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European Space Agency (ESA)
 

Background and History

Since the mid-1980s, ESA has been active in all relevant research, technology and operational aspects related to space debris. Agency expertise is mainly concentrated at the European Space Operations Centre (ESA/ESOC), Darmstadt, Germany, and the European Space Research & Technology Centre (ESA/ESTEC), Noordwijk, The Netherlands.

The team at ESOC has developed long-standing experience in the areas of:

  • Radar and optical measurements and their simulation
  • Development of space debris and meteoroid environment and risk assessment models
  • Analysis of debris mitigation measures and their effectiveness for long-term environmental stability
  • In-orbit collision risk assessments
  • Re-entry safety analyses
  • Space debris database issues

The team at ESTEC has a strong background in:

  • In-situ impact sensor technology
  • Vulnerability and impact damage analyses
  • Hypervelocity accelerator technologies
  • Hypervelocity impact shielding and protection

Since 2006, ESA has focused space debris responsibilities in ESOC's Space Debris Office, which is an independent team within the Ground Systems Engineering Department of ESA's Directorate of Operations & Infrastructure.

The Space Debris Office coordinates ESA's research activities in all major debris disciplines, including measurements, modelling, protection, and mitigation, and coordinates such activities with national research efforts with space agencies in Italy (ASI), the United Kingdom (BNSC), France (CNES) and Germany (DLR). Together with ESA, these national agencies form the European Network of Competences on Space Debris (SD NoC).

ESA's Space Debris Office provides operational services in support of planned and ongoing missions within ESA and to third parties.

These services comprise in-orbit collision avoidance (forecasts, prediction refinements and avoidance manoeuvre recommendations), re-entry prediction and risk assessment (prediction of re-entry time and location, forecast of spacecraft disintegration and demise and on-ground risk assessment) and maintenance of space situational awareness information on all trackable objects in the DISCOS database (Database and Information System Characterising Objects in Space).

The Space Debris Office has developed and maintains several engineering tools for space debris analysis.

These tools, which are available as ready-to-use, self-standing and self-installing software products, include MASTER-2005 (prediction of debris and meteoroid particle fluxes on user-defined target orbits), PROOF-2005 (planning and simulation of radar and optical debris observation campaigns) and DRAMA (verification of the compliance of space missions with mitigation guidelines).

Since 1984, ESA has organised or co-sponsored several international conferences dealing with space debris. These include the quadrennial series of European Conferences on Space Debris, COSPAR Conferences (Committee on Space Research), IAC Congresses (International Astronautical Congress) and IAASS Conferences (Internal Association for the Advancement of Space Safety)

Analysis and Prediction

The consolidation of knowledge on all known objects in space is a fundamental condition for the operational support activities of ESA's Space Debris Office. This knowledge is maintained and kept up-to-date through the DISCOS database (Database and Information System Characterising Objects in Space).

DISCOS serves as a single-source reference for information on launch details, orbit histories, physical properties and mission descriptions for about 33 500 objects tracked since Sputnik-1, including 7.4 million orbit records in total. A continuous flow of orbit data for all tracked, unclassified objects is provided by the US Space Surveillance Network (SSN).

Today, DISCOS constitutes a recognised and dependable source of space object data that is regularly used by almost 50 customers worldwide. Apart from its use for standard database queries, DISCOS generates several automated products, which include a log of upcoming re-entries, and publication-quality status reports (ESA Register of Space Objects, ESA GEO Log, ESA Fragmentation Events Log).

DISCOS is also instrumental for producing ESA's annual 'Classification of GEO Objects' report.

MASTER - a Debris and Meteoroid Environment Model

ESA maintains and distributes a number of models for the characterisation of the space debris environment and its evolution. The Agency's most prominent debris and meteoroid risk assessment tool is called MASTER (Meteoroid and Space Debris Terrestrial Environment Reference).

It was first issued in 1995 and has been continuously improved. The current release is MASTER-2005 (with MASTER-2009 and PROOF-2009 to be released in early 2011). MASTER uses sophisticated mathematical techniques to determine impact flux (number of impacts per square meter of area and per year) information with high spatial resolution for an object population derived from all known, historic debris generation events. These comprise more than 200 in-orbit fragmentation events, more than 1000 solid rocket motor firings, and 16 reactor core ejections from RORSAT satellites. The model can be used to assess the debris and meteoroid impact flux that a spacecraft would encounter on any arbitrary Earth orbit. The MASTER model covers all debris and meteoroid sizes larger than 1 micrometre.

