In January 2007, China successfully tested an anti-satellite missile, destroying one of its redundant spacecraft orbiting about 800km (1,290 miles) above the Earth. In April 2011, one of the 3,000 pieces of debris that test created passed close enough to the International Space Station, orbiting at around 400km, to raise concerns of a collision.
Those 3,000 pieces are matched by about the same number left over from the February 2009 collision between an active Iridium communications satellite and a Russian satellite whose orbit was degrading. In all, the US military's Joint Space Operations Center (JSOC) tracks some 22,000 orbiting items larger than about 10cm (3.9in) across - and, as the chart on this page shows, that number has roughly doubled since 2000. The vast majority of those items are debris, rather than payloads or rocket bodies, and there are another 700,000 or so pieces at around the 1cm size.
Through the US State Department, the JSOC issued about 1,000 warnings in 2010 to foreign governments that a space asset was at risk of collision. Many of those warnings resulted in avoidance manoeuvres, which consume precious propellant a satellite would normally use to maintain orientation.
While some debris is an unavoidable consequence of launching a rocket, one senior US official told Flight International that "if we don't make some changes, we won't be able to use space". The USA, this official says, "learned our lesson" after a 1985 anti-satellite test left debris near the ISS.
Stressing the need for all space-faring nations to take debris seriously, this official adds that China has behaved "very irresponsiblybut we are hopeful"; a January 2011 "missile defence" test by the Chinese was followed within an hour by a statement assuring the world the incident was a test - that left no debris. However, the US position is that another two or three ASAT tests would threaten the use of low-Earth orbits.
The gravity of the debris issue was highlighted earlier this month by the European Commission, whose "reinforced" space policy calls for, among other goals, the establishment of an autonomous European space situational awareness system to track debris and monitor solar weather, two hazards which do €332 million ($480 million) in damage to European assets annually.
What Europe is proposing - and actively working to define - is a system of comparable performance to what the USA is operating. Holger Krag, a space debris analyst at the European Space Agency's debris office, describes the ESA concept as a single radar installation somewhere in Europe supplemented by some 20 optical telescopes at four sites equally spaced near the Equator.
The radar would be capable of tracking, and thus calculating the orbit of, objects of about 5cm size at altitudes around 800km, with capability decreasing with altitude; 10cm objects could be tracked at 1,200-1,300km and 30-40cm objects at 2,000km. However, radar cannot track objects at the 36,000km orbits of geosynchronous communications satellites, so to protect them the optical telescopes become necessary. Some space-based telescopes could ultimately be added to the system.
Tracking capability gives spacecraft operators some warning of a collision, but Krag notes that scarce resources have to be directed to averting "catastrophic" collisions that create more debris. A 1cm object carries enough energy to disable a satellite but a 10cm object is powerful enough to break it apart, hence the focus on objects of this size or larger. The ISS is unusual in being shielded against 1cm impacts, though Krag admits there are larger objects which fall in the "gap" between what the ISS is protected against and the objects which can be detected.
Also important, says Krag, is for space-faring nations to agree operational measures to reduce debris, including designing rocket bodies so they do not explode when releasing remnant energy and, perhaps, agreeing to safely de-orbit satellites from densely populated orbits - below around 2,000km - after their useful lives.
Ultimately it may be necessary to actively remove some debris from orbit. No reasonably affordable or technically feasible scheme could address anything but the largest pieces of debris, including defunct satellites. Active debris removal would involve launching robotic missions directed at achieving controlled re-entry of individual pieces of debris. Krag says dealing with five to 10 large objects a year would be worthwhile, as they are potential targets for debris-creating collisions.
However, there will be no active debris removal in the near future. Germany is working toward a test mission, but a launch is at least 10 years away, says Krag.
A European space situational awareness (SSA) capability is much closer to reality. Nicolas Bobrinsky, head of ESA's SSA programme office, foresees a 2012 or 2013 start to a five-year first phase project in conjunction with the European Union that would both bring together existing European assets into a coherent, nascent SSA system as well as build a radar system, the first optical telescopes and related ground infrastructure. That first phase would also see the beginning of development of space weather monitoring satellites.
While the goal is to provide Europe with autonomous capability - the US system could at some point be down or otherwise unavailable - it is also important to be compatible with the US system which, as advanced as it is, cannot track everything in orbit. Much more can be achieved together, says Bobrinsky, and talks are advanced on "how the systems can evolve together", especially through a common data standard.
Critically, ESA's approach is also incremental, to accommodate investment in stages for what Bobrinsky readily admits will be an "expensive system"; the budget could run to €600-700 million over the first five years.