18000160001400012000100008000600040002000019571960 1970 1980 1990 2000 2010Catalogued Objects in Orbit as of October 2012NumberofObjectsPayloadsPayload mission-related objectsPayload debrisRocket bodiesRocket mission-related objectsRocket debris3space operations→ The Space DebrisEnvironmentDebris is generated during normal operations by the injectionof stages into orbit, the release of mission-related objects andthe eventual retirement of the satellite. Subsequently break-ups and other release events may occur and contribute furtherdebris. These combined debris sources are counteracted bynatural cleaning mechanisms, such as air drag and luni-solarperturbations. LEO satellites are continuously exposed toaerodynamic forces from the rarified upper reaches of theatmosphere. Depending on the altitude, after a few weeks,years or even centuries, this drag will decelerate the satellitesufficiently so that it reenters. At higher altitudes, above700–800 km, air drag is less effective and objects generallyremain in orbit for at least several decades.The result of the balancing effects of debris creation andorbital decay leads to maximum debris concentrations ataltitudes of 800–1000 km and near to 1400 km. Secondarypeaks of spatial densities in geostationary orbit (GEO) andnear the orbits of navigation satellite constellations aresmaller by two to three orders of magnitude.Objects as small as 5–10 cm in LEO, or 0.3–1 m at higheraltitudes, are tracked by the US Space Surveillance Network,and orbital information is maintained in a catalogue. Modelsindicate that 750 000 objects larger than 1 cm might exist.As of September 2012, all human-made space objects resultedfrom the 4900 launches that have been made since the start ofthe Space Age. The majority of the catalogued objects (about65%), however, originate from more than 250 break-ups inorbit, mainly caused by explosions, and about 10 knowncollisions. Major contributions to the population of fragmentscame from a Chinese antisatellite test targeting the FengYun-1C weather satellite on 11 January 2007, which createdmore than 3300 tracked fragments, and the more than 2200tracked fragments created from the first-ever accidentalcollision between two satellites, Iridium-33 and Kosmos-2251,on 10 February 2009. About 20% of the catalogued objects aresatellites (less than a third of which are operational), and about17% are spent rocket bodies and other mission-related objects.Fragmentation debris dominates the smaller size regimesdown to 1 mm. Below 1 mm, slag and dust residues from about2000 solid-propellant motor firings prevail. Other debrissources can be associated with the release of coolant liquidfrom 16 Buk nuclear reactors in the 1980s on Russian radarocean reconnaissance satellites, and with the release of surfacematerials from old satellites and rocket bodies due to impactsand/or surface degradation.China’s Feng Yun-1C intercept in January 2007 alone increasedthe trackable space object population by 25%. The collisionbetween the intact Iridium-33 and Cosmos-2251 added 16%.The main source of information onlarge space debris is the US SpaceSurveillance Network (SSN). As ofSeptember 2012, SSN tracked,correlated and catalogued around23 000 space objects larger than5–10 cm in Earth orbit (around17 000 of which were published).Debris environment models can beused to estimate total numbers,resulting in 30 000 larger than10 cm, 750 000 larger than 1 cm, andmore than 166 million larger than1 mm.Evolution of the tracked and published spaceobject population and its composition byobject class (status September 2012).
