STS-121 was a 2006 NASA Space Shuttle mission to the International Space Station (ISS) flown by . 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 German European Space Agency (ESA) astronaut Thomas Reiter to the ISS. 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. Crew Crew notes Thomas Reiter's position was previously planned to be filled by Sergey Volkov (Russia) before the launch of STS-121 was postponed until July 2006. British-born astronaut Piers Sellers replaced Carlos Noriega who was originally scheduled to be on the STS-121 mission NASA announced on Thursday, July 15, 2004. This was due to an undisclosed, temporary medical condition. The transfer of Reiter to the ISS returned the station to a three crew member staffing level. After the loss of Columbia and the grounding of the Shuttle fleet, only two people had been residing on the ISS. Shuttle hardware External Tank: ET-119 Solid Rocket Boosters: BI-126 & RSRM-93 SSMEs: s/n 2045, 2051, 2056 OMS engines: LP-01/35 RP-03/33 Mission background During the STS-121 mission to the International Space Station (ISS), the crew of continued to test new equipment and procedures for the inspection and repair of the thermal protection system that is designed to increase the safety of the Space Shuttles. It also delivered more supplies and cargo for future ISS expansion. After the Columbia accident, NASA decided that two test flights would be required and that activities that were originally assigned to STS-114 would need to be divided into two missions because of the addition of post-Columbia safety tests. Before the accident, Columbia had been assigned to missions STS-118 and STS-121. The STS-118 mission, also an International Space Station flight, was