental spacecraft maneuvered to avoid impact from the debris.[82]

On 20 February 2008, the U.S. launched an SM-3 missile from the USS Lake Erie to destroy a defective U.S. spy satellite thought to be carrying 450 kg (1,000 lb) of toxic hydrazine propellant. The event occurred at about 250 km (155 mi), and the resulting debris has a perigee of 250 km (155 mi) or lower.[83] The missile was aimed to minimize the amount of debris, which (according to Pentagon Strategic Command chief Kevin Chilton) had decayed by early 2009.[84]

On 27 March 2019, Indian Prime Minister Narendra Modi announced that India shot down one of its own LEO satellites with a ground-based missile. He stated that the operation, part of Mission Shakti, would defend the country's interests in space. Afterwards, US Air Force Space Command announced they were tracking 270 new pieces of debris but expected the number to grow as data collection continues.[85]

The vulnerability of satellites to debris and the possibility of attacking LEO satellites to create debris clouds has triggered speculation that it is possible for countries unable to make a precision attack. An attack on a satellite of 10 tonnes or more would heavily damage the LEO environment.[79]

Space junk can be a hazard to active satellites and spacecraft. It has been theorized that Earth orbit could even become impassable if the risk of collision grows too high.[86]

However, since the risk to spacecraft increases with the time of exposure to high debris densities, it is more accurate to say that LEO would be rendered unusable by orbiting craft. The threat to craft passing through LEO to reach higher orbit would be much lower owing to the very short time span of the crossing.

Although spacecraft are typically protected by Whipple shields, solar panels, which are exposed to the Sun, wear from low-mass impacts. Even small impacts can produce a cloud of plasma which is an electrical risk to the panels.[87]

Satellites are believed to have been destroyed by micrometeorites and (small) orbital debris (MMOD). The earliest suspected loss was of Kosmos 1275, which disappeared on 24 July 1981 (a month after launch). Kosmos contained no volatile propellant, therefore, there appeared to be nothing internal to the satellite which could have caused the destructive explosion which took place. However, the case has not been proven and another hypothesis forwarded is that the battery exploded. Tracking showed it broke up, into 300 new objects.

Many impacts have been confirmed since. For example, on 24 July 1996, the French microsatellite Cerise was hit by fragments of an Ariane-1 H-10 upper-stage booster which exploded in November 1986.[45]^:2 On 29 March 2006, the Russian Ekspress AM11 communications satellite was struck by an unknown object and rendered inoperable.[54] On October 13, 2009, Terra suffered a single battery cell failure anomaly and a battery heater control anomaly which were subsequently considered likely the result of an MMOD strike.[88] On March 12, 2010, Aura lost power from one-half of one of its 11 solar panels and this was also attributed to an MMOD strike.[89] On May 22, 2013 GOES-13 was hit by an MMOD which caused it to lose track of the stars that it used to maintain an operational attitude. It took nearly a month for the spacecraft to return to operation.[90]

The first major satellite collision occurred on 10 February 2009. The 950 kg (2,090 lb) derelict satellite Kosmos 2251 and the operational 560 kg (1,230 lb) Iridium 33 collided, 500 mi (800 km)[91] over northern Siberia. The relative speed of impact was about 11.7 km/s (7.3 mi/s), or about 42,120 km/h (26,170 mph).[92] Both satellites were destroyed, creating thousands of pieces of new smaller debris, with legal and political liability issues unresolved even years later.[93][94][95] On 22 January 2013 BLITS (a Russian laser-ranging satellite) was struck by debris suspected to be from the 2007 Chinese anti-satellite missile test, changing both its orbit and rotation rate.[96]

Satellites sometimes perform Collision Avoidance Maneuvers and satellite operators may monitor space debris as part of maneuver planning. For example, in January 2017, the European Space Agency made the decision to alter orbit of one of its three[97] Swarm mission spacecraft, based on data from the US Joint Space Operations Center, to lower the risk of collision from Cosmos-375, a derelict Russian satellite.[98]

Crewed flights are naturally particularly sensitive to the hazards that could be presented by space debris conjunctions in the orbital path of the spacecraft. Examples of occasional avoidance maneuvers, or longer-term space debris wear, have occurred in Space Shuttle missions, the MIR space station, and the International Space Station.

