Other NASA scientists worry about getting astronauts off the ground or sending interplanetary probes to Pluto. Nicholas Johnson worries about a nightmare scenario called the Kessler syndrome, named for his colleague Donald Kessler, who first described it in the 1970s. The scenario begins in an overcrowded orbit. Two massive pieces of hardware—satellites, say, or spent booster rockets—slam together at more than 20,000 miles an hour, smashing each other into hundreds of pieces. One piece then collides with another spacecraft, creating hundreds more pieces—and so on in a slowly building chain reaction that culminates in a belt of space shrapnel too dense for anything to traverse safely.
Until last year, says Johnson, chief scientist at NASA’s Orbital Debris Program Office, “the danger was purely academic.” But on February 10, 2009, the world witnessed its first full-blown hypervelocity crack-up. An Iridium communications satellite collided with a defunct Russian satellite 500 miles above Siberia. That one alarming mishap added about 2,000 large pieces to the cloud of debris orbiting Earth.