Space debris, also known as orbital debris, space junk, and space waste, is the collection of objects in orbit around Earth that were created by humans but no longer serve any useful purpose. These objects consist of everything from spent rocket stages and defunct satellites to erosion, explosion and collision fragments. As the orbits of these objects often overlap the trajectories of newer objects, debris is a potential collision risk to operational spacecraft.
The vast majority of the estimated tens of millions of pieces of space debris are small particles, less than 1 centimetre (0.39 in). These include dust from solid rocket motors, surface degradation products such as paint flakes, and coolant released by RORSAT nuclear powered satellites. Impacts of these particles cause erosive damage, similar to sandblasting. This damage can be partly mitigated through the use of the "meteor bumper", which is widely used on spacecraft such as the International Space Station. However, not all parts of a spacecraft may be protected in this manner, e.g. solar panels and optical devices (such as telescopes, or star trackers), and these components are subject to constant wear by debris, and to a much lesser extent, micrometeorites.
A much smaller number of the debris objects are larger, over 10 centimetres (3.9 in). Against larger debris, the only protection is to maneuver the spacecraft in order to avoid a collision. If a collision with larger debris does occur, many of the resulting fragments from the damaged spacecraft will be in the 1 kilogram (2.2 lb) mass range, and these objects become an additional collision risk. As the chance of collision is a function of the number of objects in space, there is a critical density where the creation of new debris occurs faster than the various natural forces remove these objects from orbit. Beyond this point a runaway chain reaction can occur that reduces all objects in orbit to debris in a period of years or months. This possibility is known as the "Kessler Syndrome", and there is debate as to whether or not this critical density has already been reached in certain orbital bands.[1]
A runaway Kessler Syndrome would render the useful polar-orbiting bands difficult to use, and greatly increase the cost of space launches and missions. Measurement, growth mitigation and active removal of space debris are major activities within the space industry today.
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In 1946, during the Giacobinid meteor shower, Helmut Landsberg collected several small magnetic particles that apparently were associated with the shower.[2] Fred Whipple was intrigued by this and wrote a paper that demonstrated that particles of this size were too small to maintain their velocity when they encountered the upper atmosphere. Instead, they quickly decelerated and then fell to Earth unmelted. In order to classify these sorts of objects, he coined the term "micro-meteorites".[3]
Whipple, in collaboration with Fletcher Watson of the Harvard Observatory, led an effort to build an observatory to directly measure the velocity of the meteors that could be seen. At the time the source of the micro-meteorites was not known. Direct measurements at the new observatory were used to locate the source of the meteors, demonstrating that the bulk of material was leftover from comet tails, and that none of it could be shown to have an extra-solar origin.[4] Today it is understood that meteors of all sorts are leftover material from the formation of the solar system, consisting of particles from the interplanetary dust cloud or other objects made up from this material, like comets.[5]
The early studies were based on optical measures only. In 1957 Hans Pettersson conducted one of the first direct measurements of the fall of space dust on the Earth, estimating it to be 14,300,000 tons per year.[6] This suggested that the meteor flux in space was much higher than the number based on telescope observations. Such a high flux presented a very serious risk to missions deeper in space, specifically the high-orbiting Apollo capsules. To determine whether the direct measure was accurate, a number of additional studies followed, including the Pegasus satellite program. These showed that the rate of meteorites passing into the atmosphere, or flux, was in line with the optical measures, at around 10,000 to 20,000 tons per year.[7]
Whipple's work pre-dated the space race and it proved useful when space exploration started only a few years later. His studies had demonstrated that the chance of being hit by a meteor large enough to destroy a spacecraft was extremely remote. However, a spacecraft would be almost constantly struck by micrometeorites, about the size of dust grains.[4]
Whipple had already developed a solution to this problem in 1946. Originally known as a "meteor bumper" and now termed the Whipple shield, this consists of a thin foil film held a short distance away from the spacecraft's body. When a micrometeorite strikes the foil, it vaporizes into a plasma that quickly spreads. By the time this plasma crosses the gap between the shield and the spacecraft, it is so diffused that it is unable to penetrate the structural material below.[8] The shield allows a spacecraft body to be built to just the thickness needed for structural integrity, while the foil adds little additional weight. Such a spacecraft is lighter than one with panels designed to stop the meteors directly.
For spacecraft that spend the majority of their time in orbit, some variety of the Whipple shield has been almost universal for decades.[9][10] Later research showed that ceramic fiber woven shields offer better protection to hypervelocity (~7 km/s) particles than aluminum shields of equal weight.[11] Another modern design uses multi-layer flexible fabric, as in NASA's TransHab expandable space habitation module.[12]
As space missions moved out from the Earth and into deep space, the question arose about the dangers posed by the asteroid belt environment, which probes would have to pass through on voyages to the outer solar system. Although Whipple had demonstrated that the near-Earth environment was not a problem for space travel, the same depth of analysis had not been applied to the belt. Starting in late 1968, Donald Kessler published a series of papers estimating the spatial density of asteroids.[13] The main outcome of this work was the demonstration that risks in transiting the asteroid belt could be mitigated, and the maximum possible flux was about the same as the flux in near-Earth space.[14] A few years later, the Pioneer and Voyager missions demonstrated this to be true by successfully transiting this region.
