Usmp Longterm Payoff

Most visible was the second United States Microgravity Payload (USMP-2), which continued the research begun on STS-52 a year-and-a-half earlier by carrying the joint US/French MEPHISTO furnace and the SAMS acceleration monitor, as well as indulging in new scientific studies with three new experiments: the Advanced Automated Directional Solidification Furnace (AADSF), the Isothermal Dendritic Growth Experiment (IDGE) and the Critical Fluid Light Scattering Experiment

(nicknamed 'Zeno' by its sponsors). Like USMP-1, the experiments were mounted on two bridge-like MPESS carriers at the midpoint of Columbia's payload bay and operated via 'telescience' by ground controllers.

''There is the potential for a number of important benefits and applications to come out of this flight,'' Casper had told journalists in a pre-flight news conference, ''but the payoff is not always right away, and that's the way it is with this flight.'' Some of the benefits Casper alluded to were advanced semiconductors, which could be used in computers, calculators and infrared detectors, as well as materials to make stronger turbine blades for aircraft and powerful electronic components. USMP-2's two directional-solidification furnaces - MEPHISTO and AADSF, both attached to the 'front' MPESS - played a pivotal role in this research.

Building on experience from its previous outing on STS-52, MEPHISTO once again conducted studies of the actual process of'directional solidification', whereby certain materials were melted, then solidified, from end-to-end. The USMP-1 investigations had traced the motion of the 'interface' between the solid part of a sample and its liquid part; on USMP-2, this would expand to look more closely at the precise location, shape and behaviour of the interface. Three rod-shaped samples of a bismuth-tin alloy were again flown and, despite a temperature sensor glitch on 5 March, the furnace was returning analysable data within a couple of days.

During the USMP-1 experiments, investigators had found regular cellular patterns on the alloy's structure close to the point at which its solidification became unstable. This, it was pointed out before the second mission, was an important discovery because it might enable adjustments to be made in order to better manipulate and control the mechanical and electrical properties of resulting materials. Such materials might possibly be used as the basis for stronger and more resilient alloys in the future. The use of telescience allowed ground controllers to adjust MEPHISTO's operating procedures and improve their data.

In fact, cross-sectional analyses of the samples after Columbia's return to Earth would provide the best-yet information on the shape of the solid-to-liquid interface front, as well as refining the directional solidification process. Meanwhile, the AADSF furnace employed directional solidification to process a cylindrical sample of mercury-cadmium telluride, a material that has seen uses in remote-sensing and astronomical detectors. ''It is difficult to grow a good [semiconducting] crystal on Earth from the molten state,'' said the experiment's co-investigator Frank Szofran, ''because of convection: the movement of fluids caused by gravity. In microgravity, the substantial reduction of convection makes it possible to grow much better crystals.'' Directional solidification and relatively rapid cooling of samples had already been identified as two methods of obtaining suitable electronic properties in them, but could not be employed effectively in terrestrial experiments because convection and the 'settling' of molten components introduce defects and imperfections. These, in turn, can then lead to physical flaws in a crystal's internal structure and an uneven distribution of its chemical constituents.

To process its sample during the USMP-2 experiment runs, the AADSF furnace moved the mercury-cadmium telluride through three separate temperature 'zones' -ranging from a 'hot' region of 870 Celsius to a 'cool' region of 340 Celsius - and, in doing so, slowly cooled and solidified them. This use of three temperature zones, and the one-way directional technique of solidification, yielded a 'flatter' solidification front and crystals which scientists could analyse with greater clarity after Columbia returned to Earth. Analysis of data downlinked from AADSF was met with delight by ground-based scientists.

''Our furnace is operating perfectly,'' said Principal Investigator Sandor Lehoczky during the mission. ''In fact, the experiment has gone so well that we were able to use telescience to create a 'demarcation', or reference point, on the crystal at a critical time in our growth process.'' This marker precisely determined the location and shape of the liquid-to-solid interface in the mercury-cadmium telluride, which moved along the length of the sample at around 0.7 mm per hour. By the time the experiment was completed on 11 March, members of the AADSF science team were overjoyed with their results.

Telescience had already been used with great fanfare on USMP-1 and investigators transmitted hundreds of commands from the ground to adjust their experiments' settings and parameters and make changes as new and unexpected data and changes emerged. The technique was being regarded as essential for operating experiments on board the International Space Station which, at the time, it was thought would operate for long periods without a permanent caretaker crew. ''Telescience is the closest thing a scientist can get, without actually being there, to the way he would conduct his experiment on Earth,'' said USMP-2 Assistant Mission Scientist Don Reiss.

After Columbia's landing, the AADSF sample was carefully polished and etched to enable investigators to better determine the position and shape of the liquid-to-solid front. As the two furnaces continued their work, mounted on the aft MPESS bridge were the IDGE and 'Zeno' experiments: the former of which grew strange, tree-like crystalline structures known as 'dendrites', while the latter investigated the behaviour of xenon as it approached its so-called 'critical point', at which it simultaneously, and temporarily, displayed the characteristics of both a liquid and a gas.