Space Shuttle Discovery's lower starboard wing and Thermal Protection System tiles, photographed on STS-114 during an R-Bar Pitch Manoeuvre where astronauts examine the TPS for any damage during ascent

From the early Space Shuttle missions, NASA used NORAD space monitoring capabilities to assess the Shuttle's orbital path for debris. In the 1980s, this used a large proportion of NORAD capacity.[18] The first collision-avoidance maneuver occurred during STS-48 in September 1991,[99] a seven-second thruster burn to avoid debris from the derelict satellite Kosmos 955.[100] Similar maneuvers were initiated on missions 53, 72 and 82.[99]

One of the earliest events to publicize the debris problem occurred on Space Shuttle Challenger's second flight, STS-7. A fleck of paint struck its front window, creating a pit over 1 mm (0.04 in) wide. On STS-59 in 1994, Endeavour's front window was pitted about half its depth. Minor debris impacts increased from 1998.[101]

Window chipping and minor damage to thermal protection system tiles (TPS) were already common by the 1990s. The Shuttle was later flown tail-first to take a greater proportion of the debris load on the engines and rear cargo bay, which are not used in orbit or during descent, and thus are less critical for post-launch operation. When flying attached to the ISS, the two connected spacecraft were flipped around so the better-armored station shielded the orbiter.[102]

Space Shuttle Endeavour had a major impact on its radiator during STS-118. The entry hole is about 5.5 mm (0.22 in), and the exit hole is twice as large.

A NASA 2009 study concluded that debris accounted for approximately half of the overall risk to the Shuttle.[102][103] Executive-level decision to proceed was required if catastrophic impact was likelier than 1 in 200. On a normal (low-orbit) mission to the ISS the risk was approximately 1 in 300, but the Hubble telescope repair mission was flown at the higher orbital altitude of 560 km (350 mi) where the risk was initially calculated at a 1-in-185 (due in part to the 2009 satellite collision). A re-analysis with better debris numbers reduced the estimated risk to 1 in 221, and the mission went ahead.[104]

Debris incidents continued on later Shuttle missions. During STS-115 in 2006 a fragment of circuit board bored a small hole through the radiator panels in Atlantis's cargo bay.[105] On STS-118 in 2007 debris blew a bullet-like hole through Endeavour's radiator panel.[106]

Impact wear was notable on Mir, the Soviet space station, since it remained in space for long periods with its original solar module panels.[107][108]

Debris impacts on Mir's solar panels degraded their performance. The damage is most noticeable on the panel on the right, which is facing the camera with a high degree of contrast. Extensive damage to the smaller panel below is due to impact with a Progress spacecraft.

The ISS also uses Whipple shielding to protect its interior from minor debris.[109] However, exterior portions (notably its solar panels) cannot be protected easily. In 1989, the ISS panels were predicted to degrade approximately 0.23% in four years due to the "sandblasting" effect of impacts with small orbital debris.[110] An avoidance maneuver is typically performed for the ISS if "there is a greater than one-in-10,000 chance of a debris strike".[111] As of January 2014, there have been sixteen maneuvers in the fifteen years the ISS had been in orbit.[111]

As another method to reduce the risk to humans on board, ISS operational management asked the crew to shelter in the Soyuz on three occasions due to late debris-proximity warnings. In addition to the sixteen thruster firings and three Soyuz-capsule shelter orders, one attempted maneuver was not completed due to not having the several days' warning necessary to upload the maneuver timeline to the station's computer.[111][112][113] A March 2009 event involved debris believed to be a 10 cm (3.9 in) piece of the Kosmos 1275 satellite.[114] In 2013, the ISS operations management did not make a maneuver to avoid any debris, after making a record four debris maneuvers the previous year.[111]

The Kessler syndrome,[115][116] proposed by NASA scientist Donald J. Kessler in 1978, is a theoretical scenario in which the density of objects in low Earth orbit (LEO) is high enough that collisions between objects could cause a cascade effect where each collision generates space debris that increases the likelihood of further collisions.[117] He further theorized that one implication if this were to occur is that the distribution of debris in orbit could render space activities and the use of satellites in specific orbital ranges economically impractical for many generations.[117]

