Had Challenger not been lost in January 1986, it is likely that Spacelab-D2 would have taken place some time in 1988, about three years after the D1 mission. The delays in getting the Shuttle back into space, however, pushed it back into 1992 and finally 1993, but the enthusiasm of West Germany - and later the unified Federal Republic - remained intact. ''It is viewed by the general public, as well as the politicians in our government, as a very important mission,'' said Heinz Stoewer, a Spacelab-D2 project manager for DARA which, with DLR (the German Aerospace Research Establishment), was financing the flight.
''We definitely have the fullest plate that's ever flown in space,'' Ross told the pre-flight press conference in February 1993. That was quite a remarkable statement from the gruff-voiced US Air Force Colonel, whose three previous Shuttle missions had included a total of five satellite deployments and four spacewalks, on one of which he had helped 'save' NASA's Compton Gamma-Ray Observatory by manually unstowing its jammed communications antenna. ''We have a large number of experiments. There are constraints on when things can be done and when they can't. We will be very challenged to keep up with the timeline.''
In fact, Ross' experience thus far had been in EVA matters and he has admitted to being more interested in building the space station than in a science mission. ''Dan Brandenstein, [then] head of the Astronaut Office, called me when I was in quarantine, getting ready to go fly STS-37, and said, 'We want you to be the Payload Commander for STS-55'. I said, 'Dan, you want a scientist for that. You don't want me, an old engineer!' He said, 'No, we want you. We want somebody that will work well with the Germans and [get] the flight pulled together.' So, literally, it was less than two weeks after I got down from STS-37 [in April 1991], I was announced as the Payload Commander for STS-55, and I didn't do a lot of the normal post-flight [public relations] activities on STS-37 because I went to Germany probably within three weeks of landing, to start working on STS-55.''
In many ways, Spacelab-D2 augmented the microgravity research begun by D1 in the autumn of 1985 with investigations into life sciences, biology, materials processing, fluid physics, technology and - mounted on the USS pallet - astronomy, atmospheric studies and Earth observations. In addition to DLR, DARA and NASA, several of these experiments were also provided with support from ESA and French and Japanese researchers.
''Our scientific methods are governed by the effect of gravity,'' said Hauke Dodeck before the mission. ''Objects fall down, lighter objects float or are carried upwards, heavier ones sink to the bottom. What happens to these processes when there is no gravitational force: no sedimentation, no thermal convections, no hydrostatic pressure? What new mixtures, structures and forms are possible? Concrete answers can only be given by space research.''
Many of these materials science experiments were housed in three custom-built facilities inside the Spacelab module: the Materials Science Experiment Double Rack for Experiment Modules and Apparatus (MEDEA), Werkstofflabor and the Holographic Optical Laboratory (HOLOP). Others, which required direct exposure
A full plate of experiments 191
to the vacuum of space, were attached to the USS in the Materials Science Autonomous Payload (MAUS) and Atomic Oxygen Exposure Tray (AOET).
These experiments studied the behaviour of 'columns' of fluid under weightless conditions, closely followed the solidification processes of different metallic alloys and undertook crystal growth of various materials. Three separate furnaces were contained within the MEDEA double rack: one that conducted long-duration crystallisation studies, another that processed metallic crystals at extremely high temperatures using the directional solidification technique and a high-precision thermostat that examined the behaviour of metals under carefully controlled temperature conditions.
During the mission, investigator Peter Saum excitedly reported that the astronauts had grown the largest-ever gallium arsenide crystals in MEDEA -measuring some 20 mm in diameter; such materials are used extensively in electronic components, such as light-emitting diodes, semiconducting lasers, photo-detectors and high-speed switching circuits. The second materials science facility, Werkstoff labor, held almost a quarter of Spacelab-D2's 88 experiments and successfully produced a monotectic alloy of bismuth-aluminium, which might one day be used to assemble ball bearings capable of withstanding higher loads and temperatures.
Other research included the growth of crystals of materials that could be used to produce stronger turbine blades and other components for aircraft and spacecraft engines. ''If the tests produce the hoped-for results,'' said Dodeck, ''turbine blades can be developed which are strongly resistant to heat and stress, thereby improving the performance and lifetime of aircraft engines.'' Meanwhile, HOLOP used holography to investigate the processes of heat- and mass-transfer and cooling in transparent materials, which are of importance in metallurgy and casting techniques. The holography, using laser light, made these processes more easily visible.
''HOLOP will transmit video pictures of experiments to the ground while they are being performed,'' Dodeck said before the mission. ''Scientists on Earth can not only watch what happens, but also may intervene in the test sequence, thus demonstrating a concept called 'telescience'.'' The technique had also been used in conjunction with the USMP-1 experiments on Columbia's previous mission, STS-52. In fact, more than 600 telescience commands were transmitted to HOLOP during the course of Spacelab-D2 from DLR's Microgravity Life Support Center at Cologne-Porz.
To emphasise the German flavour of the mission, like Spacelab-D1, the Payload Operations Control Center (POCC) had shifted from Houston to Oberpfaffenhofen, near Munich. However, during D1, several functions still had to be monitored from Houston because Oberpfaffenhofen's data-transmission capabilities were insufficient to handle all communications traffic. For this second mission, the situation had improved and satellite-transmitted data was received by ground stations on DLR premises and forwarded directly to the control centre. Sitting in the control room as Spacelab-D2's science coordinator was German astronaut Ulf Merbold, who had flown on STS-9 and served as backup Payload Specialist on D1.
'Outside' Columbia, on the USS pallet, two experiments in the MAUS facility investigated complex boiling processes and the diffusion phenomenon of gas bubbles in salt melts. Meanwhile, AOET exposed more than a hundred material samples to the harsh atomic oxygen environment of low-Earth orbit to obtain data on their reaction rates. Such data was deemed important for constructing new materials to help shield Columbus, ESA's laboratory module for the International Space Station, which would have to remain in orbit for many years.
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