The evolution of the asteroid belt had been studied as a dynamic process since it was first considered by Ernst Öpik. Öpik's seminal paper considered the effect of gravitational influence of the planets on smaller objects, notably the Mars-crossing asteroids, noting that their expected lifetime was on the order of billions of years.[15] A number of papers explored this work further, using elliptical orbits for all of the objects and introducing a number of mathematical refinements.[16] Kessler used these methods to study Jupiter's moons, calculating expected lifetimes on the order of billions of years and demonstrating that several of the outer moons were almost certainly the result of recent collisions.[17]
Since the earliest days of the space race, the North American Aerospace Defense Command (NORAD) has maintained a database of all known rocket launches and the various objects that reach orbit as a result – not just the satellites themselves, but the aerodynamic shields that protected them during launch, upper stage booster rockets that placed them in orbit, and in some cases, the lower stages as well. This was known as the Space Object Catalog when it was created with the launch of Sputnik in 1957. NASA published modified versions of the database in the now common two-line element set format via mail,[18] and starting in the early 1980s, the CelesTrak Bulletin Board System (BBS) re-published them.[19]
The trackers that fed this database were aware of a number of other objects in orbit, many of which were the result of on-orbit explosions.[20] Some of these were deliberately caused during the 1960s anti-satellite weapon (ASAT) testing, while others were the result of rocket stages that had "blown up" in orbit as leftover propellant expanded into a gas and ruptured their tanks. Since these objects were only being tracked in a haphazard manner, a NORAD employee, John Gabbard, took it upon himself to keep a separate database of as many of these objects as he could. Studying the results of these explosions, Gabbard developed a new technique for predicting the orbital paths of their products. "Gabbard diagrams" (or plots) have since become widely used. Along with Preston Landry, these studies were used to dramatically improve the modelling of orbital evolution and decay.[21]
When NORAD's database first became publicly available in the 1970s, Kessler applied the same basic technique developed for the asteroid belt study to the database of known objects. In 1978, Kessler and Burton Cour-Palais co-authored the seminal Collision Frequency of Artificial Satellites: The Creation of a Debris Belt,[22] which showed that the same process that controlled the evolution of the asteroids would cause a similar collisional process in low Earth orbit (LEO), but instead of billions of years, the process would take just decades. The paper concluded that by about the year 2000, the collisions from debris formed by this process would outnumber micrometeorites as the primary ablative risk to orbiting spacecraft.[23]
At the time this did not seem like cause for major concern, as it was widely held that drag from the upper atmosphere would de-orbit the debris faster than it was being created. However, Gabbard was aware that the number of objects in space was under-represented in the NORAD data, and was familiar with the sorts of debris and their behaviour. Shortly after Kessler's paper was published, Gabbard was interviewed on the topic, and he coined the term "Kessler syndrome" to refer to the orbital regions where the debris had become a significant issue. The reporter used the term verbatim,[23] and when it was picked up in a Popular Science article in 1982,[24] the term became widely used. The article won the Aviation/Space Writers Association's 1982 National Journalism Award.[23]
A lack of good data about the debris problem prompted a series of studies to better characterize the LEO environment. In October 1979 NASA provided Kessler with additional funding for further studies of the problem.[23] Several approaches were used by these studies.
Optical telescopes or short-wavelength radars were used to more accurately measure the number and size of objects in space. These measurements demonstrated that the published population count was too low by at least 50%.[25] Before this it was believed that the NORAD database was essentially complete and accounted for at least the majority of large objects in orbit. These measurements demonstrated that some objects (typically U.S. military spacecraft) were deliberately eliminated from the NORAD list, while many others were not included because they were considered unimportant, and the list could not easily account for objects under 20 centimetres (7.9 in) in size. In particular, the debris left over from exploding rocket stages and several 1960s anti-satellite tests were only tracked in a haphazard way with the main database.[23]
Spacecraft that had returned to Earth were examined with microscopes, looking for impacts that had already taken place and had gone unnoticed. Sections of Skylab and the Apollo CSMs that had been recovered in the 1960s and '70s were shown to be heavily pitted by debris. Every study demonstrated that the debris flux was much higher than expected, and that the debris was already the primary source of collisions in space. LEO was shown to be subject to the Kessler Syndrome, as originally defined.[23] These results were refined by scrutiny of returning spacecraft, including the Solar Maximum Mission, the Long Duration Exposure Facility, numerous Space Shuttle missions, and many others.
One discovery that was particularly disconcerting was that 42% of all cataloged debris was the result of only 19 events, which were all caused by explosions of spent rocket stages, mostly from U.S. Delta rockets.[26] Kessler made this discovery using Gabbard's methods against known debris fields, which overturned the previously held belief that most unknown debris was from formerly unknown ASAT tests.[27] The Delta remained a workhorse of the U.S. space program, and there were numerous other Delta components in orbit that had not yet exploded.
Through the 1980s, the US Air Force ran an experimental program to determine what would happen if debris collided with satellites or other debris. The study demonstrated that the process was entirely unlike the micrometeor case, and that many large chunks of debris would be created that would themselves be a collisional threat.[23] This leads to a worrying possibility – instead of the density of debris being a measure of the number of items launched into orbit, it was that number plus any new debris caused when they collided. If the new debris did not decay from orbit before impacting another object, the number of debris items would continue to grow even if there were no new launches.
