Another name for minefield is junk - space junk. Satellites that no longer work, pieces of spent rockets, debris from collision of man-made objects - together they form a floating landfill that orbits the Earth at speeds of over 18000 miles per hour. The European Space Agency estimates that there are hundreds of thousands of pieces of space junk.

Today, we'll look first at the history of space junk, then at the way it's affecting space exploration, and finally at some proposals scientists have advanced to clean it up.

I first learnt about space junk last year in my Physics class. Now, after doing research for this speech, I understand why there's an international effort to clean up the landfill whirling around our planet.

Tonight, after the sun goes down, I'd like you to step outside, lift your eyes to the heavens, and gaze at the night sky. If it's clear, you'll be able to see the moon, the stars, and distant planets. Yet right outside our atmosphere is something not so beautiful, something that Scientific America calls "a minefield in Earth orbit".