In 2009, nearly 500 miles above Siberia, two satellites collided at some 22,300 mph, bursting into a cloud of thousands of pieces of debris. The culprits of this high-speed accident were the inactive Russian satellite Cosmos 2251 and the active U.S.-based communication satellite Iridium 33. Their catastrophic end was the first known time that two satellites collided in space, and a startling reminder of the growing problem of space junk.