We are still waiting to find out what happened to make the Columbia break up as it entered Earth's atmosphere.
There have been three other major accidents which have happened to Nasa since it was formed in 1958:
Date:27 January 1967
Mission: Apollo 1, part of the attempt to go to the moon.
What happened: A fire in a capsule in a test on the ground.
Casualties: Astronauts Roger B Chaffee, Virgil 'Gus' Grissom and Edward H White Jr.
- Date:13 April 1970
Mission: Apollo 13, part of the attempt to go to the moon.
What happened: An oxygen tank burst on the way to the moon.
Casualties: None. All three crew members made it back to Earth.
- Date:28 January 1986
Mission: Challenger space shuttle on a mission into space
What happened: A leak made the fuel tanks explode.
Casualties: All seven crew were lost.