Earth's orbit is already littered with bits of debris, which have been accumulating since space exploration began. Space junk can reach relative velocities of 30,000 mph-speeds at which it can cause serious damage. U.S. Space Command tracks and catalogs debris that's larger than 10 square centimeters-of which there are roughly 8,900 individual pieces. Debris bigger than 1 square centimeter can destroy a space shuttle or a satellite, which means NASA can only cross its fingers. The agency replaces, on average, one shuttle window per flight because of collisions with small bits of space debris, and some argue that it's only a matter of time before something larger causes a serious problem. Indeed, in 1996 the French satellite CERISE was knocked off its orbit in 1996 when a chunk of an old rocket smashed into its stabilization boom. CERISE was able to return to orbit, but other craft may not be so fortunate.