At small (sub-millimetre) particle sizes, meteoroids can prevail over space debris in some orbital regions, in particular during intense seasonal meteoroid streams, with peak activities close to perihelion passes of the related source comets (e.g. the Leonids in 1966 and 1999). The ESA Interplanetary Meteoroid Model (IMEM) describes the resulting meteoroid environment, also outside Earth-bound orbits.

DELTA - a Tool for Assessing the Effectiveness of Mitigation Measures

In order to study the effectiveness of debris mitigation measures on the debris population stability, long-term forecasts are required to determine future trends as a function of individual mitigation actions. This kind of analysis can be performed with ESA's DELTA tool (Debris Environment Long-Term Analysis).

DELTA is a 2- to 3-dimensional, time-dependent, dynamic debris model, with detailed traffic model and release event data, and with statistically generated collision events, based on local object densities and collision probabilities. It is built on the mathematical principles of MASTER. Starting from a current MASTER population, DELTA analyses the effects of traffic variations and different debris mitigation measures (e.g. explosion prevention, re-orbiting at end of mission, de-orbiting at end of mission, lifetime reduction) on the future evolution and stability of the space debris environment up to super-GEO altitudes.

The time spans covered in such projections are typically 100 to 200 years. The particle sizes considered by DELTA are larger than 1 mm (as compared to 1 micrometre for MASTER), since only these are relevant for risk assessments. DELTA uses a corresponding MASTER debris population as a starting point for its projections. An underlying traffic model statistically distributes the contributions of launches, explosions and solid rocket motor firing events into different orbit regimes of the Earth environment.

The resulting collision risk is dynamically determined from the actual status of the environment at each epoch, and in each altitude regime. Collision events are statistically triggered - and fragments are added to the environment - if a given kinetic energy threshold is exceeded. The so-called 'business-as-usual' scenario (unchanged operational practises) forms the baseline for comparisons with alternative approaches in which mitigation measures are applied. It can be shown that business-as-usual space activities lead to a progressive, uncontrolled increase of object numbers, with collisions becoming the primary debris source within less than 50 years. The removal of mass from orbit turns out to be the most effective way of preventing this collisional cascading process from setting in.

To make their analyses more accessible by mission planners, and by spacecraft designers and manufacturers, ESA has published a "Space Debris Mitigation Handbook." This comprehensive document provides an overview of all major space debris research disciplines, with a large number of tables and charts to characterise the space debris environment, to determine related risks, and to outline effective protection and mitigation measures.

Ground-Based and In-Situ Measurements

Radar Measurements

Space object catalogues, as generated and maintained by space surveillance networks, are limited to larger objects, typically greater than 10cm in low Earth orbits and greater than 1m at geostationary altitudes. These sensitivity thresholds are a compromise between system cost and performance. Knowledge of the meteoroid and space debris environment at sub-catalogue sizes is normally acquired in a statistical manner through experimental sensors with higher sensitivities.

Ground-based telescopes can detect GEO debris down to 10cm in size, ground-based radars can detect LEO debris down to a few mm in size, and in-situ impact detectors can sense objects down to a few micrometres in size. While telescopes are mainly suited for GEO and high-altitude debris observations, radars are advantageous in the low-Earth orbit (LEO) regime, below 2000 km.

ESA collaborates primarily with the operators of the German TIRA system (Tracking and Imaging Radar), located near Bonn, Germany. TIRA has a 34-metre dish antenna operating in L-band for debris detection and tracking (1.333 GHz, 0.45° beam width, at 1 MW peak power). Apart from tracking campaigns, the radar also conducts regular beam park experiments, where the radar beam is pointed in a fixed direction for 24 hours, so that the beam scans 360° in a narrow strip on the celestial sphere, during a full Earth rotation. In such experiments, TIRA can detect debris and determine coarse orbit information for objects of diameters down to 2 cm at 1000 km range. In a bi-static mode, together with the 100m receiver antenna of the nearby Effelsberg radio telescope, the overall sensitivity increases toward 1-cm objects. A special seven-horn receiver, developed for the Effelsberg radio telescope, allows better resolution of object passages, permitting a reliable assessment of the object's radar cross-section.

