Extended Duration Orbiter

[1][2] Endeavour's EDO modifications were removed in 1996 as part of routine maintenance, to reduce the orbiter's weight prior to STS-89.

Although Atlantis was partially upgraded to accommodate the EDO, only Columbia and Endeavour actually flew with the pallet.

[4] 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.

[1][5] 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.

[1][6] The EDO pallet was designed to augment the orbiter's endurance for prolonged missions by supplying additional hydrogen and oxygen for its fuel cells.

These fuel cells, in turn, converted hydrogen and oxygen into electrical energy essential for the orbiter's operations.

Since space travelers may faint when they stand up (orthostatic intolerance) after returning to normal gravity even after short flights, and muscle strength may be reduced, the EDOMP project focused on ensuring that the crew could land the orbiter, and exit from it without help after a 16-day flight.