(Spoiler Alert for the movie Gravity)
If you've seen the movie Gravity, it shows the potential dangers of space debris where it takes out the Space Shuttle that Sandra Bullock and George Clooney needed to get back to Earth. Scientist have said that the film represents a scientific possibility. As space debris collides with other space debris it causes them to break apart; creating even more space debris, which increases the chance of debris hitting other debris.
This is a serious threat to the future of space travel and would require it to be dealt with in the next 20 years. If it isn't dealt with it could cause the destruction of $billion satellites.