The growth in the number of objects as a result of the late-1990s studies sparked debate in the space community on the nature of the problem and the earlier dire warnings. According to Kessler's 1991 derivation and 2001 updates,[118] the LEO environment in the 1,000 km (620 mi) altitude range should be cascading. However, only one major satellite collision incident has occurred: the 2009 satellite collision between Iridium 33 and Cosmos 2251. The lack of obvious short-term cascading has led to speculation that the original estimates overstated the problem.[119] According to Kessler in 2010 however, a cascade may not be obvious until it is well advanced, which might take years.[120]

Saudi officials inspect a crashed PAM-D module in January 2001.

Although most debris burns up in the atmosphere, larger debris objects can reach the ground intact. According to NASA, an average of one cataloged piece of debris has fallen back to Earth each day for the past 50 years. Despite their size, there has been no significant property damage from the debris.[121]

Notable examples of space junk falling to Earth and impacting human life include:

• 1969: five sailors on a Japanese ship were injured by space debris.[122]
• 1997: an Oklahoma woman, Lottie Williams, was injured when she was hit in the shoulder by a 10 cm × 13 cm (3.9 in × 5.1 in) piece of blackened, woven metallic material confirmed as part of the propellant tank of a Delta II rocket which launched a U.S. Air Force satellite the year before.[123][124]
• 2001: a Star 48 Payload Assist Module (PAM-D) rocket upper stage re-entered the atmosphere after a "catastrophic orbital decay",[125] crashing in the Saudi Arabian desert. It was identified as the upper-stage rocket for NAVSTAR 32, a GPS satellite launched in 1993.
• 2003: Columbia disaster, large parts of the spacecraft reached the ground and entire equipment systems remained intact.[126] More than 83,000 pieces, along with the remains of the six astronauts, were recovered in an area from three to 10 miles around Hemphill in Sabine County, Texas.[127] More pieces were found in a line from west Texas to east Louisiana, with the westernmost piece found in Littlefield, TX and the easternmost found southwest of Mora, Louisiana.[128] Debris was found in Texas, Arkansas and Louisiana. In a rare case of property damage, a foot-long metal bracket smashed through the roof of a dentist office.[129] NASA warned the public to avoid contact with the debris because of the possible presence of hazardous chemicals.[130] 15 years after the failure, people were still sending in pieces with the most recent, as of February 2018, found in the spring of 2017.[131]
• 2007: airborne debris from a Russian spy satellite was seen by the pilot of a LAN Airlines Airbus A340 carrying 270 passengers whilst flying over the Pacific Ocean between Santiago and Auckland. The debris was reported within 9.3 kilometres (5 nmi) of the aircraft.[132]

Radar and optical detectors such as lidar are the main tools for tracking space debris. Although objects under 10 cm (4 in) have reduced orbital stability, debris as small as 1 cm can be tracked,[133][134] however determining orbits to allow re-acquisition is difficult. Most debris remain unobserved. The NASA Orbital Debris Observatory tracked space debris with a 3 m (10 ft) liquid mirror transit telescope.[135] FM Radio waves can detect debris, after reflecting off them onto a receiver.[136] Optical tracking may be a useful early-warning system on spacecraft.[137]

The U.S. Strategic Command keeps a catalog of known orbital objects, using ground-based radar and telescopes, and a space-based telescope (originally to distinguish from hostile missiles). The 2009 edition listed about 19,000 objects.[138] Other data come from the ESA Space Debris Telescope, TIRA,[139] the Goldstone, Haystack,[140] and EISCAT radars and the Cobra Dane phased array radar,[141] to be used in debris-environment models like the ESA Meteoroid and Space Debris Terrestrial Environment Reference (MASTER).

The Long Duration Exposure Facility (LDEF) is an important source of information on small-particle space debris.