4→ Space DebrisMeasurementsESA collaborates primarily with the Fraunhofer Institute forHigh Frequency Physics and Radar Techniques (FHR), near Bonn,Germany, which operates the Tracking and Imaging Radar (TIRA)system. Apart from dedicated tracking campaigns, TIRA alsoregularly conducts surveys that detect debris and determinecoarse orbit information for objects of diameters down to 2 cmat 1000 km range. The nearby Effelsberg radio telescope,equipped with a 100 m-diameter parabolic antenna and aseven-horn receiver, allows experiments where the sensitiveradio telescope can receive the radar echoes. This mode canincrease the overall sensitivity towards the 1 cm object size. Inaddition, using the ionospheric research radars of the EuropeanIncoherent Scatter (EISCAT) scientific association, statisticalobservations of LEO debris down to 2 cm can be acquiredwithout compromising the main scientific objectives of EISCAT.Space object catalogues, such as maintained by SSN, arelimited to sizeable objects, typically larger than 5–10 cm inLEO and larger than 0.3–1 m in GEO. These sensitivitythresholds are a compromise between system cost andperformance. Telescopes are suited mainly to GEO and high-altitude debris observations, whereas radars areadvantageous in the LEO regime, below 2000 km.Knowledge of the meteoroid and debris environment atsubcatalogue sizes is normally acquired in a statisticalmanner through experimental sensors with highersensitivities. Ground-based telescopes can detect debris inGEO down to 10 cm, ground-based radars can detect LEOdebris down to a few millimetres, and in situ impact detectorsin orbit can sense objects down to a few micrometres.ESA’s 1 m-diameter Zeiss space debris telescope at the Optical Ground Station on Tenerife.esa and space debris
5space operationsESA also gains information on the submillimetre meteoroidand debris environment through analysing retrieved spacehardware and through active in situ sensors by analysingimpact fluxes.Monitoring and model validation of the space debrisenvironment requires regular radar and optical observationcampaigns.The acquired data on space debris are essential for developingand validating environment models. To validate modelpredictions and to optimise observation times and sensorsensitivity in the planning of observation campaigns, ESA hasdeveloped the Program for Radar and Optical ObservationsForecasting, or PROOF.At the Teide Observatory on Tenerife in the Canary Islands,Spain, ESA operates the Optical Ground Station – dubbedESA’s space debris telescope. Its 1 m-diameter Zeiss telescope,equipped with highly efficient cameras, is used to survey andcharacterise objects at high altitudes, often in collaborationwith other telescopes, such as those operated by theUniversity of Bern, Switzerland. The ESA telescope can detectand track near-GEO objects up to magnitudes of +19 to +21(equivalent to down to 15 cm in size). With this performance,the ESA telescope is among the world’s top-ranking sensors.ESA data indicate that up to 10 fragmentation events musthave occurred in GEO, causing faint, extremely lightweightobjects with high area-to-mass ratios, such as pieces ofthermal blankets, and with strongly perturbed orbits thatrequire frequent re-observations.Close-up of DEBIE-2 sensors on the International Space Station (right).The Optical Ground Station on Tenerife.ESA’s DEBIE-2 impact detector is mounted onthe Agency’s Columbus science module on theInternational Space Station.
6→ Space Debris ModellingTo study the effectiveness of debris mitigation measures onthe stability of the debris population, long-term forecasts arerequired to determine trends as a function of individualmitigation actions. This kind of analysis can be performedwith ESA’s Debris Environment Long-Term Analysis (DELTA)tool. DELTA is a time-dependent, dynamic debris model, withdetailed traffic model and release event data, and withstatistically generated collision events, based on local objectconcentrations and collision probabilities.The consolidation of our knowledge on all known objects in spaceis a fundamental condition for the operational support activitiesof ESA’s Space Debris Office.This knowledge is maintained andkept up to date through the Database and Information SystemCharacterising Objects in Space (DISCOS) .Today, DISCOS is arecognised, reliable and dependable source of space object datathat is regularly used by almost 40 customers worldwide.ESA maintains models for characterising the debrisenvironment and its evolution. The Agency’s prominent debrisand meteoroid risk assessment tool is MASTER: Meteoroidand Space Debris Terrestrial Environment Reference. It wasfirst issued in 1995 and is continually improved. The currentrelease is MASTER-2009. MASTER provides impact fluxinformation (number of impacts per surface area and time)with high spatial resolution for an object population derivedfrom all known, historical debris generation events. Atsubmillimetre particle sizes, meteoroids can prevail over spacedebris in some orbital regions, in particular during intenseseasonal meteoroid streams.MASTER is ESA’s prominent debris and meteoroid riskassessment tool covering all debris and meteoroid sizes largerthan 0.001 mm, and predictions of the debris environment forobjects larger than 1 mm up to 2055.