There's a lot of space junk—or, as NASA calls it, "orbital debris"—circling high above our heads: around 19,000 objects larger than 10 centimeters, 500,000 between 1 and 10 cm in size, and tens of millions of pieces smaller than 1 cm. Generally, all that junk isn't much of a problem. If it falls towards Earth, it burns up before it reaches the surface; no one has ever been harmed by falling space debris and no property on the surface has ever been destroyed. Spacecraft are often hit by tiny bits of junk without any damage, and they have shields to protect from larger debris. The likelihood of collision with one of the larger pieces is low. It does happen, though: in 2009, a U.S. Iridium satellite was destroyed in a collision with a Russian communications satellite that had been deactivated in 1995.
How long a piece stays in orbit depends on its altitude: at 600 kilometers, a piece will stay up for only a few years, but at 1,000 km or higher it can orbit the Earth for a century. The build-up of greenhouse gases in the atmosphere is lengthening the time that all that junk stays in the atmosphere, according to new research. At higher altitudes, excess greenhouse gases cools the air and makes it less dense and decreases the drag on the debris. (This doesn't mean greenhouse gases are causing global cooling. The upper altitudes are cooler because greenhouse gases are trapping heat closer to the surface.) Space.com explains:
Even though air density at these altitudes is only about a billionth of that of the Earth's surface, it still provides sufficient drag to slow down objects in low-Earth orbit (LEO), causing their eventual re-entry. As the atmospheric density in the thermosphere decreases, however, debris can remain in orbit up to 25 percent longer, said Hugh Lewis, from the School of Engineering Sciences.
There are plenty of ideas on how to clean up the skies: laser brooms, giant helium-filled balloons, rockets filled with water, nanosatellites with solar sails, a ballistic orbital debris removal system. (That list doesn't even include any of the proposed solutions from the science fiction world.) DARPA, the Defense Department's research arm, recently added orbital debris cleaners to its wish list.
NASA notes that the best way to deal with space junk it "to prevent the unnecessary creation of additional orbital debris." That isn't always a successful goal, however, as the tale of the astronaut who dropped a $100,000 tool kit illustrates. So if you've got a good solution for getting rid of the millions of pieces that are orbiting overhead, NASA or DARPA just might be interested.