Returned space hardware is a valuable source of information on the directional distribution and composition of the (sub-millimetre) debris flux. The LDEF satellite deployed by mission STS-41-C Challenger and retrieved by STS-32 Columbia spent 68 months in orbit to gather debris data. The EURECA satellite, deployed by STS-46 Atlantis in 1992 and retrieved by STS-57 Endeavour in 1993, was also used for debris study.[142]

The solar arrays of Hubble were returned by missions STS-61 Endeavour and STS-109 Columbia, and the impact craters studied by the ESA to validate its models. Materials returned from Mir were also studied, notably the Mir Environmental Effects Payload (which also tested materials intended for the ISS[143]).[144][145]

A debris cloud resulting from a single event is studied with scatter plots known as Gabbard diagrams, where the perigee and apogee of fragments are plotted with respect to their orbital period. Gabbard diagrams of the early debris cloud prior to the effects of perturbations, if the data were available, are reconstructed. They often include data on newly observed, as yet uncatalogued fragments. Gabbard diagrams can provide important insights into the features of the fragmentation, the direction and point of impact.[15][146]

Spatial density of LEO space debris by altitude, according to 2011 a NASA report to the United Nations Office for Outer Space Affairs[150]

Spatial density of space debris by altitude according to ESA MASTER-2001, without debris from the Chinese ASAT and 2009 collision events

A variety of technical approaches to the mitigation of the growth of space debris are typically undertaken as of the 2010s. To avoid excessive creation of artificial space debris, many—but not all—satellites are launched initially into elliptical orbits with perigees inside Earth's atmosphere so the orbit will quickly decay and the satellites then will destroy themselves upon reentry into the atmosphere. Other methods are used for spacecraft in higher orbits. These include passivation of the spacecraft at the end of its useful life; as well as use of upper stages that can reignite to decelerate the stage to intentionally deorbit it, often on the first or second orbit following payload release; satellites that can, if they remain healthy for years, deorbit themselves from the lower orbits around Earth. Other satellites (such as many CubeSats) in low orbits below approximately 400 km orbital altitude depend on the energy-absorbing effects of the upper atmosphere to reliably deorbit a spacecraft within weeks or months.

Increasingly, spent upper stages in higher orbits—orbits for which low-delta-v deorbit is not possible, or not planned for—and architectures that support satellite passivation, at end of life are passivated at end of life. This removes any internal energy contained in the vehicle at the end of its mission or useful life. While this does not remove the debris of the now derelict rocket stage or satellite itself, it does substantially reduce the likelihood of the spacecraft destructing and creating many smaller pieces of space debris, a phenomenon that was common in many of the early generations of US and Soviet[61] spacecraft.

Upper stage passivation (e.g. of Delta boosters[18]) by releasing residual propellants reduces debris from orbital explosions; however even as late as 2011, not all upper stages implement this practice.[151] SpaceX used the term "propulsive passivation" for the final maneuver of their six-hour demonstration mission (STP-2) of the Falcon 9 second stage for the US Air Force in 2019, but did not define what all that term encompassed.[152]

When originally proposed in 2015, the OneWeb constellation, initially planned to have ~700 satellites anticipated on orbit after 2018, would only state that they would re-enter the atmosphere within 25 years of retirement,[153] which would comply with the Orbital Debris Mitigation Standard Practices (ODMSP) issued by the US government in 2001.[154] By October 2017, both OneWeb—and also SpaceX, with their large Starlink constellation—had filed documents with the US FCC with more aggressive space debris mitigation plans. Both companies committed to a deorbit plan for post-mission satellites which will explicitly move the satellites into orbits where they will reenter the Earth's atmosphere within approximately one year following end-of-life.[155]

With a "one-up, one-down" launch-license policy for Earth orbits, launchers would rendezvous with, capture and de-orbit a derelict satellite from approximately the same orbital plane.[156] Another possibility is the robotic refueling of satellites. Experiments have been flown by NASA,[157] and SpaceX is developing large-scale on-orbit propellant transfer technology.[158]

Another approach to debris mitigation is to explicitly design the mission architecture to always leave the rocket second-stage in an elliptical geocentric orbit with a low-perigee, thus ensuring rapid orbital decay and avoiding long-term orbital debris from spent rocket bodies. Such missions will often complete the payload placement in a final orbit by the use of low-thrust electric propulsion or with the use of a small kick stage to circularize the orbit. The kick stage itself may be designed with the excess-propellant capability to be able to self-deorbit.[159]