In Tromsø, Norway, the EISCAT Scientific Association (European Incoherent Scatter Radar) is operating a 930-MHz UHF radar and a 225-MHz VHF radar. Furthermore, they own a 500-MHz radar system consisting of a steerable 32-metre dish and a fixed 42-metre dish in Longyearbyen, Svalbard. The primary mission of the EISCAT network is to perform ionospheric measurements. However, following the development of a dedicated space-debris computer to run at the back-end of the processing units, these radars are now capable of statistical observations of LEO debris down to 2 cm, without compromising the main EISCAT objectives.

The EISCAT radars now allow a continuous monitoring of the LEO debris population in a beam park-type configuration. As an example, EISCAT was able to monitor and characterise China's Feng-Yun 1C debris cloud, generated at 800-km altitude in January 2007, following the worst single fragmentation event in space history.

Optical Measurements

ESA operates a Zeiss telescope of 1-metre aperture that is used for the detection and survey of objects near the geostationary ring. The telescope is equipped with Ritchey-Crétien optics, with an FOV (field of view) of about 0.7°, and a liquid-nitrogen-cooled CCD camera with a 2 x 2 mosaic of 2k x 2k CCD chips.

The telescope can detect and track near-GEO objects up to magnitudes of +19 to +21 (i.e. down to 15 cm in size). With this performance, the ESA telescope is top-ranked worldwide. During GEO observation campaigns, typically 75 percent of all detections are new objects that are not contained in the US Space Surveillance Catalogue.

The data provided by the telescope are a major input for space debris environment models, indicating a much larger number of GEO fragmentation events than confirmed so far (a Soviet Ekran 2 satellite explosion in 1978 and a US Titan Transtage break-up in 1992). Observations of highly eccentric orbits passing through GEO led to the discovery of a class of faint, lightweight objects with high area-to-mass ratios. Orbital characteristics indicate that they could be pieces of thermal blankets of satellites.

Impact Measurements on Retrieved Space Hardware

ESA also gains information on the small-size, sub-millimetre meteoroid-and-space-debris environment through the analysis of retrieved space hardware, such as the EURECA satellite, and the three solar arrays retrieved from the Hubble Space Telescope through the Space Shuttle.

The total exposed surface that was analysed exceeded 300 square metres. The samples contained several thousand impact craters from a few micrometres up to a 7 mm in diameter. An analysis of chemical residues in the craters allowed a discrimination of possible progenitors of the impacts.

In-Situ Impact Detectors

For orbits above 600 km, and for inclinations outside the Shuttle's reach, a retrieval of space hardware is not possible at present, and active in-situ sensors are required to measure impact fluxes. In 1996, the ESA-funded Geostationary Orbit Impact Detector (GORID) was launched into GEO on board the Russian Ekspress-2 satellite. During five years of operation, an average of 2.4 impacts/day were detected, with peak counts of 50 per day.

In 2001, a newly designed Debris in-Orbit Evaluator (DEBIE) was launched into LEO on ESA's PROBA-1 satellite. DEBIE used a combination of impact ionisation, momentum and foil penetration detection for the active monitoring of sub-millimetre particles impacting on the detector surfaces. In 2008, DEBIE II was launched together with the Columbus science module, now docked to the International Space Station (ISS). It is now operated via EUTEF (European Technology Exposure Facility), one of the external Columbus payloads.

PROOF - a Tool for Predicting Sensor Detection Performances

Debris environment models require measurement data of well-defined regions in space, at specific observation times, to validate their predictions. On the other hand, the data return from measurement campaigns can be optimised with respect to search regions, observation times and sensor sensitivity if some a-priori information on the space debris population is available.

To support the planning of observation campaigns and the exploitation of data for improved debris environment models, ESA has sponsored the development of the PROOF software (Program for Radar and Optical Observations Forecasting).

PROOF simulates space debris observation campaigns for ground- and space-based sensors using available population data from the MASTER model. The simulation generates expected detection statistics and characteristics for given radar or telescope systems and for given campaign parameters. For catalogued objects, PROOF performs predictions with a level of accuracy that allows a clear correlation of recorded detections with catalogued objects.

Hypervelocity Impacts and Protection

The consequences of meteoroid and debris impacts on spacecraft can range from small surface pits due to micrometre-size impactors, via clear hole penetrations for millimetre-size objects, to mission-critical damage for projectiles larger than one centimetre. Any impact of a 10-cm catalogue object on a spacecraft or orbital stage will most likely entail a catastrophic disintegration of the target. This destructive energy is a consequence of high impact velocities, which can reach 15 km/second for space debris and 72 km/second for meteoroids.