Dendrites, the primary focus of the IDGE investigation, are known to develop as materials solidify under certain conditions; their name derives directly from their tree-like shape, which comes from the Greek word for 'tree'. By increasing scientists' understanding of solidification processes, it was hoped to improve the industrial manufacturing of a wide range of different materials, including steel and super-alloys used in applications ranging from making tin foil to cars and jet engines. Each of these materials are formed under conditions that yield dendrites.

During the course of the USMP-2 mission, dendrites were grown and photographed under a variety of pre-programmed experiment parameters on more than 30 occasions. As each growth 'cycle' ended, the tiny tree-like structures were re-melted and another dendrite produced at a different temperature. In total, they were grown at no fewer than 20 different temperature levels and the experiment as a whole performed beyond expectations. ''I couldn't be more pleased,'' IDGE's Principal Investigator Marty Glicksman told journalists on 8 March. ''It's working like a charm!''

Throughout the growth processes, a pair of cameras photographed the dendrites for post-flight analysis. About five pictures were taken during each process and slow-scan television images were recorded and downlinked in real time to ground controllers; this enabled them to make adjustments and changes to their operating procedures where necessary. Compared to Earth-processed specimens, the USMP-2 photographs and video showed that space-grown dendrites grew much faster and were larger. This allowed investigators to spend additional time studying their forms and structures.

''These discoveries and their subsequent development simply could not have been accomplished without going into low-Earth orbit, where gravity is reduced up to a million times,'' said Glicksman. ''We are certain that the basis of current theories about dendritic growth are seriously corrupted by convective effects due to gravity. We are confident that the images and data collected during the IDGE experiment will become the 'standard' for the scientific field for some time to come.'' By the time IDGE operations ended on 13 March, he added that it had delivered ''a goldmine of scientific data.''

Zeno, meanwhile, examined the behaviour of xenon at its critical point, where its properties change back and forth from a liquid to a gas so rapidly that neither state is fully distinguishable. Such points are difficult to achieve in terrestrial experiments, because the fluid becomes highly compressible or 'elastic'; its own weight compressed part of the sample to a density greater than that of the critical density and ultimately caused it to collapse. This meant that, if it was possible to reach the critical point, it could not be maintained long enough to conduct detailed studies of it.

In space, however, the virtual 'absence' of gravity and its limitations served to 'widen' the critical zone and provided clearer insights into the phenomenon. Xenon had been chosen as the sample gas for IDGE, because it was known to develop telltale patches of 'milky' iridescence as it neared its critical point. A somewhat slower-than-expected cooling of the Zeno unit shortly after activation put the experiment behind its timeline, but by 5 March - after calibration - investigators reported closing in to within 20 millionths of a degree Celsius of xenon's critical point.

Ultimately, according to Zeno Project Scientist Jeff Shaumeyer, the closest point the experiment reached was within 500 millionths of a degree Celsius. On 7 March his colleagues reported that they were seeing behaviour in the xenon unlike any ever observed on Earth, indicating that they were nearing the critical point. ''We have gone where no-one has gone before,'' said Principal Investigator Robert Gammon. After pinpointing the critical point, a temperature search and optical laser measurements were performed to obtain the best possible measurements of the region surrounding the phenomenon.

''These light-scattering optical measurements will help to test theories at temperatures closer to the critical point than is possible on Earth,'' Zeno Project Manager Richard Laurer told journalists on 10 March. As this research went on, astronaut Andy Allen described the purpose and accomplishments of the experiment in a televised downlink. Using a vial filled with freon, he explained to audiences how gases reached their critical points, and added that, knowing for example the critical point of water led to the development of new techniques to decaffeinate coffee.

Similar understanding of other substances, Allen noted, could yield new knowledge about them.

The fifth and final 'experiment' on USMP-2 was the SAMS accelerometer, which included two sensor heads on each MPESS pallet and another deep inside the IDGE unit. In general, its data revealed that no major disturbances were caused to the sensitive microgravity experiments by the movements of Columbia's crew. It did find, however, that larger-scale accelerations, including thruster firings by the vehicle itself, did have a direct impact on the quality of the MEPHISTO and AADSF crystals and was able to precisely measure these effects.

Unlike many flights, which operate scientific payloads alongside one another, that of STS-62 was virtually two separate Shuttle missions in one. The USMP-2 operations were scheduled to consume the first 10 or so days, after which they would be shut down and operations on a second payload, sponsored by NASA's Office of Aeronautics and Space Technology and dubbed 'OAST-2', would take precedence. This would also be highlighted by the deliberate lowering of Columbia's orbital altitude early on 14 March to enhance the data-gathering capabilities of the six OAST-2 experiments.

By the time USMP-2 operations drew to a close on the evening of 13 March, virtually all of the scientific personnel involved praised it as a superb success, made possible only through the cooperation of thousands of individuals on the ground and five astronauts in orbit. ''One factor that has really contributed to the success of this flight,'' said USMP-2 Assistant Mission Manager Sherwood Anderson, ''is the teamwork among all the investigators to maximise total science return.'' Added Mission Scientist Peter Curreri: ''We have made basic science discoveries that we had not anticipated. It has been a fantastically rich mission.''

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