STS-121

STS-121 was a 2006 Space Shuttle mission to the International Space Station (ISS) flown by Space ShuttleDiscovery on its 32nd flight. 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 (ESA) astronaut Thomas Reiter to the ISS. The European segment of the mission was called "Astrolab".

After two weather-related delays, the shuttle successfully launched on Tuesday, July 4, 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 July 17, 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.