Since only larger space objects can be catalogued and tracked, only these can be avoided through active measures or by evasive manoeuvres. Smaller, uncatalogued objects can only be defeated by passive protection techniques, as used with the International Space Station (ISS).

The effects of hypervelocity impacts are a function of projectile and target material, impact velocity, incident angle and the mass and shape of the projectile. At low velocities, plastic deformation normally prevails. With increasing velocities the impactor will leave a crater on the target. Beyond 4 km/s (depending on the materials), an impact will lead to a complete break-up and melting of the projectile, and an ejection of crater material to a depth of typically two to five times the diameter of the projectile.

In hypervelocity impacts, the projectile velocity exceeds the speed of sound within the target material. The resulting shock wave that propagates across the material is reflected by the surfaces of the target, and reverses its direction of travel. The superimposition of progressing and reflected waves can lead to local stress levels that exceed the material's strength, thus causing cracks and/or the separation of spalls at significant velocities.

With decreasing target thickness, the effects range from cratering, via internal cracks, to spall detachment, and finally to clear hole perforations.

ESA's space projects use damage assessment tools in combination with debris and meteoroid environment models to predict potential damage from hypervelocity impacts, and to define effective protection measures through shielding and design.

ESA experts have been actively involved in the development and testing of protective shields for the Columbus manned modules of the ISS.

Protection is achieved through stuffed Whipple shields with aluminium and Nextel-Kevlar bumper layers. The shields are composed of an external, thin bumper shield that is exposed to the debris flux and causes the impactors to completely disintegrate during impact. The resulting cloud of liquid projectile and target material that forms behind the bumper leads to a much wider spatial and temporal distribution of momentum, allowing the back wall of the shield to withstand the impact pressure.

Intermediate fabric layers further slow down the cloud particles. Today, these shields have reached a mature state of development.

Today, ESA's impact protection research activities concentrate on quantifying the expected failure rates and failure characteristics of unmanned spacecraft due to space debris and meteoroid impacts. The aim is to reduce the design margins required for no structural perforation, as required by manned modules.

Material models for composite materials under very high strain rates have been developed for Nextel and Kevlar. These models have been used to verify the structural protection of several ESA spacecraft, including Columbus and ATV.

Collision Avoidance

Apart from protection and shielding, the effects of debris impacts can be best mitigated by avoiding their occurrence in the first place. This, however, can only be done, if the orbits of the debris and target object are known with sufficient accuracy. For initial assessments, the information provided by the US Space Surveillance Network catalogue is sufficient to predict all close fly-bys (conjunctions) of a target satellite with any of the known catalogue objects (more than 20,000 in 2010).

ESA's Space Debris Office routine screens close conjunctions between the Agency's LEO spacecraft (currently ERS-2, Envisat and CryoSat-2) and all known catalogue objects. Conjunction event predictions are performed every day, for seven days ahead, using automatically retrieved catalogue data, operational orbit files and environmental data for the orbit propagation.

The collision risk is determined as a function of the object sizes, the predicted miss distance, the fly-by geometry and the orbit uncertainties of the two objects involved. Results of this process are provided daily, by email, in the form of automatically generated conjunction event bulletins, indicating all relevant data for the assessment of the 10 top-ranking risk events. This includes approach geometry, miss distance, collision probability, detailed information on the chaser object (geometry, mass, origin, type) and information on orbits and orbit uncertainties.

If a customer-defined collision risk level is exceeded, an alert message is automatically issued. In such cases, improved orbit information of the chaser object is generated from dedicated radar campaigns (e.g. using the TIRA radar). The resulting orbit uncertainties are mostly reduced by two orders of magnitude. Hence, in most cases, an avoidance manoeuvre is not necessary even if the fly-by distance remains small (e.g. within 300m). As a consequence of the FengYun-1C anti-satellite test in 2007 and the collision between Iridium-33 and Cosmos-2251 in 2009 the collision risk for ESA spacecraft in LEO has significantly increased. As a results, in total 9 avoidance manoeuvres were conducted in 2010 (4 for Envisat, 4 for ERS-2, and 1 for CryoSat-2).

Re-Entry Predictions

Every day satellites, rocket stages or fragments thereof re-enter into the denser layers of the atmosphere, where they usually burn up. Shortly before re-entry, at about 120 km altitude, spacecraft have velocities of typically 28 000 km/hour.