↑ Objects larger than 10 cm (left) and 1 cm (right) according to MASTER. The sizes of debris are exaggerated in relation to Earth.→ Mitigation Spacefaring nations are focusing their efforts on controllingthe debris environment. The ultimate goal is to prevent thecollisional cascading process from setting in over the next fewdecades. Initial steps aim at reducing the generation ofhazardous debris by avoiding in-orbit explosions or collisionswith operational satellites, and by removing satellites fromdensely populated altitude regions at the end of theirmissions.The Inter-Agency Space Debris Coordination Committee(IADC), the most-recognised international entity on spacedebris, has produced a set of mitigation guidelines, whichalso served as input to a set of Space Debris MitigationGuidelines adopted by the UN Committee on the PeacefulUses of Outer Space (UNCOPUOS). The related keyrecommendations are:• Limit debris release during normal operations;• Minimise the potential for break-ups during operationalphases;With today’s annual launch rate of 60–70 and with futurebreak-ups continuing at mean historical rates of four to fiveper year, the number of objects in space will steadily increase.As a consequence of the rising object count, the probability ofcatastrophic collisions will also grow in a progressive manner.Collision fragments will collide with collision fragmentsleading to a self-sustained process known as the Kesslersyndrome, which is particularly critical for LEO and mayseriously endanger spaceflight within a few decades incertain orbit altitudes. Ultimately, in more than 100 years, theentire debris population could grind itself to subcritical sizes.‘Business-as-usual space activities lead to a progressive,uncontrolled increase in debris objects, with collisionsbecoming the primary debris source within less than50 years.esa and space debris
space operations7Effects of hypervelocityimpacts: Whipple shield(above) and impact on a solidaluminium block (below).The International Space Station isequipped with debris shieldsaround the inhabited modules.These shields are composed of twometal sheets, separated by about10 cm. Between the walls, fabricwith the same purpose as in bullet-proof vests is used. This designenables the shield to defeat debrisobjects of up to 1 cm.The protection of an unmannedsatellite can be efficiently increasedby moving sensitive equipmentaway from the most probableimpact direction, and/or bycovering sensitive partswith protective fabriclayers. Such measurescan significantly increasethe survivability of asatellite against debrisof up to 1 mm.The consequences of impacts on satellites can range fromsmall surface pits owing to micrometre-size objects, via clear-hole penetrations for millimetre-size objects, to mission-critical damage for projectiles larger than a centimetre. Thedestructive energy is a consequence of high impact speeds,which can reach more than 15 km/s. Smaller, uncataloguedobjects can only be defeated by passive protectiontechniques, as used by the International Space Station in theform of ‘stuffed Whipple shields’. At more than 7 km/s,depending on the materials, an impact on the bumper wallwill lead to a clear-hole penetration with a complete break-upand melting of the projectile, such that the dispersedfragment cloud can be withstood by a back wall.Any impact of a 10 cm catalogued object on a satellite ororbital stage will most likely entail a catastrophicdisintegration of the target.ESA’s space projects use hypervelocity impact tests inassociation with damage-assessment tools to predictpotential risks from hypervelocity impacts of debris andmeteoroids, and to define effective protection measuresthrough shielding and design.→ Protectionbudgets for collision avoidance, optimisation of disposalstrategies along with estimation of orbital lifetimes, andanalysis of on-ground casualty expectations.Recent analyses, however, suggest that, in the long term, theproposed mitigation measures are not sufficient to suppresscollisional cascading fully. In addition to mitigation,remediation measures such as the removal of existing in-orbitmass, which fuels the collisional cascading process, also needto be considered. Active removal can effectively targetsatellites and rocket stages, in which most of the in-orbitmass and collision risk is concentrated. Long-termenvironment projections indicate that this is a mandatorystep to maintain the space debris at a safe level for futurespace operations.Mitigation measures need to be accompanied by remediationmeasures: the removal of 5–10 large objects per year fromregions with high object densities and long orbital lifetimesprovides an effective long-term means of stabilising the spacedebris environment at a safe level.