Although the ITU requires geostationary satellites to move to a graveyard orbit at the end of their lives, the selected orbital areas do not sufficiently protect GEO lanes from debris.[50] Rocket stages (or satellites) with enough propellant may make a direct, controlled de-orbit, or if this would require too much propellant, a satellite may be brought to an orbit where atmospheric drag would cause it to eventually de-orbit. This was done with the French Spot-1 satellite, reducing its atmospheric re-entry time from a projected 200 years to about 15 by lowering its altitude from 830 km (516 mi) to about 550 km (342 mi).[160][161]

The Iridium constellation—95 communication satellites launched during the five-year period between 1997 and 2002—provides a set of data points on the limits of self-removal. The satellite operator—Iridium Communications—remained operational (albeit with a company name change through a corporate bankruptcy during the period) over the two-decade life of the satellites, and by December 2019, had "completed disposal of the last of its 65 working legacy satellites."[162] However, this process left nearly one-third of the mass of this constellation (30 satellites, 20,400 kg (45,000 lb) of materiel) in LEO orbits at approximately 700 km (430 mi) altitude, where self-decay is quite slow. 29 of these satellites simply failed during their time in orbit and were thus unable to self-deorbit, while one—Iridium 33—was involved in the 2009 satellite collision with the derelict Russian military Kosmos-2251 satellite.[162] No "Plan B" provision was designed in for removal of the satellites that were unable to remove themselves. However, in 2019, Iridium CEO Matt Desch said that Iridium would be willing to pay an active-debris-removal company to deorbit its remaining first-generation satellites if it were possible for a sufficiently low cost, say "US$10,000 per deorbit, but [he] acknowledged that price would likely be far below what a debris-removal company could realistically offer. 'You know at what point [it’s] a no-brainer, but [I] expect the cost is really in the millions or tens of millions, at which price I know it doesn’t make sense'"[162]

Passive methods of increasing the orbital decay rate of spacecraft debris have been proposed. Instead of rockets, an electrodynamic tether could be attached to a spacecraft at launch; at the end of its lifetime, the tether would be rolled out to slow the spacecraft.[163] Other proposals include a booster stage with a sail-like attachment[164] and a large, thin, inflatable balloon envelope.[165]

A variety of approaches have been proposed, studied, or had ground subsystems built to use other spacecraft to remove existing space debris. A consensus of speakers at a meeting in Brussels in October 2012, organized by the Secure World Foundation (a U.S. think tank) and the French International Relations Institute,[166] reported that removal of the largest debris would be required to prevent the risk to spacecraft becoming unacceptable in the foreseeable future (without any addition to the inventory of dead spacecraft in LEO). To date in 2019, removal costs and legal questions about ownership and the authority to remove defunct satellites have stymied national or international action. Current space law retains ownership of all satellites with their original operators, even debris or spacecraft which are defunct or threaten active missions.

Moreover, as of 2006, the cost of any of the proposed approaches for external removal is about the same as launching a spacecraft and, according to NASA's Nicholas Johnson, not cost-effective.[24]

This is beginning to change in the late 2010s, as some companies have made plans to begin to do external removal on their satellites in mid-LEO orbits. For example, OneWeb will utilize on-board self-removal as "plan A" for satellite deorbiting at the end of life, but if a satellite is unable to remove itself within one year of end of life, OneWeb will implement "plan B" and dispatch a reusable (multi-transport mission) space tug to attach to the satellite at an already built-in capture target via a grappling fixture, to be towed to a lower orbit and released for reentry.[167][168]

A well-studied solution uses a remotely controlled vehicle to rendezvous with, capture and return debris to a central station.[169] One such system is Space Infrastructure Servicing, a commercially developed refueling depot and service spacecraft for communications satellites in geosynchronous orbit originally scheduled for a 2015 launch.[170] The SIS would be able to "push dead satellites into graveyard orbits."[171] The Advanced Common Evolved Stage family of upper stages is being designed with a high leftover-propellant margin (for derelict capture and de-orbit) and in-space refueling capability for the high delta-v required to de-orbit heavy objects from geosynchronous orbit.[156] A tug-like satellite to drag debris to a safe altitude for it to burn up in the atmosphere has been researched.[172] When debris is identified the satellite creates a difference in potential between the debris and itself, then using its thrusters to move itself and the debris to a safer orbit.

