STS-121 was a 2006 NASA Space Shuttle mission to the International Space Station flown by Space Shuttle Discovery The main purposes of the mission were to test new safety and repair techniques introduced following the Columbia disaster of February 2003 as well as to deliver supplies, equipment and European Space Agency astronaut Thomas Reiter from Germany to the ISS.
After two weather-related delays, the shuttle successfully launched on Tuesday, 4 July 2006 at 14:37:55 EDT. It was the first and only shuttle launch on the United States' Independence Day. The mission lasted for 13 days before landing at the Kennedy Space Center on 17 July 2006 at 09:14:43 EDT.
STS-121 was also designated the ISS Assembly Mission ULF 1.1. As the mission followed on from STS-114 in carrying out the recommendations made in response to the Columbia Accident Investigation Board report, it was considered a Return to Flight test mission. Its successful launch and landing led NASA to fully resume regular Space Shuttle launches in the construction of the ISS.