Extended Duration Orbiter

The Extended Duration Orbiter (EDO) program was a project by NASA to prepare for long-term (months) microgravity research aboard Space Station Freedom, which later evolved into the International Space Station. Scientists and NASA needed practical experience in managing progressively longer times for their experiments. The original space shuttle configuration usually provided a week to ten days of spaceflight. Several research projects and hardware components were part of the project, of which the EDO-pallet was one of the most visible, contracted by Rockwell International.

The first orbiter outfitted with the EDO hardware configuration was Endeavour, during its construction, and its last EDO flight was STS-67, in 1995.Endeavour's EDO modifications were removed in 1996 as part of routine maintenance, to reduce the orbiter's weight prior to STS-89.Columbia was outfitted for EDO flight during its maintenance period from August 10, 1991, through February 9, 1992, prior to STS-50, which was the first EDO flight. From 1992, through 1994, Atlantis went through a maintenance period, during which Atlantis was modified to have the provisions needed for EDO capability, but NASA chose not to proceed with the final modifications, and Atlantis does not have EDO capability. The EDO-pallet used in these orbiter configurations was destroyed in the Columbia accident.

The Extended Duration Orbiter Cryogenic kit (EDO-pallet or CRYO) was a 15-foot-diameter (4.6 m) equipment assembly which attached vertically to the payload bay rear bulkhead of an orbiter, and allowed the orbiter to support a flight of up to 16 days duration. The equipment included cryogenic tanks, associated control panels, and avionics equipment. Although Atlantis was partially upgraded to accommodate the EDO, only Columbia and Endeavour actually flew with the pallet. The pallet made its debut on STS-50, and was lost on mission STS-107, the Space Shuttle Columbia disaster.

...
Wikipedia