Documentary revealing the scale of the problem of debris in space, with Earth surrounded by hundreds of millions of items moving at 17,000 miles per hour. The International Space Station had to move three times in 2014 to avoid potentially lethal chunks of junk and the Horizon team reveals how the US government is investing $1 billion to track space debris, while companies around the world are developing ways to clear up the mess, from nets and robot arms to harpoons.