A variation of this approach is for the remotely controlled vehicle to rendezvous with debris, capture it temporarily to attach a smaller de-orbit satellite and drag the debris with a tether to the desired location. The "mothership" would then tow the debris-smallsat combination for atmospheric entry or move it to a graveyard orbit. One such system is the proposed Busek ORbital DEbris Remover (ORDER), which would carry over 40 SUL (satellite on umbilical line) de-orbit satellites and propellant sufficient for their removal.[16]

Cleanspace One

On 7 January 2010 Star, Inc. reported that it received a contract from the Space and Naval Warfare Systems Command for a feasibility study of the ElectroDynamic Debris Eliminator (EDDE) propellantless spacecraft for space-debris removal.[173] In February 2012 the Swiss Space Center at École Polytechnique Fédérale de Lausanne announced the Clean Space One project, a nanosatellite demonstration project for matching orbit with a defunct Swiss nanosatellite, capturing it and de-orbiting together.[174] The mission has seen several evolutions to reach a pac-man inspired capture model.[175] In 2013, Space Sweeper with Sling-Sat (4S), a grappling satellite which captures and ejects debris was studied.[176]

In December 2019, the European Space Agency awarded the first contract to clean up space debris. The €120 million mission dubbed ClearSpace-1 (a spinoff from the EPFL project) is slated to launch in 2025. It aims to remove a 100 kg VEga Secondary Payload Adapter (Vespa)[177] left by Vega flight VV02 in an 800 km orbit in 2013. A "chaser" will grab the junk with four robotic arms and drag it down to Earth's atmosphere where both will burn up.[178]

The laser broom uses a ground-based laser to ablate the front of the debris, producing a rocket-like thrust which slows the object. With continued application, the debris would fall enough to be influenced by atmospheric drag.[179][180] During the late 1990s, the U.S. Air Force's Project Orion was a laser-broom design.[181] Although a test-bed device was scheduled to launch on a Space Shuttle in 2003, international agreements banning powerful laser testing in orbit limited its use to measurements.[182] The Space Shuttle Columbia disaster postponed the project and according to Nicholas Johnson, chief scientist and program manager for NASA's Orbital Debris Program Office, "There are lots of little gotchas in the Orion final report. There's a reason why it's been sitting on the shelf for more than a decade."[183]

The momentum of the laser-beam photons could directly impart a thrust on the debris sufficient to move small debris into new orbits out of the way of working satellites. NASA research in 2011 indicates that firing a laser beam at a piece of space junk could impart an impulse of 1 mm (0.039 in) per second, and keeping the laser on the debris for a few hours per day could alter its course by 200 m (660 ft) per day.[184] One drawback is the potential for material degradation; the energy may break up the debris, adding to the problem. A similar proposal places the laser on a satellite in Sun-synchronous orbit, using a pulsed beam to push satellites into lower orbits to accelerate their reentry.[16] A proposal to replace the laser with an Ion Beam Shepherd has been made,[185] and other proposals use a foamy ball of aerogel or a spray of water,[186] inflatable balloons,[187] electrodynamic tethers,[188] boom electroadhesion,[189] and dedicated anti-satellite weapons.[190]

On 28 February 2014, Japan's Japan Aerospace Exploration Agency (JAXA) launched a test "space net" satellite. The launch was an operational test only.[191] In December 2016 the country sent a space junk collector via Kounotori 6 to the ISS by which JAXA scientists experiment to pull junk out of orbit using a tether.[192][193] The system failed to extend a 700-meter tether from a space station resupply vehicle that was returning to Earth.[194][195] On 6 February the mission was declared a failure and leading researcher Koichi Inoue told reporters that they "believe the tether did not get released".[196]

Since 2012, the European Space Agency has been working on the design of a mission to remove large space debris from orbit. The mission, e.Deorbit, is scheduled for launch during 2023 with an objective to remove debris heavier than 4,000 kilograms (8,800 lb) from LEO.[197] Several capture techniques are being studied, including a net, a harpoon and a combination robot arm and clamping mechanism.[198]

The RemoveDEBRIS mission plan is to test the efficacy of several ADR technologies on mock targets in low Earth orbit. In order to complete its planned experiments the platform is equipped with a net, a harpoon, a laser ranging instrument, a dragsail, and two CubeSats (miniature research satellites).[199] The mission was launched on 2 April 2018.