In 1991 Kessler published a new work using the best data then available. In "Collisional cascading: The limits of population growth in low earth orbit" he mentioned the USAF's conclusions about the creation of debris. Although the vast majority of debris objects by number was lightweight, like paint flecks, the majority of the mass was in heavier debris, about 1 kilogram (2.2 lb) or heavier. This sort of mass would be enough to destroy any spacecraft on impact, creating more objects in the critical mass area.[28] As the National Academy of Sciences put it:
A 1-kg object impacting at 10 km/s, for example, is probably capable of catastrophically breaking up a 1,000-kg spacecraft if it strikes a high-density element in the spacecraft. In such a breakup, numerous fragments larger than 1 kg would be created.[29]
Kessler's analysis led to the conclusion that the problem could be categorized into three regimes. With a low enough density, the addition of debris through impacts is slower than their rate of decay, and the problem does not become significant. Beyond that is a critical density where additional debris lead to additional collisions. At densities greater than this critical point, the rate of production is greater than decay rates, leading to a "cascade", or chain reaction, that reduces the on-orbit population to small objects on the order of a few cm in size, making any sort of space activity very hazardous.[28] This third condition, the chain reaction, became the new use of the term "Kessler Syndrome".[23]
In a historical overview written in early 2009, Kessler summed up the situation bluntly:
Aggressive space activities without adequate safeguards could significantly shorten the time between collisions and produce an intolerable hazard to future spacecraft. Some of the most environmentally dangerous activities in space include large constellations such as those initially proposed by the Strategic Defense Initiative in the mid-1980s, large structures such as those considered in the late-1970s for building solar power stations in Earth orbit, and anti-satellite warfare using systems tested by the USSR, the U.S., and China over the past 30 years. Such aggressive activities could set up a situation where a single satellite failure could lead to cascading failures of many satellites in a period of time much shorter than years.[23]
Faced with this scenario, as early as the 1980s NASA and other groups within the U.S. attempted to limit the growth of debris. One particularly effective solution was implemented by McDonnell Douglas on the Delta booster, by having the booster move away from their payload and then venting any remaining propellant in the tanks. This eliminated the pressure build-up in the tanks that had caused them to explode in the past.[30] Other countries, however, were not as quick to adopt this sort of measure, and the problem continued to grow throughout the 1980s, especially due to a large number of launches in the Soviet Union.[31]
A new battery of studies followed as NASA, NORAD and others attempted to better understand exactly what the environment was like. Every one of these studies adjusted the number of pieces of debris in this critical mass zone upward. In 1981 when Schefter's article was published it was placed at 5,000 objects,[20] but a new battery of detectors in the Ground-based Electro-Optical Deep Space Surveillance system quickly found new objects within its resolution. By the late 1990s it was thought that the majority of 28,000 launched objects had already decayed and about 8,500 remained in orbit.[32] By 2005 this had been adjusted upward to 13,000 objects,[33] and a 2006 study raised this to 19,000 as a result of an ASAT test and a satellite collision.[34] In 2011, NASA said 22,000 different objects were being tracked.[35]
The growth in object count as a result of these new studies has led to intense debate within the space community on the nature of the problem and earlier dire warnings. Following Kessler's 1991 derivation, and updates from 2001,[36] the LEO environment within the 1,000 kilometres (620 mi) altitude range should now be within the cascading region. However, only one major incident has occurred: the 2009 satellite collision between Iridium 33 and Cosmos 2251. The lack of any obvious cascading in the short term has led to a number of complaints that the original estimates overestimated the issue.[37] Kessler has pointed out that the start of a cascade would not be obvious until the situation was well advanced, which might take years.[38]
A 2006 NASA model suggested that even if no new launches took place, the environment would continue to contain the then-known population until about 2055, at which point it would increase on its own.[39][40] Richard Crowther of Britain's Defence Evaluation and Research Agency stated that he believes the cascade will begin around 2015.[41] The National Academy of Sciences, summarizing the view among professionals, noted that there was widespread agreement that two bands of LEO space, 900 to 1,000 kilometres (620 mi) and 1,500 kilometres (930 mi) altitudes, were already past the critical density.[42]
In the 2009 European Air and Space Conference, University of Southampton, UK researcher, Hugh Lewis predicted that the threat from space debris would rise 50 percent in the coming decade and quadruple in the next 50 years. Currently more than 13,000 close calls are tracked weekly.[43]
A report in 2011 by the National Research Council in the USA warned NASA that the amount of space debris orbiting the Earth was at critical level. Some computer models revealed that 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 has called for international regulations to limit debris and research into disposing of the debris.[44]
Any discussion of space debris generally categorizes large and small debris. "Large" is defined not by its size so much as the current ability to detect objects of some lower size limit. Generally, large is taken to be 10 centimetres (3.9 in) across or larger, with typical masses on the order of 1 kilogram (2.2 lb).[45] Logically it would follow that small debris would be anything smaller than that, but in fact the cutoff is normally 1 centimetre (0.39 in) or smaller. Debris between these two limits would normally be considered "large" as well, but goes unmeasured due to our inability to track them.[45]
