Ross, an astronaut since 1980, knew exactly what he was talking about: not only had he already flown three times before STS-55, but he would also fly three more times after the German mission, becoming the first human being to chalk up seven trips into space overall. His expertise would also, after his return from Spacelab-D2, be rewarded with the post of deputy chief of NASA's Astronaut Office. Yet even Ross has described this mission as one of the most demanding of his career.
''The Payload Commander is the guy that's responsible basically for interfacing with the payload sponsors and the crew to make sure that [what] the payload sponsors want to happen on orbit is things that the crew can physically do, both from the interfaces to the payloads, the checklists [and] the timeline. I had to do all the coordination, all the dealings - everything - with the safety community, the medical community, the science community. I had to work all that in addition to trying to get three rookies ready to go fly on 90 different, very complex experiments.''
It was no mean feat. Nagel had also flown three times, including a previous mission with Ross and also as Pilot on Spacelab-D1. ''They [the Germans] were hoping he'd come back and fly again on D2,'' said Ross. ''I was also hoping he may come back and we could fly together again, so I could harass him some more! So he said, yes, the system thought that would be a good thing to do to have that kind of continuity and he [came] back as Commander.''
It was becoming common practice on complex research flights like this one to 'carry over' a crew member from one mission to the next to provide expertise and ensure a smooth transition. Similarly, Ken Bowersox - who flew as Pilot on STS-50, the first United States Microgravity Laboratory - would command USML-2. However, as Nagel would say later, the second German Spacelab was somewhat different to the first. ''The complement of experiments was more biased in the direction of life sciences. It was [also] going to be a little longer: it was a 10-day mission [whereas] the other one was seven days.''
The life sciences bias was further illustrated by the inclusion of a physician, Harris, on the crew. ''The Germans had requested a medical doctor Mission Specialist,'' said Ross, ''and since I wasn't one, they were certainly hoping the next one would be. While I didn't personally think that a medical doctor was mandatory, I did think that it was not a bad idea, because probably over 50% of the work we were going to do was life sciences [and] human research type of experiments.''
The EDO pallet, which could have extended STS-55's time aloft to a fortnight, was not carried this time, although according to Nagel the Germans were offered this option. ''They had a tradeoff they could've had there. Could've put [an EDO] pallet in the back with more cryo[genic tanks] for fuel cells and flown even longer, but they chose to have experiments back there on an outside rack instead of additional days in orbit.'' That 'rack' was a Unique Support Structure (USS) - a bridge in Columbia's payload bay - which carried a set of astronomy, atmospheric physics and materials science experiments.
One of the most important of these USS-mounted experiments was the second Modular Optoelectronic Multispectral Scanner (MOMS-2), an improved version of a device previously carried on Spacelab-D1. It was capable of simultaneously acquiring high-resolution images of Earth's surface for remote-sensing purposes. Although the relatively short length of the D2 mission meant that its full potential could not be achieved, another version was later flown to Mir in 1996, providing better data coverage over longer periods of time.
The instrument's multispectral coverage allowed geologists to better discriminate between different classes of vegetation and rock or soil surface coverings. Columbia's 28.5-degree-inclination orbit allowed photography to take place of terrestrial features between 28.5 degrees North and 28.5 degrees South latitude which, coupled with less-than-ideal solar illumination and cloud cover, initially restricted MOMS-2's studies to North and South America and parts of Africa. However, the 'daytime' ground track drifted westward later in the mission and images were acquired of the Near East, India, Southeast Asia, Indonesia, Australia and the Pacific islands.
NASA's Earth Observations Laboratory at JSC also pushed up the instrument's scientific yield by providing real-time information on individual targets' characteristics from geosynchronous- and polar-orbiting satellite data. This allowed MOMS-2 to take more high-resolution pictures over cloud-free regions. Among the instrument's notable successes were incredibly detailed images of irrigation ditches,
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