Measuring 4.6 m wide, the wafer-like pallet was to be situated in an upright configuration at the rear of Columbia's payload bay on extended missions and be equipped with additional oxygen and hydrogen tanks to supplement her electricity-generating fuel cells. Eight tanks were attached to the pallet - four for oxygen, two for hydrogen and two for helium - although more could be fitted for longer missions.
When fully loaded, the pallet could store 1,420 kg of liquid oxygen and 167 kg of liquid hydrogen, which pushed its weight to almost 3,180 kg. Coupled with the cryogenic tanks already on board each Shuttle - which can ordinarily support missions of around 10 days - this provided Columbia and, for a time, Endeavour, with the option of spending 16 days aloft. Some consideration was given to adding a second EDO pallet to Endeavour, to permit even longer missions, but such plans were abandoned to save weight as the International Space Station effort gathered pace.
Although Endeavour did fly an EDO mission lasting almost 17 days in the spring of 1995, the system has since been removed and she is no longer capable of such flights. Similarly, Atlantis was equipped with some of the electrical provisions needed to make her EDO-capable, but ultimately NASA decided not to proceed with the final changes. The result was that Columbia became the only vehicle to routinely employ the EDO pallet and, to date, she holds - and, most likely, will continue to hold - the record for having flown the longest Shuttle mission.
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