Nagel thinks it was the presence of the time-critical biological samples that influenced NASA's decision to bring Columbia home on 6 May, lest the second freezer also failed. In all other respects, the flight had proved so successful - "We've really got this laboratory cranked-up,'' Ross had told ground controllers on 27 April - and the crew's use of onboard consumables was so economical that NASA decided to extend it from nine to 10 days.
In fact, both the Americans and the Germans had been working towards the 'extra' day since the start of the mission. Over a three-day period, they had worked with the astronauts to conserve as many of Columbia's consumables as possible and easily accumulated 25 hours' worth of additional margin in the cryogenic fuel cells. On the morning of 3 May, it was official: the Mission Management Team told a happy Nagel that his crew would now return to Earth around 1:00 pm on the 6th.
Early on the morning of 'landing day', the Red Team took charge of deactivating the Spacelab module and storing all of the biological specimens and materials samples from the many experiments. They then bade the German ground controllers ''Aufwiedersehen'', before shutting the hatch and returning to the middeck, closing the tunnel and packing away the remainder of their equipment in readiness for reentry. Both payload bay doors were closed and latched without incident by 9:25 am, with the de-orbit burn scheduled for around midday. It was not to happen.
Unacceptable weather at KSC led forecasters to 'wave-off the crew for one orbit and, when it did not improve, the landing site was changed to Edwards Air Force Base. Nagel was sure Columbia could have stayed aloft an extra day or two to wait for conditions in Florida to improve, but added that this mission was ''a big life sciences flight with lots of samples. They [the Germans] were hanging it all on one freezer to save these samples, and I'm sure that's why, when it came time to land, we were weathered out of Florida and went to Edwards.
''If it hadn't been for that, we'd have stayed up an extra day waiting for good weather in Florida, but the Germans were deathly afraid that their second freezer was going to fail and they'd lose all these samples.'' Nagel performed the almost-three-minute de-orbit burn at 1:29 pm to begin the hour-long glide home. The astronauts landed on Runway 22 at Edwards at 2:29:59 pm, after a mission lasting a few minutes short of 10 full days. During STS-55, Columbia also achieved a personal milestone: becoming the first Shuttle to spend more than a hundred cumulative days in space.
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