Practising for the space station

materials at their critical points because such knowledge could provide greater insights into physical problems ranging from phase changes in fluids to magnetisation changes in solids. It had long been recognised that temperature control is extremely important in critical point experiments and the CPF, which was delivered to KSC for IML-2 pre-flight processing in September 1993, included five interchangeable, high-precision thermostats.

One experiment, provided by Dutch scientist Antonius Michels, measured the wave motion of heat within sulphur hexafluoride as it neared its critical point. ''The facility functioned flawlessly, especially in providing stability to our sample,'' he said as his experiment ended on 13 July. Other studies included Richard Farrell's examination of how energy was transported through fluids that had reached their critical points. At one stage in the experiment, a wire was charged inside the test cell to 500 volts to simulate the pressures induced by terrestrial gravity. ''The effect was like turning the gravity on and off,'' Farrell said later.

Elsewhere in the Spacelab module, the Electromagnetic Containerless Processing Facility - known by its German acronym of 'TEMPUS' - provided a levitation melting device for processing metals in an ultra-clean environment. 'Containerless' processing was highly desirable because, on Earth, properties of liquids are known to be affected by the properties of whatever vessel holds them. In microgravity, on the other hand, positioning and control of liquids can be accomplished more accurately and precisely and the amount of power needed for positioning is greatly reduced. This, in turn, reduces motions within the liquid and is less intrusive on the physical phenomena under investigation.

For its first flight on board IML-2, the facility was loaded with 22 spherical specimens, each measuring up to 10 mm in diameter. These were mounted on a storage disk, which rotated until the desired specimen was positioned over a mechanism that transferred it to a processing area where it could be processed in either a vacuum or an ultra-pure helium-argon 'atmosphere'. During processing, TEMPUS provided researchers with the option to manipulate their samples by applying a direct-current magnetic field to change rotation speeds and oscillation rates.

Most of the experiment runs were controlled almost entirely by computer, requiring little in the way of crew interaction, other than starting the facility and shutting it down; TEMPUS was reprogrammed from the ground via telescience, although the astronauts on Columbia could adjust experiment parameters if necessary. In general, the IML-2 operations proved highly successful, although Julian Szekely's study of viscosity, internal friction and surface tension - the forces which keep liquid in a 'drop' - using a 10-mm-diameter sphere of copper had to be halted earlier than planned when the sample inadvertently made contact with its containment cage.

On another occasion, Hieb kept watch over a zirconium-cobalt alloy as the TEMPUS ground team sent commands to levitate, then melt, the small metal sphere inside the facility's processing chamber. It was hoped that such research could aid the production of near-perfect metallic 'glasses' with unique mechanical and physical properties. ''The sample looks extremely stable today,'' Hieb told investigators. In an unexpected moment of serendipity, on 16 July Thomas began processing a niobiumnickel alloy sample and Principal Investigator William Johnson discovered that it had an unknown 'metastable phase'.

In such phases, materials can be quite different from what they are in their 'stable' phases; for example, a diamond is a metastable phase of carbon. ''People have been wondering for a long time about the special behaviour of this alloy, but there was no explanation for it,'' said TEMPUS team member Knut Urban. ''The excellent quality of the space images allowed us to detect a phase which had been masked by other forces on Earth.'' The niobium-nickel sample was solidified and returned to Earth for a detailed microstructural analysis.

Other materials that were melted, levitated and solidified in TEMPUS included an 8-mm-diameter nickel-tin alloy and an aluminium-copper-cobalt sample; all were preserved for analysis after Columbia's landing to determine their structures. Researchers were particularly interested in a newly discovered type of atomic arrangement, known as 'quasi-crystals', which it was hoped could yield materials with high degrees of 'hardness', together with novel electrical and physical properties. This research was of such importance that, during a particularly sensitive TEMPUS run with a sample of pure zirconium on 17 July, Columbia's crew suspended all thruster firings to provide it with an ultra-stable platform.

European participation in the IML-2 mission was represented still further by a facility called RAMSES - a French acronym for Applied Research on Separation Methods Using Space Electrophoresis - which had been developed by the French Space Agency (Centre Nationale d'Etudes Spatiales, or 'CNES') in conjunction with several European industrial partners. Electrophoresis - a process that involves the separation and collection of ultra-pure components of biological substances according to their electrical charges - had been pioneered on several of Columbia's early missions and continues to be an important area of research for the pharmaceutical industry.

Such separation processes are hindered on Earth by our planet's strong gravitational influences, including sedimentation and convection, which have a tendency to 'remix', and thus ruin, the compounds during terrestrial experiments. Chiao activated RAMSES on 13 July, initiating and monitoring a sample of haemoglobin and bovine serum albumin that was being flown to evaluate the degree of protein purification possible in microgravity. ''This investigation went better than expected,'' said Bernard Schoot, whose highly concentrated protein-extract experiment flew on board RAMSES. ''Because of the high concentration of protein in this sample, we cannot do this investigation on Earth.''

Also provided by Europe was the Bubble, Drop and Particle Unit (BDPU), a multi-user facility designed to study fluid behaviour and interactions, including bubble growth, evaporation, condensation and temperature-induced thermocapil-lary flows. Previous experiments had revealed the unusual behavioural patterns exhibited by fluids in microgravity - instead of forming teardrop shapes, they become perfectly spherical as their shape is dominated by surface-tension effects rather than the influence of terrestrial gravity. A clear understanding of fluid behaviour in space was considered important for, among other reasons, the design of new and improved spacecraft life-support and fuel-management systems.

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