A US-owned Iridium satellite has hit a defunct Russian satellite in an unprecedented space collision. The crash occurred some 790km (491 miles) over Siberia on Tuesday, according to NASA, and produced a "massive" cloud of debris. About 600 pieces are being tracked from the debris field in hopes of understanding the risk they present to other satellites and the international space station. The Russian craft was identified as the 950kg (2,094 pound) Cosmos 2251, a communications relay station launched in 1993 and believed to have been non-operational for the last 10 years or so. The Iridium telecommunications satellite was estimated to weigh about 560kg (1,234 pounds).
