Extended Shuttle missions, possibly lasting up to a month, had been in the back of NASA's mind since long before the Challenger disaster. They were considered a useful way of evaluating group dynamics in a closed environment for extended
periods of time, as well as conducting research for longer than the week or so that had previously been possible. In fact, not since Skylab in the early 1970s had NASA had the facility to have astronauts tend experiments in space for more than a few days.
By the beginning of 1990, the plans had reached fruition: Columbia, the longest-serving Shuttle, would be decommissioned for six months in the summer of the following year to be extensively modified for long-duration trips. Initially, these were planned for up to 16 days, although the option of longer, month-long stays was kept open for the future. The modifications began almost immediately after Columbia returned to Earth from her highly successful SLS-1 mission in mid-1991. After three refuelling stops, the 747 carrier aircraft dropped her back at KSC on 21 June. She would not, however, remain in Florida for long.
After several weeks of 'deservicing' activities - draining her APUs, repairing her damaged thermal insulation material, taking out her forward RCS unit and removing her tyres - she was finally transferred from OPF Bay 2 to the new OPF Bay 3 (formerly the Orbiter Maintenance and Refurbishment Facility) for fit checks in preparation for her ferry flight to Palmdale. On 10 August 1991, secured to the top of the 747, she left KSC and, after two refuelling stops and two days on the ground due to bad weather, arrived at Rockwell International's Shuttle plant on the afternoon of the 13th.
For almost six months, she underwent over 150 modifications; the most notable of which were the addition of a new Regenerative Carbon Dioxide Removal System (RCRS) and the 1,630-kg Rockwell-built Extended Duration Orbiter (EDO) pallet.
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