The robots that will sweep Earth's skies

After six years in space, China's first orbital station, the Tiangong-1 (aka the "Heavenly Palace") has finally outlived its operational limits and begun its descent back to Earth. It's expected to re-enter the atmosphere in a few months, whereupon a majority of the 9.3-ton station should burn up before reaching the surface. This is how defunct satellites are supposed to be disposed of. Unfortunately, until very recently, that hasn't often been the case. For the past 50 years, we've been filling Low Earth Orbit with defunct satellites, launch vehicle upper stages, and various bits of broken spacecraft (including frozen water, coolant and paint flecks).