aware of the threat an orbital impact during space missions and monitors this junk constantly.The agency estimates that there might be more than 3000 satellites orbiting Earth. In Februaly 2008,for example,the United States blew up a satellite that contained toxic fuel.Is was exploded over the Pacific Ocean so that it could not fall and harm humans.Some scientists speculate that one day there might be orbiting "garbage collector" machines to pick up the debris.