There is no international treaty minimizing space debris. However, the United Nations Committee on the Peaceful Uses of Outer Space (COPUOS) published voluntary guidelines in 2007,[200] using a variety of earlier national regulatory attempts at developing standards for debris mitigation. As of 2008, the committee was discussing international "rules of the road" to prevent collisions between satellites.[201] By 2013, a number of national legal regimes existed,[202][203][204] typically instantiated in the launch licenses that are required for a launch in all spacefaring nations.[205]

The U.S. issued a set of standard practices for civilian (NASA) and military (DoD and USAF) orbital-debris mitigation in 2001.[154][206][203] The standard envisioned disposal for final mission orbits in one of three ways: 1) atmospheric reentry where even with "conservative projections for solar activity, atmospheric drag will limit the lifetime to no longer than 25 years after completion of mission;" 2) maneuver to a "storage orbit:" move the spacecraft to one of four very broad parking orbit ranges (2,000–19,700 km (1,200–12,200 mi), 20,700–35,300 km (12,900–21,900 mi), above 36,100 km (22,400 mi), or out of Earth orbit completely and into any heliocentric orbit; 3) "Direct retrieval: Retrieve the structure and remove it from orbit as soon as practicable after completion of mission."[202] The standard articulated in option 1, which is the standard applicable to most satellites and derelict upper stages launched, has come to be known as the "25-year rule."[207] The US updated the ODMSP in December 2019, but made no change to the 25-year rule even though "[m]any in the space community believe that the timeframe should be less than 25 years."[154] There is no consensus however on what any new timeframe might be.[154]

In 2002, the European Space Agency (ESA) worked with an international group to promulgate a similar set of standards, also with a "25-year rule" applying to most Earth-orbit satellites and upper stages. Space agencies in Europe began to develop technical guidelines in the mid-1990s, and ASI, UKSA, CNES, DLR and ESA signed a "European Code of Conduct" in 2006,[204] which was a predecessor standard to the ISO international standard work that would begin the following year. In 2008, ESA further developed "its own "Requirements on Space Debris Mitigation for Agency Projects" which "came into force on 1 April 2008."[204] Germany and France have posted bonds to safeguard the property from debris damage.[208] The "direct retrieval" option (option no. 3 in the US "standard practices" above) has rarely been done by any spacefaring nation (exception, USAF X-37) or commercial actor since the earliest days of spaceflight due to the cost and complexity of achieving direct retrieval, but the ESA has scheduled a 2025 demonstration mission (Clearspace-1) to do this with a single small 100 kg (220 lb) derelict upper stage at a projected cost of €120 million not including the launch costs.[178]

By 2006, the Indian Space Research Organization (ISRO) had developed a number of technical means of debris mitigation (upper stage passivation, propellant reserves for movement to graveyard orbits, etc.) for ISRO launch vehicles and satellites, and was actively contributing to inter-agency debris coordination and the efforts of the UN COPUOS committee.[209]

In 2007, the ISO began preparing an international standard for space-debris mitigation.[210] By 2010, ISO had published "a comprehensive set of space system engineering standards aimed at mitigating space debris. [with primary requirements] defined in the top-level standard, ISO 24113." By 2017, the standards were nearly complete. However, these standards are not binding on any party by ISO or any international jurisdiction. They are simply available for use in any of a variety of voluntary ways. They "can be adopted voluntarily by a spacecraft manufacturer or operator, or brought into effect through a commercial contract between a customer and supplier, or used as the basis for establishing a set of national regulations on space debris mitigation."[207] The voluntary ISO standard also adopted the "25-year rule" for the "LEO protected region" below 2000 km altitude that has been previously (and still is, as of 2019) used by the US, ESA, and UN mitigation standards, and identifies it as "an upper limit for the amount of time that a space system shall remain in orbit after its mission is completed. Ideally, the time to deorbit should be as short as possible (i.e. much shorter than 25 years)".[207]

Holger Krag of the European Space Agency states that as of 2017 there is no binding international regulatory framework with no progress occurring at the respective UN body in Vienna.[86]