The near-Earth space is filled by more than 300000 artificial debris particles with diameter larger than 1 cm. This population is similar to the asteroidal one because its long term evolution is affected by high-velocity mutual collisions. The space where the debris is placed can be divided into three main regions: the Low Earth Orbit (LEO), below about 2000 km, the Medium Earth Orbit (MEO), between 2000 km and about 36000 km, and the Geosynchronous Earth Orbit (GEO) at about 36000 km of altitude.
As dramatically evidenced by the impact between the satellites Iridium 33 and Cosmos 2251 on Feb. 10, 2009, the possibility of collisions in space is a very serious risk for all the space assets. After the Iridium event the need for active collision avoidance, space traffic management and effective mitigation measures became apparent to all the space operators.
All the un-classified spacecrafst currently in orbit are catalogued by the United States Strategic Command (USSTRATCOM) in the Two–Line Element (TLE) catalogue. In this catalogue about 15000 objects are listed along with their current orbital parameters. The limiting size of the objects included in the catalogue is about 5 to 10 cm below a few thousand kilometers of altitude and about 0.5 - 1 m in higher orbits (up to geostationary). Only about 6% of the objects in the TLE catalogue are operative satellites. Approximately 24% are composed of non-operative spacecraft; around 17% of upper stages and about 13% of mission related debris . Finally, some 40% are fragments. To produce and maintain such a catalogue a large number of optical and radar observations are routinely performed by the United States Space Surveillance Network. Nowadays also Europe has launched its Space Situational Awareness (SSA) initiative aimed at increasing the knowledge of the circumterrestrial environment. In this context the availability of efficient methods and algorithms for accurate orbit determination is extremely important.
The team of SpaceDyS has a longstanding experience and excellence in the field of space debris. It participated, since the early 90s', in the development of the first European models for the study of the long term evolution of the space debris population, pointing out the risk of proliferation of collisions and the need for mitigation measures. As an example, back in 1999, more than 10 year before the actual event, in a seminal paper on the journal Nature, it warned about the risk and consequences of a fragmentation in the Iridium constellation. Since then, the team continued to develop theories and tools in the field, reaching peaks of excellence in the orbit determination and correlation of space objects. Many publications in international journals and conferences testify its continued interest in the subject.