STS-40, the eleventh launch of Space Shuttle Columbia, was a nine-day mission. It carried the Spacelab module for Spacelab Life Sciences 1 (SLS-1), the fifth Spacelab mission and the first dedicated solely to biology. STS-40 was the first spaceflight that included three women crew members. Launch originally set for 22 May 1991. Mission postponed less than 48 hours before launch when it became known that a leaking liquid hydrogen transducer in orbiter main propulsion system which was removed and replaced during leak testing in 1990, had failed an analysis by vendor. Engineers feared that one or more of the nine liquid hydrogen and liquid oxygen transducers protruding into fuel and oxidizer lines could break off and be ingested by the engine turbopumps, causing engine failure.
It was the fifth dedicated Spacelab mission, Spacelab Life Sciences-1, and first dedicated solely to life sciences, using the habitable module. Mission featured most detailed and interrelated physiological measurements in space since 1973–1974 Skylab missions. Subjects were humans, 30 rodents and thousands of tiny jellyfish. Primary SLS-1 experiments studied six body systems; of 18 investigations, ten involved humans, seven involved rodents, and one used jellyfish.