The great majority of debris consists of smaller objects, 1 centimetre (0.39 in) or less. The mid-2009 update to the NASA debris FAQ places the number of large debris items over 10 centimetres (3.9 in) at 19,000, between 1 and 10 centimetres (3.9 in) approximately 500,000, and that debris items smaller than 1 centimetre (0.39 in) exceeds tens of millions.[46] In terms of mass, the vast majority of the overall weight of the debris is concentrated in larger objects, using numbers from 2000, about 1,500 objects weighing more than 100 kilograms (220 lb) each account for over 98% of the 1,900 tons of debris then known in low earth orbit.[47]
Since space debris comes from man-made objects, the total possible mass of debris is easy to calculate: it is the total mass of all spacecraft and rocket bodies that have reached orbit. The actual mass of debris will be necessarily less than that, as the orbits of some of these objects have since decayed. As debris mass tends to be dominated by larger objects, most of which have long ago been detected, the total mass has remained relatively constant in spite of the addition of many smaller objects. Using the figure of 8,500 known debris items from 2008, the total mass is estimated at 5,500 tonnes.[48]
Every satellite, space probe and manned mission has the potential to create space debris. Any impact between two objects of sizeable mass can spall off shrapnel debris from the force of collision. Each piece of shrapnel has the potential to cause further damage, creating even more space debris. With a large enough collision (such as one between a space station and a defunct satellite), the amount of cascading debris could be enough to render Low Earth Orbit essentially unusable.[23]
The problem in LEO is compounded by the fact that there are few "universal orbits" that keep spacecraft in particular rings, as opposed to GEO, a single widely-used orbit. The closest would be the sun-synchronous orbits that maintain a constant angle between the sun and orbital plane. But LEO satellites are in many different orbital planes providing global coverage, and the 15 orbits per day typical of LEO satellites results in frequent approaches between object pairs. Since sun-synchronous orbits are polar, the polar regions are common crossing points.[49]
After space debris is created, orbital perturbations mean that the orbital plane's direction will change over time, and thus collisions can occur from virtually any direction. Collisions thus usually occur at very high relative velocities, typically several kilometres per second.[50] Such a collision will normally create large numbers of objects in the critical size range, as was the case in the 2009 collision. It is for this reason that the Kessler Syndrome is most commonly applied only to the LEO region. In this region a collision will create debris that will cross other orbits and this population increase that leads to the cascade effect.
At the most commonly-used low earth orbits for manned missions, 400 kilometres (250 mi) and below, residual air drag helps keep the zones clear. Collisions that occur under this altitude are less of an issue, since they result in fragment orbits having perigee at or below this altitude. The critical altitude also changes as a result of the space weather environment, which causes the upper atmosphere to expand and contract. An expansion of the atmosphere leads to an increased drag to the fragments, resulting in a shorter orbit lifetime. An expanded atmosphere for some period of time in the 1990s is one reason the orbital debris density remained lower for some time.[25] Another was the rapid reduction in launches by Russia, which conducted the vast majority of launches during the 1970s and 80s.[51]
At higher altitudes, where atmospheric drag is less significant, orbital decay takes much longer. Slight atmospheric drag, lunar perturbations, and solar radiation pressure can gradually bring debris down to lower altitudes where it decays, but at very high altitudes this can take millennia.[52] Thus while these orbits are generally less used than LEO, and the problem onset is slower as a result, the numbers progress toward the critical threshold much more quickly.
The issue is especially problematic in the valuable geostationary orbits (GEO), where satellites are often clustered over their primary ground "targets" and share the same orbital path. Orbital perturbations are significant in GEO, causing longitude drift of the spacecraft, and a precession of the orbit plane if no maneuvers are performed. Active satellites maintain their station via thrusters, but if they become inoperable they become a collision concern (as in the case of Telstar 401). There has been estimated to be one close (within 50 meters) approach per year.[53]
On the upside, relative velocities in GEO are low, compared with those between objects in largely random low earth orbits. The impact velocities peak at about 1.5 kilometres per second (0.93 mi/s). This means that the debris field from such a collision is not the same as a LEO collision and does not pose the same sort of risks, at least over the short term. It would, however, almost certainly knock the satellite out of operation. Large-scale structures, like solar power satellites, would be almost certain to suffer major collisions over short periods of time.[54]
In response, the ITU has placed increasingly strict requirements on the station-keeping ability of new satellites and demands that the owners guarantee their ability to safely move the satellites out of their orbital slots at the end of their lifetime. However, studies have suggested that even the existing ITU requirements are not enough to have a major effect on collision frequency.[55] Additionally, GEO orbit is too distant to make accurate measurements of the existing debris field for objects under 1 metre (3 ft 3 in), so the precise nature of the existing problem is not well known.[56] Others have suggested that these satellites be moved to empty spots within GEO, which would require less maneuvering and make it easier to predict future motions.[57] An additional risk is presented by satellites in other orbits, especially those satellites or boosters left stranded in geostationary transfer orbit, which are a concern due to the typically large crossing velocities.
