The growing amount of defunct satellites in Earth's orbit, i.e. space junk, could complicate spaceflight missions in the future: "The U.S. military currently tracks about 20,000 pieces of junk in low-Earth orbit, most of which are discarded bits of spacecraft or debris from collisions in orbit. The atmosphere naturally drags a portion of this refuse down to Earth every year. But in 1978, N.A.S.A. astronomer Don Kessler predicted a doomsday scenario: As collisions drive up the debris, we’ll hit a point where the amount of trash is growing faster than it can fall out of the sky."