at first reassigned to Discovery, but was later assigned to . 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 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 May 12, 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 May 19, 2006, ahead of the planned launch, during the July 2006 launch window, which existed for about ten minutes each day between July 1 and 19. Equipment delivered to the ISS Key items delivered, as part of over 2 tons of cargo included: Multi Purpose Logistics Module (MPLM) Leonardo on its 4th flight, 7th MPLM overall, carrying: −80 °C Freezer This freezer is known as the Minus Eighty Degree Laboratory Freezer for ISS (MELFI). The French-built unit comprises four independent drawers which can be set to operate at different temperatures. Initially, temperatures of will be used during on-orbit ISS operations. Both reagents and samples will be stored in the freezer. As well as storage, the freezer is designed to be used to transport samples to and from the ISS in a temperature controlled environment. The total capacity of the unit is . The European Modular Cultivation System (EMCS) for biological experiments. This consists of a gas tight incubator in which there are two centrifuges, each able to carry four experimental cartridges. Two "Ground controls"—exact copies of the equipment and experiments—will be run on the