Mission extended 261

Project Scientist Steve Davison. The investigator concerned, Marty Glicksman of Rensselaer Polytechnic Institute in Troy, New York, was joined for the historic commanding session by one of his team members from Lewis. The instructions were part of a third science phase for the dendritic-growth team, as part of continuing efforts to study how metals solidified in the virtually convection-free environment of low-Earth orbit.

Overall, Glicksman's team collected twice as much data as they needed, which helped to significantly improve post-flight statistical analyses. It was hoped that knowledge from IDGE might aid Earth-based processes such as metal casting, welding and the production of aluminium, steel and copper. The ability to perform real-time materials science in space had already shown experiment investigators that variations in Columbia's microgravity environment did not affect variations in the speed of dendrite growth. ''This was an important operational experiment, as well as a significant science experiment,'' Glicksman told journalists.

''An incredible atomic journey'', meanwhile, was going on elsewhere on USMP-3, according to Zeno's Principal Investigator, Robert Gammon. As with its previous run on USMP-2 in March 1994, this experiment sought to precisely identify the 'critical point' of xenon - the peculiar temperature-and-pressure point at which its characteristics rapidly drift between those of a liquid and a gas - as part of efforts to better understand the phenomenon. At such 'supercritical' temperatures, fluids make remarkable solvents as well as having other applications. Carbon dioxide, at its critical point, for example, is routinely used to extract caffeine from coffee beans.

As it approaches its critical point, the normally clear xenon ''turns milky-white because it does not know whether it really wants to be like a liquid or like a vapour. It just can't decide,'' said Gammon. In order to determine exactly where the critical point lay, the Zeno team simultaneously monitored more than 100 channels of data, updated each second, to plot statistical curves that measured the sample's critical temperature. These plots, known as 'correlograms', enabled researchers to show the relationship between temperature and pressure and provide clues as to when the critical point was reached.

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