• Limit the probability of accidental collisions;• Avoid intentional destruction and other harmful activities;• Minimise the potential for post-mission break-ups resultingfrom stored energy;• Limit the long-term presence of spacecraft and launchvehicle orbital stages in protected regions after the end oftheir missions.Further recommendations on space debris environmentremediation are discussed at the International Academy ofAstronautics (IAA) and in an ad hoc international workinggroup on the Long-term Sustainability of Activities in OuterSpace at UNCOPUOS.To facilitate analyses by mission planners, spacecraftdesigners and space system manufacturers, ESA haspublished the Debris Risk Assessment and Mitigation Analysis(DRAMA) Software. DRAMA allows the estimation of delta-V
8→ Collision and ReentryRisk ControlThe first accidental collision between two satellites occurredat 16:56 UTC on 10 February 2009. An operational UScommercial communications satellite, Iridium-33, and aretired Russian military satellite, Kosmos-2251, collided at11.7 km/s. Both were destroyed and more than 2200 trackedfragments were generated.The first collision between two intact satellites occurred inFebruary 2009, 776 km above Siberia, generating more than2200 tracked fragments.Avoiding collisions is an important mitigation measure butrequires that orbits of the chaser objects are known withsufficient accuracy. For initial assessments, ESA uses theinformation provided by the US Strategic Commandcatalogue, which is sufficient to indicate all close flybys(conjunctions) of a target satellite with any of thecatalogued objects. This information is further refined byConjunction Summary Messages provided by the US JointSpace Operations Center and by ESA’s own orbit data. Thecollision risk is determined as a function of the object sizes,the predicted miss distance, the flyby geometry, the orbituncertainties and the time to conjunction.ESA executes a few collision avoidance manoeuvres per year,in cases where the estimated collision risk is above atolerable probability threshold.ESA’s Space Debris Office offers conjunction predictions andcollision risk estimation as an operational service.ESA’s Automated Transfer Vehicle carried out a collision avoidance manoeuvre for the International Space Station in 2008.esa and space debrisESA–D.Ducros
scale uncontrolled reentries. In such cases, 20–40% of thespacecraft mass may impact the ground. Examples ofuncontrolled reentries in 2011–12 that gained a lot of publicinterest were the US Upper Atmosphere Research Satellite, theGerman Röntgensatellit Rosat and the Russian Phobos–Grunt.ESA’s Space Debris Office also maintains a web-based reentrydata exchange service that is used by the 12 IADC members tomonitor the reentry of risk objects and to exchange orbitdetermination and reentry prediction results.ESA’s Automated Transfer Vehicle (ATV) missions all perform acontrolled and safe reentry into an uninhabited area of theSouth Pacific Ocean. The reentry break-up process of the firstATV mission on 29 September 2008 was monitored from twoobservation aircraft.A 1 in 10 000 probability threshold for the casualty risk in asingle uncontrolled reentry is commonly accepted. ESA canprovide analysis of both controlled and uncontrolledreentries.Only a few large objects reenter Earth’s atmosphere everyyear. In total, about 75% of all larger objects ever launchedhave already reentered. Objects of moderate size (1 m orlarger) reenter about once per week, while about two smalltracked debris objects reenter per day. In general, reenteringobjects do not pose a risk and burn up to a large extent. In thelast 10 minutes before reaching the ground, the denseatmosphere starts to heat up and decelerate the object. In thecase of very compact and massive satellites, and if a largeamount of high melting-point material is involved, such asstainless steel or titanium, fragments of the vehicle may reachthe ground. As these are rare events, and as about 75% ofEarth’s surface is covered by water and large portions of theland mass are uninhabited, the risk for a single individual isseveral orders of magnitude smaller than commonly acceptedrisks in daily life. No injuries resulting from reentering spacedebris are known.Skylab (74 tonnes, July 1979) and Salyut-7/Kosmos-1686(40 tonnes, February 1991) are well-known examples of large-9Reentry of ATV Jules Verne in 2008.space operations
