A defunct spacecraft (or pieces of spacecraft) that remain in orbit after the end of a mission. This space junk ranges from lens covers to spent upper stages (see multistage rocket) of craft and decommissioned satellites. There is concern that high-speed collisions between such objects will eventually produce even more debris, as well as posing a potential hazard to new spacecraft and to astronauts' spacesuits during extravehicular activities. In 1996, the antenna of a French satellite, Cerise, was severed by a piece of an Ariane 4 third-stage rocket. The first-ever accidental in-orbit collision between two satellites occurred on 10 February 2009, between an Iridium satellite and a Russian military Kosmos satellite. Both were destroyed, and more than 2 300 trackable fragments were generated, some of which have since re-entered and burned up.

In almost 60 years of space activities, more than 5 250 launches have resulted in some 42 000 tracked objects in orbit, of which about 23 000 remain in space and are regularly tracked. About 24% of the catalogued objects are satellites (less than a third of which are operational), and about 18% are spent upper stages and mission-related objects such as launch adapters and lens covers. Fragments often come from explosions of upper stages and spacecraft, adding to some 750 000 pieces of space debris larger than 1 cm. More than 290 in-orbit fragmentation events have been recorded since 1961.

http://www.esa.int/Our_Activities/Operations/Space_Debris/About_space_debris The European Space Agency (ESA) coordinates research into space debris through the Mission Analysis Section of its European Space Operations Centre (ESOC) at Darmstadt, Germany. As well as assessing the risk of collisions between operational spacecraft and human-made or natural objects, ESOC provides forecasts of re-entry times and locations for space debris.