In February 2003 the space shuttle Columbia broke up as it returned to Earth. All seven crew were killed. The accident was caused by insulating foam from the external fuel tank breaking off during launch and damaging the wing. This allowed superheated gas to penetrate the craft during re-entry.
Since the accident, Nasa has been working to modify both the shuttle and its working practices to cut the risk of damage from debris, improve detection of damage should it occur, and develop ways of making in-flight repairs.
The launch of Discovery in July 2005 marked a successful return to flight, but the mission was beset by problems when images revealed that once again, foam had broken away from the fuel tank during launch, prompting the need for further changes.