For decades, space experts have worried that a speeding bit of orbital debris might one day smash a large spacecraft into hundreds of pieces and start a chain reaction, a slow cascade of collisions that would expand for centuries, spreading chaos through the heavens. In the last decade or so, as scientists came to agree that the number of objects in orbit had surpassed a critical mass or, in their terms, the critical spatial density, the point at which a chain reaction becomes inevitable they grew more anxious. Early this year, after a half-century of growth, the federal list of detectable objects (four inches wide or larger) reached 10,000, including dead satellites, spent rocket stages, a camera, a hand tool and junkyards of whirling debris left over from chance explosions and destructive tests. So our billion dollars of satellites are at risk. As space experts have worried that orbital debris might one day smash a large spacecraft into pieces and start a chain reaction, the scientist recently came to agree that the number of orbital debris had surpassed the critical spatial density, which will inevitably lead to a chain reaction that puts our billion dollars of satellites at risk.