10→ Regulations and TreatiesIADC is internationally recognised as a space debris centre ofcompetence and it also influences mitigation activities at theUNCOPUOS Scientific and Technical Subcommittee and at theSubcommittee for Space Systems and Operations (ISO-TC20/SC14) of the International Organization for Standardization.Today, the global dimension of the problem is internationallyrecognised, and space system designers, operators and policy-makers share the view that active control of the space debrisenvironment is necessary to sustain safe space activities inthe future. In order to guarantee an effective and balancedimplementation of debris mitigation practices, identifiedcontrol measures need to be based on an internationalconsensus. As an example, the 2011 ISO standard 24113 definesprimary debris mitigation requirements. This standard wasadopted by the European Cooperation for SpaceStandardization, which, via a formal ESA ‘ADMIN/IPOL’instruction, is applicable to ESA projects. Verification ofcompliance with the ISO standard can be supported by usingESA’s Debris Risk Assessment and Mitigation Analysis(DRAMA) tool.Space debris is a problem to which all spacefaring nationshave contributed. Likewise, debris poses a risk to missions ofall spacefaring nations. Since analysts first became aware ofan emerging space debris problem in the early 1970s, theunderstanding of debris sources, the resulting debrisenvironment and the associated risks has significantlyimproved.Research results are regularly discussed at the quadrennialseries of ESA-organised European Conferences on Space Debris,and at dedicated sessions of the International AstronauticalCongress and COSPAR (Committee on Space Research) ScientificAssemblies.The most prominent body for information exchange on spacedebris is the 12-member Inter-Agency Space DebrisCoordination Committee.Since its foundation in 1993, IADC has conducted annualmeetings to discuss research results in debris measurements,modelling, protection and mitigation.ESA Council at Ministerial Level in Naples on 21 November 2012.esa and space debrisESA–S.Corvaja
11space operations→ Active Debris RemovalStudies at NASA and ESA show that with a removal strategyfocusing on large target masses, the environment can bestabilised, if about 10 objects are removed from LEO per yearwith the following priorities:• objects with a high mass (largest environmental impact interms of critical-size fragments);• objects with a high collision probability (orbiting in denselypopulated regions);• objects at high altitudes (long orbital lifetime of fragments).High-ranking hotspot regions have been identified at around1000 km altitude and 82° inclination, at 800 km and 98°, and at850 km and 71°.The concentration of critical-size objects innarrow orbital bands would allow multi-target removal missions.Legal constraints associated with the ownership of spacedebris and related liability issues cannot be neglected. Whileremoval technology should be generic, the responsibility for acoupled remover/target is shared between the object owners.Actions to counter the exponential growth of space debris,such as mitigation and active removal, are most effective whenthey are applied early. The further the number of critical-size,intact objects in the debris environment deviates from asustainable level, the more objects will have to be removed tosuppress the additional growth and the multiplying effects.ESA’s internal studies show that continuous removal actionsstarting in 2060 would have only 75% of the effect incomparison to an immediate start.ESA, as a space technology and operations agency, hasidentified active removal technologies as a strategic goal.ESA’s CleanSpace Initiative will look at the requiredtechnology developments, including advanced guidance andnavigation, capture and berthing technologies, and deorbitingtechniques and operations. Technologies for a wide range ofremoval targets will be studied, including real applications.Mitigation measures have been established in order to controlthe growth of the number of space objects. Over the past 10years, however, only about half of all satellites initiallyoperating in the GEO region have been reorbited to a graveyarddisposal orbit in line with IADC recommendations. For the LEOregime, which is most sensitive for an onset of collisionalcascading, there are no comprehensive statistics yet oncompliance regarding the clearing of the region within 25 yearsof mission completion.It has been shown that a business-as-usual scenario leads to aprogressive, uncontrolled increase of object numbers in LEOwith collisions becoming the primary debris source. Themitigation measures as proposed by IADC reduce the growth,but long-term proliferation is still expected, even with fullmitigation compliance, and even if all launch activities arehalted. This is an indication that the population of large andmassive objects has reached a critical concentration in LEO. As aconsequence, the number of large and massive objects (3300physically intact objects) must be controlled. An ESA analysisshowed that on reaching a number of around 2500 intactobjects (equivalent to the status in the mid-1990s) there is a50% probability that the debris environment will become stable.A limitation of the launch rate – currently about 70 objects peryear into LEO – or a further reduction of the allowed orbitallifetime cannot by themselves prevent collisional cascading.Active debris removal requires navigatingin the close vicinity of the target.
