page_title: STS-121
section_title: Mission background
hierarchical_section_title: STS-121 / Mission background
caption_attribution_description: KENNEDY SPACE CENTER, FLA. – Accompanied by Center employees, the orbiter Discovery, sitting on an orbiter transporter, rolls to NASA's Vehicle Assembly Building (VAB). The rollover marks the start of the journey to the launch pad and, ultimately, launch. Once inside the VAB, Discovery will be raised to vertical and lifted up and over into high bay 3 for stacking with its redesigned external tank and twin solid rocket boosters. The rollout of Space Shuttle Discovery to Launch Pad 39B is expected in approximately a week. Launch of Discovery on mission STS-121 is scheduled to take place in a window extending July 1 to July 19. Photo credit: NASA/Jack Pfaller
context_page_description: 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.
context_section_description: During the STS-121 mission to the International Space Station (ISS), the crew of Space Shuttle Discovery 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 at first reassigned to Discovery, but was later assigned to Space Shuttle Endeavour.
  The STS-121 mission was originally to have sent Columbia to service the Hubble Space Telescope. However, that servicing mission was given another designation on the manifest before the disaster and the designation of STS-121 once again became available. Since STS-115 through STS-120 were already delegated to existing missions, NASA selected the lowest available mission designation for the second test flight. Hence, the mission following STS-114 is STS-121.
  The STS-121 test flight mission was originally to be flown aboard Atlantis in September 2005, after Space Shuttle Discovery flew STS-114, but a problem with the landing gear of Atlantis moved Discovery ahead to fly STS-121. After the return of Discovery to California following the completion of STS-114, scheduling again changed. Atlantis was moved up to fly the STS-115 mission (whose launch was planned for August 2006) and Discovery was to fly the STS-121 mission as originally planned. The launch of the STS-121 mission was delayed until July 2006 as well, due to an unresolved foam debris and the Engine Cut Off (ECO) sensor issue from STS-114.
  On 12 May 2006, Discovery was moved from the Orbiter Processing Facility to the Vehicle Assembly Building, where it was mated to its External Tank and SRBs. Rollout to Pad 39B occurred on 19 May 2006 ahead of the planned launch, during the July 2006 launch window, which existed for about ten minutes each day between 1 and 19 July.
caption: Discovery is moved from the OPF to VAB for STS-121.
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