European Space Agency (ESA)   Introduction to Space Debris IN 43 YEARS OF SPACE ACTIVITIES SOME 4100 LAUNCHES led to more than 26000 observable space objects (larger than 10 cm) of which currently 8700 are still on orbit. Only 7% of the catalogued orbit population are operational spacecraft, while 50% can be attributed to decommissioned satellites, spent upper stages, and mission related objects (launch adapters, lens covers, etc.) The remainder of 43% is originating from 160 on-orbit fragmentations which have been recorded since 1961. These events, all but 1 or 2 of them explosions of spacecraft and upper stages, are assumed to have generated a population of objects larger than 1 cm on the order of 100000 to 150000. Only near sizes of 0.1 mm the sporadic flux from meteoroids prevails over man-made debris. General Space Debris Objectives To assess the risk potential of collisions of man-made or natural particulates with operational spacecraft, one must resort to statistical models of the particle population for all size regimes except for man-made debris above 10 cm. In the latter case, collision events or near-miss events can be predicted on the basis of orbital data from operational surveillance networks of the USA or of the CIS. In the former case, collision fluxes can only be estimated statistically. - For the small-size debris current research concentrates on better estimates of the spatial distribution, and on improved mathematical fragmentation models to generate such a population. Supporting data come from in-situ measurements (e.g. LDEF, EURECA, HST), from ground-based experimental radar and optical observations (e.g. Haystack, FGAN radars-, ESA's 1 meter ZEISS telescope and NASA's liquid mirror telescope), and from on-ground fragmentation tests (both collisions and explosions). Currently, space debris between 1 cm and 10 cm are neither observable, nor are they shieldable with available on-orbit technology. Hyper-Velocity Impact (HVI) tests are used to experimentally verify and improve shields for on-orbit use (e.g. for the International Space Station), with the aim to increase the shieldable impactor size beyond 1 cm. Experimental radar observations are used on the other hand to decrease the threshold of observable objects below 10 cm, and to further narrow down the range of unobservable, undefeatable objects. Ground Operations The Mission Analysis Section of ESOC is coordinating all Space Debris Research Activities within ESA. ESA's activities are harmonised with European national space agencies, and with specialists from national organisations and institutes in Europe (via the Space Debris Advisory Group SDAG) and outside Europe (via the Inter-Agency Space Debris Coordination Committee IADC, consisting of ESA, Japan, NASA, the Russian Avation and Space Agency Rosaviakosmos, the Chinese National Space Administration CNSA, ASI of Italy, BNSC of the UK, CNES of France, DLR of Germany, ISRO of India and NSAU of the Ukraine).< ESOC's main debris activities are oriented towards space debris and meteoroid environment models for the present and for the future, optical and radar observation campaigns of debris, prediction of re-entries of risk objects, maintenance of a database on known ("catalog") space objects called DISCOS, and risk assessment for meteor showers like the Leonids in 1998 and 1999. The MASTER Debris and Meteoroid Environment Model: ESOC's Meteoroid and Space Debris Terrestrial Environment Reference (MASTER) model can be used to assess the debris or meteoroid flux imparted on a spacecraft on an arbitrary earth orbit. MASTER uses quasi-deterministic techniques to determine flux information with high spatial resolution from a debris population derived from historic fragmentation events. The software, data, and documentation of MASTER is available on CD ROM for many different computer platforms. Long-term forecasts of future trends in the evolution of the space debris population can be performed with softwares like CHAINEE and SDM, which allow to study the effectiveness of debris mitigation measures (e.g. reduction of on-orbit explosions, de-orbiting of satellites at end of life) with respect to debris population stability. The most recent release is the MASTER99 model. Radar and Optical Observation Campaigns: ESOC is preparing optical observations of high altitude orbits (e.g. GEO) with a 1 meter telescope located at the Teide observatory (Canary Islands). Advanced Radar measurement techniques for the detection of sub-decimeter size objects in Low-Earth Orbits (LEO) are developed and applied by the German FGAN radar (at Wachtberg). Prediction of Re-Entries of Risk Objects: ESOC has been providing forecasts of re-entry time and location of risk objects which decay into the earth atmosphere since Kosmos 1402 A and C (re-entry of a reactor core in Jan/Feb 1983). Further campaigns were initiated for Salyut-7/Kosmos-1686 in Feb'91 (40 ton spacestation), for Progress M-17 in Mar'94, and for Kosmos-398 in Dec'95. Re-entry forecasts are distributed to national points of contact within ESA Member States. The DISCOS Database: ESOC maintains a database on spacecraft launches since 1957 (with currently 4000 entries) and on the catalogued on-orbit population since 1989 (with currently 3.3 million orbit states). DISCOS is a relational database with high flexibility on search criteria, and with statistical and graphical post processing capabilities. The database includes bibliographic references of space debris publications. - DISCOS can be accessed by registered users via the Internet. The ESA Space Debris Mitigation Handbook: The purpose of ESA's Space Debris Mitigation Handbook, issued in 1999, is to provide technical support to projects in the following areas: - description of the current space debris and meteoroid environment - risk assessment due to debris and meteoroid impacts - future evolution of the space debris population - hyper-velocity impacts and shielding - cost-efficient debris mitigation measures. European Space Debris Safety and Mitigation Standards: A joint development of ESA, ASI, BNSC, CNES and DLR is the elaboration of European Space Debris Safety and Mitigation Standards. Drafting of the standards has started in 1999. Other Activities At ESA's Space Research and Technology Centre ESTEC (Noordwijk/NL) hyper-velocity impacts, shielding- and damage assessment are investigated. ESTEC is also concerned with in-situ measurements and post-flight impact analysis. Jointly with specialized research institutes the impacts of micro-debris and meteoroids have been analyzed and the derived particulate fluxes compared with existing debris and meteoroid models. A database has been developed which contains the major results, reports and images of the EURECA and HST impact analyses (http://www.estec.esa.nl/madweb/). A standard small-size particle detector DEBIE was developed which will be flown on the PROBA spacecraft in 2001. ESA's Launcher Directorate (at ESA Headquarter, Paris) coordinates the implementation of debris mitigation measures for the ARIANE launcher.