Part one of six. On the 1 February 2003, and only minutes from the end of its 16-day mission, Space Shuttle Columbia exploded over Texas, USA instantly killing the seven astronauts on board and leaving a mass of controversy in its wake. Horizon investigates what really happened on that fateful day with revealing insights from the people who actually dealt with the tragedy, beginning with how the shuttle was initially modified for scientific testing. Clip taken from the BBC Horizon programme The Last Flight of the Columbia Watch more high quality videos on the new BBC Worldwide YouTube channel here: http://www.youtube.com/bbcworldwide