12→ ESA’s Space Debris Teamand simulation of radar and optical observations from the groundor in space, and the DRAMA software for verifying the complianceof space missions with mitigation guidelines.The Office also coordinates ESA’s research activities into spacedebris.Since 1984, ESA has organised or supported severalinternational conferences dealing with space debris. Theseinclude the quadrennial series of European Conferences onSpace Debris, COSPAR conferences, InternationalAstronautical Congresses and symposia of the InternationalAssociation for the Advancement of Space Safety.ESA’s Space Debris Office provides operational services insupport of planned and current missions within ESA and tothird parties. These services comprise debris mitigationplanning and verification, in-orbit collision avoidance, reentryprediction and risk assessment, and maintenance of DISCOS.The Office has a long-standing, recognised expertise inmeasurements, modelling, risk assessment, mitigation, long-term projection, reentry safety and databases.The Office has developed and maintains several engineering toolsfor debris analyses, which are available as ready-to-use softwarepackages.They include the MASTER model for predicting debrisand meteoroid particle fluxes, the PROOF tool for the planningHeiner Klinkrad is ESA’s senior advisor on space debris and is head of ESA’s Space Debris Office. He joined ESA in1980 and has worked on debris research since 1988. He coordinates ESA’s space debris research activities andrepresents the Agency at several international space debris forums, including IADC and UNCOPUOS. His main technicalinterest is with orbital mechanics, conjunction event and reentry risk assessments, debris environment models andspace object databases. Prof. Klinkrad chaired the programme committees of the European Conferences on SpaceDebris and has co-organised space debris sessions at COSPAR and IAC congresses. He is author of the textbook SpaceDebris – Models and Risk Analysis and is an adjoint professor at the Technical University of Braunschweig, Germany.Tim Flohrer joined ESA’s Space Debris Office in 2007 and has been working on space surveillance and space debristopics for more than 10 years. Today, his main tasks include operational collision avoidance activities for ESAmissions, reentry assessments and maintaining the Office’s database infrastructure. His main area of interest liesin the development and validation of ground- and space-based observation techniques, in particular using opticalsensor systems, also in context with Space Situational Awareness applications.Holger Krag has been working on space debris environment modelling and the simulation of debris sensorsystems since 1998. He joined ESA’s Space Debris Office in 2006. He operates the system for the prediction andassessment of in-orbit conjunctions and reentry events, and he is analysing the implementation of mitigationmeasures for several missions. He is also in charge of a number of development and maintenance contracts relatedto ESA’s debris software packages, such as MASTER and DRAMA, and of studies on environment evolution anddebris observation. He represents the team on matters related to active space object removal.Klaus Merz joined ESA in 2002. He works on space object trajectories covering various areas such as orbitprediction (spanning short- to long-term), orbit control and operational orbit determination. In 2010 he joinedESA’s Space Debris Office. His main tasks include operational conjunction analysis and collision avoidance activitiesfor ESA and third-party missions. This includes Launch and Early Orbit Phase support, upgrades of the supportinganalysis software, and support to standardisation at an international level. He is also responsible for studies ofpotential disposal orbits of ESA missions.esa and space debrisPortrait images: ESA–J. Mai
ContactProf. Dr Heiner KlinkradHead of Space Debris OfficeESA/ESOC, Robert-Bosch-Str. 5D-64293 Darmstadt, GermanyTel: +49 6151 90 2295Fax: +49 6151 90 2625Email: Heiner.Klinkrad@esa.intwww.esa.int/debrisESA–P.Carril