In spite of these efforts at risk reduction, spacecraft collisions have taken place. The ESA telecommunications satellite Olympus-1 was hit by a meteor on 11 August 1993 and left adrift.[58] On 24 July 1996, Cerise, a French microsatellite in a sun-synchronous LEO, was hit by fragments of an Ariane-1 H-10 upper-stage booster that had exploded in November 1986.[27] On 29 March 2006, the Russian Express-AM11 communications satellite was struck by an unknown object which rendered it inoperable. Luckily, the engineers had enough time in contact with the spacecraft to send it to a parking orbit out of GEO.[59]
In 1958 the United States launched Vanguard I into a medium Earth orbit (MEO). It became one of the longest surviving pieces of space junk and as of October 2009[update] remains the oldest piece of junk still in orbit.[60]
In a catalog listing known launches up to July 2009, the Union of Concerned Scientists listed 902 operational satellites.[61] This is out of a known population of 19,000 large objects and about 30,000 objects ever launched. Thus, operational satellites represent a small minority of the population of man-made objects in space. The rest are, by definition, debris.
One particular series of satellites presents an additional concern. During the 1970s and 80s the Soviet Union launched a number of naval surveillance satellites as part of their RORSAT (Radar Ocean Reconnaissance SATellite) program. These satellites were equipped with a BES-5 nuclear reactor in order to provide enough energy to operate their radar systems. The satellites were normally boosted into a medium altitude graveyard orbit, but there were several failures that resulted in radioactive material reaching the ground (see Kosmos 954 and Kosmos 1402). Even those successfully disposed of now face a debris issue of their own, with a calculated probability of 8% that one will be punctured and release its coolant over any 50 year period. The coolant self-forms into droplets up to around some centimeters in size[62] and these represent a significant debris source of their own.[63]
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 (EVA); a camera Michael Collins lost near the spacecraft Gemini 10; garbage bags jettisoned by the Soviet cosmonauts throughout the Mir space station's 15-year life;[60] a wrench and a toothbrush. Sunita Williams of STS-116 lost a camera during EVA. In an EVA to reinforce a torn solar panel during STS-120, a pair of pliers was lost and during STS-126, Heidemarie Stefanyshyn-Piper lost a briefcase-sized tool bag in one of the mission's EVAs.[64]
Lower stages, like the solid rocket boosters of the Space Shuttle, or the Saturn IB stage of the Apollo program era, do not reach orbital velocities and do not add to the mass load in orbit.[65] Upper stages, like the Inertial Upper Stage, start and end their productive lives in orbit. Boosters that remain on orbit are a serious debris problem, and one of the major known impact events was due to an Ariane booster.[27] During the initial attempts to characterize the space debris problem, it became evident that a good proportion of all debris was due to the breaking up of rocket stages. Although NASA and the USAF quickly made efforts to improve the survivability of their boosters, other launchers did not implement similar changes.
On 11 March 2000, a Chinese Long March 4's CBERS-1/SACI-1 upper stage exploded in orbit and created a debris cloud.[66][67]
An event of similar magnitude occurred on 19 February 2007, when a Russian Briz-M booster stage exploded in orbit over South Australia. The booster had been launched on 28 February 2006 carrying an Arabsat-4A communication satellite but malfunctioned before it could use all of its propellant. 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. As of 21 February 2007, over 1,000 fragments had been identified.[68][69] A third break-up event occurred on 14 February 2007 as recorded by Celes Trak.[70] Eight break-ups occurred in 2006, the most break-ups since 1993.[71]
One major source of debris in the past was the testing of anti-satellite weapons carried out by both the U.S. and Soviet Union in the 1960s and '70s. The NORAD element files only contained data for Soviet tests, and it was not until much later that debris from U.S. tests was identified.[20] By the time the problem with debris was understood, widespread ASAT testing had ended. The U.S.'s only active weapon, Program 437, was shut down in 1975.[72]
The U.S. re-started 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 kilometres (326 mi) altitude, creating thousands of pieces of space debris larger than 1 centimetre (0.39 in). Because it took place at relatively low altitude, atmospheric drag caused the vast majority of the large debris to decay from orbit within a decade. Following the U.S. test in 1985, there was a de-facto moratorium on such tests.[73]
China was widely condemned after their 2007 anti-satellite missile test, both for the military implications as well as the huge amount of debris it created.[74] This is the largest single space debris incident in history in terms of new objects, estimated to have created more than 2,300 pieces (updated 13 December 2007) of trackable debris (approximately golf ball size or larger), over 35,000 pieces 1 cm (0.4 in) or larger, and 1 million pieces 1 mm (0.04 in) or larger. The test took place in the part of near Earth space most densely populated with satellites, as the target satellite orbited between 850 kilometres (530 mi) and 882 kilometres (548 mi).[75] Since the atmospheric drag is quite low at that altitude, the debris will persist for decades. In June 2007, NASA's Terra environmental spacecraft was the first to perform a maneuver in order to prevent impacts from this debris.[76]
On 20 February 2008, the U.S. launched an SM-3 Missile from the USS Lake Erie specially to destroy a defective U.S. spy satellite thought to be carrying 1,000 pounds (450 kg) of toxic hydrazine propellant. Since this event occurred at about 250 km (155 mi) altitude, all of the resulting debris have a perigee of 250 km (155 mi) or lower.[77] The missile was aimed to deliberately reduce the amount of debris as much as possible, and they had decayed by early 2008.[78]
The vulnerability of satellites to a collision with larger debris and the ease of launching such an attack against a low-flying satellite, has led some to speculate that such an attack would be within the capabilities of countries unable to make a precision attack like former U.S. or Soviet systems. Such an attack against a large satellite of 10 tonnes or more would cause enormous damage to the LEO environment.[73]
