Space Debris.

Space debris, also known as orbital debris, space junk, and space waste, is the collection of objects in orbit around Earth that were created by humans but no longer serve any useful purpose. These objects consist of everything from spent rocket stages and defunct satellites to erosion, explosion and collision fragments. As the orbits of these objects often overlap the trajectories of newer objects, debris is a potential collision risk to operational spacecraft.Whipple's work pre-dated the space race and it proved useful when space exploration started only a few years later. His studies had demonstrated that the chance of being hit by a meteor large enough to destroy a spacecraft was extremely remote. However, a spacecraft would be almost constantly struck by micrometeorites, about the size of dust grains.Additionally, the momentum of the photons in the laser beam could be used to impart thrust on the debris directly. Although this thrust would be tiny, it may be enough to move small debris into new orbits that do not intersect those of working satellites. NASA research from 2011 indicates that firing a laser beam at a piece of space junk could impart an impulse of 0.04 inches (1.0 mm) per second. Keeping the laser on the debris for a few hours per day could alter its course by 650 feet (200 m) per day. A similar proposal replaces the laser with a beam of ions.
A number of other proposals use more novel solutions to the problem, from foamy ball of aerogel or spray of water inflatable balloons, electrodynamic tethers, On January 7, 2010 Star Inc. announced that it had won a contract from Navy/SPAWAR for a feasibility study of the application of the ElectroDynamic Debris Eliminator (EDDE), or dedicated "interceptor satellites.
As of 2006, the cost of launching any of these solutions is about the same as launching any spacecraft. Johnson stated that none of the existing solutions are currently cost-effective.