Usmp Torchbearer Of The Semiconductor

With the completion of 'direct' satellite operations late on 26 February, the STS-75 astronauts returned to their dual-shift - Red and Blue - system of activities and focused on their other mission objectives. The most important of these was the third United States Microgravity Payload (USMP-3), which included a reflight of the AADSF and MEPHISTO semiconductor-processing furnaces, the Zeno xenon-critical-point experiment and the IDGE dendritic-growth investigation, together with the SAMS and OARE accelerometers. All of these had previously been on board USMP-2 in March 1994 and a few had also ridden on the payload's first flight in October 1992.

Unlike the tethered satellite, USMP-3 performed without problems and, like its two previous outings, was operated more-or-less-autonomously by ground controllers via 'telescience'. Chang-Diaz activated the payload late on 23 February and it was one of Cheli's responsibilities to keep a close eye on it throughout more than two weeks of experiment runs. Within hours of activation, IDGE Principal Investigator Marty Glicksman reported ''the best images ever sent down'' from his experiment. Elsewhere, Archie Fripp's AADSF grew a large crystal of infrared-detecting lead-tin telluride, while French MEPHISTO scientists monitored the solidification and remelting of three tin-bismuth alloy samples.

''The ages of humanity are known for the type of materials humans used, such as Stone Age [and] Bronze Age,'' said Fripp. ''If we can contribute to the development of the 'Semiconductor Age', then we really have achieved something!'' By 2 March his furnace had finished processing its first crystal: it grew 'vertically', pointing away from Earth, with the 'hot' end at the top and the 'cool' end at the bottom, while the second sample grew in the opposite direction. This helped scientists to ascertain the importance of growth in different directions before committing hardware to the International Space Station.

''As the station structure grows, the orientation of its microgravity environment could change and affect the direction of samples growing in an onboard furnace,'' Fripp explained. ''If there's a measurable difference between the uniformity of semiconductor samples grown in different directions on this mission, then we know that this will be significant on the space station.'' After Columbia's landing, the AADSF samples were compared with Earth-grown ones, in order to shed more light on how to improve ground-based processing methods.

Meanwhile, the ability to command each of the USMP-3 experiments via telescience was proving enormously successful. Commenting on its effectiveness, IDGE Project Scientist Ed Winsa of NASA's Lewis Research Center, said ''Time is money. In our limited time in orbit, typical pre-programmed experiments would have missed out on a tremendous amount of science return if we had not been able to adjust [our experiment's] growth cycles.'' Significantly, on 4 March, the mission made history when IDGE received direct commands from scientists on a US college campus.

''This is the first time a Principal Investigator has commanded a microgravity science experiment on the Space Shuttle from his home institution,'' said USMP-3

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