Space junk cluttering earth
Old satellites, old boosters and other orbital rubbish has been estimated at 22000 pieces large enough to track from the ground. Some debris flies around the earth at speeds of up to 17500 mph / 28164 km. This can lead to serious problems for space travel as even smaller objects can cause serious damage to space vessels. A secondary problem is that this space junk can collide resulting in more, smaller, objects orbiting our earth.
See BBC‘s article for more on this topic.
Although there seems to currently be no recommendations on how to control our space environment, there have been a few suggested solutions. One main idea is to move the debris closer to earth where it will burn up in our atmosphere. Personally, I think this is risky. Just a nudge too hard could send that debris bypasing the atmosphere and crashing to earth instead.