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.