In the last 10 minutes before reaching ground, the dense atmosphere starts to heat up and decelerate the spacecraft. In the case of very compact and massive spacecraft, and if a large amount of high-melting material is involved (e.g. stainless steel or titanium), fragments of the vehicle may reach the Earth's surface.

Well-known examples of large-scale re-entry events were Skylab (74 tonnes, July 1979), Salyut-7/Kosmos-1686 (40 tonnes, February 1991) and Mir (135 tonnes, March 2001). In such cases, up to 20 to 40 percent of the spacecraft mass may impact the surface.

ESA's ATV (Automated Transport Vehicle) performed a controlled and safe re-entry into an uninhabited area in the South Pacific Ocean on 29 September 2008. The re-entry break-up process was monitored from two observation aircraft.

For people and property on the ground, the hazards posed by re-entering spacecraft or debris are extremely small. So far, there has been only one reported injury and no fatality (except for crew fatalities during manned vehicle re-entry).

The controlled or uncontrolled re-entry of space systems is, however, associated with a number of legal and safety aspects that must be considered. This risk due to re-entries can be determined through analysis of surviving fragments (if any), their dispersion across a ground swath, and the resulting casualty risk for the underlying ground population distribution.

Re-entry manoeuvres can be optimised to control the impact footprint (ideally over an ocean area), and thus maintain the casualty probability below an acceptable risk threshold (e.g. less than 1 in 10,000 for a single re-entry).

In the case of uncontrolled re-entries, the re-entry time window and impact footprint can be predicted and monitored. The quality of this process can be improved through tracking data and sophisticated orbit prediction tools. ESA has all necessary capabilities to provide analysis of both controlled and uncontrolled re-entries. This includes detailed simulations of the aero-thermal and structural break-up of satellites or orbital stages, the prediction of the orbit and attitude of each re-entry fragment, the identification of objects reaching ground and the analysis of associated risk potentials for the population in the entry ground swath. These tools have been used, for instance, for re-entry assessments for ATV, Beppo SAX, TerraSAR, GOCE, Ariane-4 and Ariane-5.

ESA's Space Debris Office also maintains a Web-based re-entry data exchange service that is used by the 12 members of the Inter-Agency Debris Coordination Committee (IADC) to monitor the re-entry of risk objects and to exchange orbit determination and re-entry prediction results.

Contributions to ESA's Space Situational Awareness Programme

ESA's Space Debris Office has also been a forerunner in the definition of a European Space Surveillance System. This project is now part of the preparatory program of the more comprehensive Space Situational Awareness programme. The Space Debris Office supports related research activities on sensor design options, system performance requirements and catalogue maintenance concepts.

Space Debris Mitigation Policies and Guidelines

Space Debris Mitigation Requirements at ESA

In 2002, the Inter-Agency Debris Coordination Committee published the "IADC Space Debris Mitigation Guidelines," and presented these to the UNCOPUOS Scientific & Technical Subcommittee (STSC), where they served as a baseline for the "UN Space Debris Mitigation Guidelines."

In 2007 these guidelines were approved by the 63 STSC member nations as voluntary high-level mitigation measures. Since the mid-1990s, space agencies in Europe have developed more technically oriented guidelines as a "European Code of Conduct," which was signed by ASI, BNSC, CNES, DLR and ESA in 2006.

The core elements of this Code of Conduct are in line with the IADC and UN guidelines. In order to tailor the Code of Conduct to the needs of ESA projects, ESA developed their own "Requirements on Space Debris Mitigation for Agency Projects" (ESA/ADMIN/IPOL(2008)2 Annex 1). These instructions came into force on 1 April 2008. They are applicable to all future procurements of space systems (launchers, satellites and inhabited objects).

Cooperation on International Debris Mitigation Standards

Space debris mitigation guidelines provide a framework for 'what' needs to be done. The way 'how' mitigation measures must be implemented is specified in a more formal manner, via international standards - or via binding national requirements for the design and operation of space systems. Such common standards guarantee a level field for industrial competition and for safe access to space into the future. International debris mitigation standards are presently being developed at ISO.

Experts from ESA regularly support these developments and their harmonisation with existing guidelines and requirements. The ultimate ISO standards on space debris mitigation, however, will remain non-binding (as is true for any ISO standard).

Contact

Prof. Heiner Klinkrad
Head of Space Debris Office
ESA/ESOC
Robert-Bosch-Str. 5
64293 Darmstadt, Germany

Tel: +49-6151-90-2295
Fax: +49-6151-90-2625
email: Heiner.Klinkrad@esa.int