Last November cosmonaut Alexander Kaleri was onboard the International Space Station (ISS) when he heard a loud bang. Kaleri didn't believe the sound was from balky equipment; rather it seemed to originate from outside. This past April the ISS crew reported hearing a similar clang. NASA has doubts whether the sounds really came from space junk hitting the station. But the noises have engineers paying renewed attention to the threat of orbital debris, which can act as missiles.

Space junk dates back to the beginning of the Space Age. The oldest known hunk is Vanguard 1, launched by the U.S. on March 17, 1958. Forty-six years later the number of known orbital objects at least 10 centimeters wide has grown to nearly 11,000, and only several hundred of those are operational satellites, according to the U.S. Space Command in Cheyenne Mountain, Colo., which monitors these objects. Material in the lowest altitudes flies at around seven to eight kilometers a second. At that velocity, debris just a few millimeters wide would have the impact of a bowling ball moving at highway speeds.

This article was originally published with the title Eye on the Junk.

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