Undated (AP) _ Most of the thousands of visible objects orbiting Earth are junk, and most of that junk is rocket fragments.

Other manmade trash includes dead satellites, spent rocket bodies, paint chips, particles from rocket firings, even frozen drops of urine that melt in orbit a few days after being flushed from spaceships.

The oldest piece of junk still being tracked by the U.S. Space Command is the Vanguard 1 satellite, launched in 1958. The reason it's junk is it no longer works; its radio transmitter went dead in 1964.

Anything that's tossed overboard or lost also becomes space junk, like Gemini astronaut Edward White II's glove, which floated out of his spacecraft in 1965, and a camera that got away from an American spacewalker in 1966. Both objects should have re-entered the atmosphere long ago.

Most junk burns up when it eventually plunges through the atmosphere. One notable exception was Skylab, which re-entered the atmosphere in 1979, raining debris into the Indian Ocean and on remote areas of Australia.