Part four of six. Having exploded at over 200,000 feet, the shuttle wreckage had been spread over a vast area. Yet even without the recovery of much of the debris, investigators have still been able to pinpoint when and why the disaster happened. Clip taken from the BBC Horizon documentary The Last Flight of the Columbia. Watch more high quality videos on the new BBC Worldwide YouTube channel here: http://www.youtube.com/bbcworldwide