Spacecraft in a debris field are subject to constant wear as a result of impacts with small debris. Critical areas of a spacecraft are normally protected by Whipple shields, eliminating most damage. However, low-mass impacts have a direct impact on the lifetime of a space mission, if the spacecraft is powered by solar panels. These panels are difficult to protect because their front face has to be directly exposed to the sun. As a result, they are often punctured by debris. When hit, panels tend to produce a cloud of gas-sized particles that, compared to debris, does not present as much of a risk to other spacecraft. This gas is generally a plasma when created and consequently presents an electrical risk to the panels themselves.[79]
The effect of the many impacts with smaller debris was particularly notable on Mir, the Soviet space station, as it remained in space for long periods of time with the panels originally launched on its various modules.[80][81]
Impacts with larger debris normally destroy the spacecraft. To date there have been several known and suspected impact events. The earliest on record was the loss of Kosmos 1275, which disappeared on 24 July 1981 only a month after launch. Tracking showed it had suffered some sort of breakup with the creation of 300 new objects. Kosmos did not contain any volatiles and is widely assumed to have suffered a collision with a small object. However, proof is lacking, and an electrical battery explosion has been offered as a possible alternative. Kosmos 1484 suffered a similar mysterious breakup on 18 October 1993.[82]
Several confirmed impact events have taken place since then. Olympus-1 was hit by a meteor on 11 August 1993 and left adrift.[58] On 24 July 1996, the French microsatellite Cerise was hit by fragments of an Ariane-1 H-10 upper-stage booster that had exploded in November 1986.[27] On 29 March 2006 the Russian Express-AM11 communications satellite was struck by an unknown object which rendered it inoperable. Luckily, the engineers had enough time in contact with the spacecraft to send it to a parking orbit out of GEO.[59]
The first major space debris collision was on 10 February 2009 at 16:56 UTC. The deactivated 950 kilograms (2,100 lb) Kosmos 2251 and an operational 560 kilograms (1,200 lb) Iridium 33 collided 500 miles (800 km)[83] over northern Siberia. The relative speed of impact was about 11.7 kilometres per second (7.3 mi/s), or approximately 42,120 kilometres per hour (26,170 mph).[84] Both satellites were destroyed and the collision scattered considerable debris, which poses an elevated risk to spacecraft.[85] The collision created a debris cloud, although accurate estimates of the number of pieces of debris is not yet available.[86]
In a Kessler Syndrome cascade, satellite lifetimes would be measured on the order of years or months. New satellites could be launched through the debris field into higher orbits or placed in lower ones where natural decay processes remove the debris, but it is precisely because of the utility of the orbits between 800 and 1,500 kilometres (930 mi) that this region is so filled with debris.[38]
From the earliest days of the Space Shuttle missions, NASA has turned to NORAD's database to constantly monitor the orbital path in front of the Shuttle to find and avoid any known debris. During the 1980s, these simulations used up a considerable amount of the NORAD tracking system's capacity.[30] The first official Space Shuttle collision avoidance maneuver was during STS-48 in September 1991.[87] A 7-second reaction control system burn was performed to avoid debris from the Cosmos satellite 955.[88] Similar maneuvers followed on missions 53, 72 and 82.[87]
One of the first events to widely publicize the debris problem was Space Shuttle Challenger's first flight on STS-7. A small fleck of paint impacted Challenger's front window and created a pit over 1 millimetre (0.04 in) wide. Endeavour suffered a similar impact on STS-59 in 1994, but this one pitted the window for about half its depth: a cause for much greater concern. Post-flight examinations have noted a marked increase in the number of minor debris impacts since 1998.[89]
The damage due to smaller debris has now grown to become a significant problem in its own right. Chipping of the windows became common by the 1990s, along with minor damage to the thermal protection system tiles (TPS). To mitigate the impact of these events, once the Shuttle reached orbit it was deliberately flown tail first in an attempt to intercept as much of the debris load as possible on the engines and rear cargo bay. These were not used on orbit or during descent and thus were less critical to operations after launch. When flown to the ISS, the Shuttle was placed where the station provided as much protection as possible.[90]
The sudden increase in debris load led to a re-evaluation of the debris issue and a catastrophic impact with large debris was considered to be the primary threat to Shuttle operations on every mission.[90][91] Mission planning required a thorough discussion of debris risk, with an executive level decision to proceed if the risk is greater than 1 in 200 of destroying the Shuttle. On a normal low-orbit mission to the ISS the risks were estimated to be 1 in 300, but the STS-125 mission to repair the Hubble Space Telescope at 350 miles (560 km) was initially calculated at 1 in 185 due to the 2009 satellite collision, and threatened to cancel the mission. However, a re-analysis as better debris numbers became available reduced this to 1 in 221, and the mission was allowed to proceed.[92]
In spite of their best efforts, however, there have been two serious debris incidents on more recent Shuttle missions. In 2006, Atlantis was hit by a small fragment of a circuit board during STS-115, which bored a small hole through the radiator panels in the cargo bay (the large gold colored objects visible when the doors are open).[93] A similar incident followed on STS-118 in 2007, when Endeavour was hit in a similar location by unknown debris which blew a hole several centimetres in diameter through the panel.[94]
The International Space Station (ISS) uses extensive Whipple shielding to protect itself from minor debris threats.[95] However, large portions of the ISS cannot be protected, notably its large solar panels. In 1989 it was predicted that the International Space Station's panels would suffer about 0.23% degradation over four years, which was dealt with by overdesigning the panel by 1%.[96] New figures based on the increase in collisions since 1998 are not available.
