EDO Pallet
The Extended Duration Orbiter Cryogenic kit (EDO-pallet or CRYO) is a 15-foot-diameter (4.6 m) assembly of equipment 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 includes 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.
Initially, NASA considered adding a second EDO pallet to Endeavour, placed in front of the first, for a total of thirteen tank sets, that would have allowed an orbiter to remain in space for 28 days, but managers decided against it when the International Space Station assembly began, and instead removed the EDO capability from the orbiter, to reduce its weight and allow it to carry more cargo to the ISS.
No replacement for the pallet was planned, since the Station-to-Shuttle Power Transfer System provided much of the same abilities, and the 2011 retirement of the shuttle fleet made it redundant.
Read more about this topic: Extended Duration Orbiter