Wake Shield Facility
The WSF was deployed using the Space Shuttle's robotic arm. The WSF then used nitrogen gas thrusters to position itself about 55 kilometres (34 mi) behind the Space Shuttle which was at an orbital altitude of over 300 kilometres (190 mi), within the thermosphere, where the atmosphere is exceedingly tenuous. The WSF's orbital speed was at least three to four times faster than the speed of thermospheric gas molecules in the area which resulted in a cone behind the WSF that was entirely free of gas molecules. The WSF thus created an ultrahigh vacuum in its wake. The resulting vacuum was used to study epitaxial film growth. The WSF operated at a distance from the Space Shuttle to avoid contamination from the Shuttle's rocket thrusters plus water dumped overboard from the Shuttle's Waste Collection System. After two days, the Space Shuttle would rendezvous with the WSF and again use its robotic arm to collect the WSF and to store it in the Shuttle's payload bay for return to Earth.
The WSF was flown into space three times, on board shuttle flights STS-60, STS-69 and STS-80. During STS-60, some hardware issues were experienced, and, as a result, the WSF was only deployed at the end of the shuttle's robotic arm. During the later missions, the WSF was deployed as a free-flying platform in the wake of the shuttle.
These flights proved the vacuum wake concept, and realized the space epitaxy concept by growing the first-ever crystalline semiconductor thin films in the vacuum of space. These included gallium arsenide (GaAs) and aluminum gallium arsenide (AlGaAs) depositions. These experiments have been used to develop better photocells and thin films. Among the potential resulting applications are artificial retinas made from tiny ceramic detectors.
Pre-flight calculations suggested that the pressure on the wake side could be decreased by some 6 orders of magnitude over the ambient pressure in low Earth orbit (from 10−8 to 10−14 Torr). Analysis of the pressure and temperature data gathered from the two flights concluded that the decrease was some 2 orders of magnitude (4 orders of magnitude less than expected).
The Wake Shield was sponsored by the Space Processing Division in NASA's Office of Life and Microgravity Sciences and Applications. Wake Shield was designed, built and is operated by the Center for Advanced Materials (formerly Space Vacuum Epitaxy Center) at the University of Houston—a NASA Commercial Space Center—in conjunction with its industrial partner, Space Industries, Inc., also in Houston.
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