Like the Shuttle, the only protection against larger debris is avoidance. On two occasions the crew have been forced to abandon work and take refuge in the Soyuz capsule while the threat passed.[97][98] This close call is a good example of the potential Kessler Syndrome; the debris is believed to be a small 10 centimetres (3.9 in) portion of the former Cosmos 1275,[99] which is the satellite that is considered to be the first example of an on-orbit impact with debris.
If the Kessler Syndrome comes to pass, the threat to manned missions may be too great to contemplate operations in LEO. Although the majority of manned space activities take place at altitudes below the critical 800 to 1,500 kilometres (310 mi) regions, a cascade within these areas would result in a constant rain down into the lower altitudes as well. The time scale of their decay is such that "the resulting debris environment is likely to be too hostile for future space use."[28][100]
Although most debris will burn up in the atmosphere, larger objects can reach the ground intact and present a risk.
The original re-entry plan for Skylab called for the station to remain in space for 8 to 10 years after its final mission in February 1974. Unexpectedly high solar activity expanded the upper atmosphere resulting in higher than expected drag on space station bringing its orbit closer to Earth than planned. On 11 July 1979, Skylab re-entered the Earth's atmosphere and disintegrated, raining debris harmlessly along a path extending over the southern Indian Ocean and sparsely populated areas of Western Australia.[101][102]
On 12 January 2001, a Star 48 Payload Assist Module (PAM-D) rocket upper stage re-entered the atmosphere after a "catastrophic orbital decay".[103] The PAM-D stage crashed in the sparsely populated Saudi Arabian desert. It was positively identified as the upper-stage rocket for NAVSTAR 32, a GPS satellite launched in 1993.
The Columbia disaster in 2003 demonstrated this risk, as large portions of the spacecraft reached the ground. In some cases entire equipment systems were left intact.[104] NASA continues to warn people to avoid contact with the debris due to the possible presence of hazardous chemicals.[105]
On 27 March 2007, wreckage from a Russian spy satellite was spotted by Lan Chile (LAN Airlines) in an Airbus A340, which was travelling between Santiago, Chile, and Auckland, New Zealand carrying 270 passengers.[106] The pilot estimated the debris was within 8 km of the aircraft, and he reported hearing the sonic boom as it passed.[107] The aircraft was flying over the Pacific Ocean, which is considered one of the safest places in the world for a satellite to come down because of its large areas of uninhabited water.
In 1969, five sailors on a Japanese ship were injured by space debris, probably of Russian origin; see p. 11 (p. 3 in the document's numbering system) of [108] In 1997 an Oklahoma woman named Lottie Williams was hit in the shoulder by a 10 x 13 centimetres (5.1 in) piece of blackened, woven metallic material that was later confirmed to be part of the propellant tank of a Delta II rocket which had launched a U.S. Air Force satellite in 1996. She was not injured.[109][110]
Radar and optical detectors such as lidar are the main tools used for tracking space debris. However, determining orbits to allow reliable re-acquisition is problematic. Tracking objects smaller than 10 cm (4 in) is difficult due to their small cross-section and reduced orbital stability, though debris as small as 1 cm (0.4 in) can be tracked.[111][112] NASA Orbital Debris Observatory tracked space debris using a 3 m (10 ft) liquid mirror transit telescope.[113]
The U.S. Strategic Command maintains a catalogue containing known orbital objects. The list was initially compiled in part to prevent misinterpretation as hostile missiles. The version compiled in 2009 listed about 19,000 objects. 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.[114] Nevertheless, the majority of expected debris objects remain unobserved – there are more than 600,000 objects larger than 1 cm (0.4 in) 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 (System),[115] Goldstone radar, Haystack radar,[116], the EISCAT radars, and the Cobra Dane phased array radar.[117] 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 a valuable source of information on the (sub-millimetre) space debris environment. The LDEF satellite deployed by STS-41-C Challenger and retrieved by STS-32 Columbia spent 68 months in orbit. Close examination of its surfaces allowed an analysis of the directional distribution and composition of the debris flux. The EURECA satellite deployed by STS-46 Atlantis in 1992 and retrieved by STS-57 Endeavour in 1993 was similarly used for debris studies.[118]
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 a means for validating debris environment models. Similar materials returned from Mir were extensively studied, notably the Mir Environmental Effects Payload which studied the environment in the Mir area.[119][120]
Space debris groups resulting from satellite breakups are often studied using scatter plots known as Gabbard diagrams. In a Gabbard diagram, the perigee and apogee altitudes of the individual debris fragments resulting from a collision are plotted with respect to the orbital period of each fragment. The distribution can be used to infer information such as direction and point of impact.[21][121]
Manmade space debris have been dropping out of orbit at an average rate of about one object per day for the past 50 years.[122] Substantial variation in the average rate occurs as a result of the 11-year solar activity cycle, averaging closer to three objects per day at solar max due to the heating, and resultant expansion, of the Earth's atmosphere. At solar min, five and one-half years later, the rate averages about one every three days.[122]
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 propellants is aimed at reducing the risk of on-orbit explosions that could generate thousands of additional debris objects.[124] The modification of the Delta boosters, at a time when the debris problem was first becoming apparent, essentially eliminated their further contribution to the problem.[30]
There is no international treaty mandating behavior to minimize space debris, but the United Nations Committee on the Peaceful Uses of Outer Space (COPUOS) did publish voluntary guidelines in 2007.[125] As of 2008, the committee is discussing international "rules of the road" to prevent collisions between satellites.[126] NASA has implemented its own procedures for limiting debris production[127] as have some other space agencies, such as the European Space Agency. Starting in 2007, the ISO has been preparing a new standard dealing with space debris mitigation.[128]
One alternative that has been envisioned to ensure launch vehicle operators absorb the cost of debris mitigation is to implement a "one-up/one-down" launch license policy to Earth orbits. In this conception, launch operators would need to build the capability into their launch vehicle-robotic capture, navigation, mission duration extension, and substantial additional propellant – to be able to rendezvous with, capture and deorbit an existing derelict satellite from approximately the same orbital plane.[129]
It is an ITU requirement that geostationary satellites be able to remove themselves to a "graveyard orbit" at the end of their lives. It has been demonstrated that the selected orbital areas do not sufficiently protect GEO lanes from debris, although a response has not yet been formulated.[55]
Rocket stages or satellites that retain enough propellant can allow power themselves into a decaying orbit. In cases when a direct (and controlled) de-orbit would require too much propellant, a satellite can 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, bringing its time to atmospheric re-entry down from a projected 200 years to about 15 years by lowering its perigee from 830 km (516 mi) to about 550 km (342 mi).[130][131]
Instead of using rockets, an electrodynamic tether can be attached to the spacecraft on launch. At the end of its lifetime it is rolled out and slows down the spacecraft.[132] Although tethers of up to 30 km have been successfully deployed in orbit the technology has not yet reached maturity.[40] It has been proposed that booster stages include a sail-like attachment to the same end.[133]
A well-studied solution is to use a remotely controlled vehicle to rendezvous with debris, capture it, and return to a central station.[134] The commercially-developed MDA Space Infrastructure Servicing vehicle is a refueling depot and service spacecraft for communication satellites in geosynchronous orbit, slated for launch in 2015.[135] The SIS includes the vehicle capability to "push dead satellites into graveyard orbits."[136] The Advanced Common Evolved Stage family of upper-stages is being explicitly designed to have the potential for high leftover propellant margins so that derelict capture/deorbit might be accomplished, as well as with in-space refueling capability that could provide the high delta-V required to deorbit even heavy objects from geosynchronous orbits.[129]
The laser broom uses a powerful ground-based laser to ablate the front surface off of debris and thereby produce a rocket-like thrust that slows the object. With a continued application the debris will eventually decrease their altitude enough to become subject to atmospheric drag.[137][138] In the late 1990s, US Air Force worked on a ground-based laser broom design under the name "Project Orion".[139] Although a test-bed device was scheduled to launch on a 2003 Space Shuttle, numerous international agreements, forbidding the testing of powerful lasers in orbit, caused the program to be limited to using the laser as a measurement device.[140] In the end, the Space Shuttle Columbia disaster led to the project being postponed and, as Nicholas Johnson, Chief Scientist and Program Manager for NASA's Orbital Debris Program Office, later noted, "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."[141]
Additionally, the momentum of the photons in the laser beam could be used to impart thrust on the debris directly. Although this thrust would be tiny, it may be enough to move small debris into new orbits that do not intersect those of working satellites. NASA research from 2011 indicates that firing a laser beam at a piece of space junk could impart an impulse of 0.04 inches (1.0 mm) per second. Keeping the laser on the debris for a few hours per day could alter its course by 650 feet (200 m) per day.[142] A similar proposal replaces the laser with a beam of ions.[143]
A number of other proposals use more novel solutions to the problem, from foamy ball of aerogel or spray of water,[144] inflatable balloons,[145] electrodynamic tethers,[146] boom electroadhesion,[147] or dedicated "interceptor satellites".[148] On January 7, 2010 Star Inc. announced that it had won a contract from Navy/SPAWAR for a feasibility study of the application of the ElectroDynamic Debris Eliminator (EDDE).[149]
As of 2006[update], the cost of launching any of these solutions is about the same as launching any spacecraft. Johnson stated that none of the existing solutions are currently cost-effective.[40]