Wake Shield Facility

Wake Shield Facility (WSF) was an experimental science platform that was placed in low Earth orbit by the Space Shuttle. It was a 3.7 meters (12 ft) diameter, free-flying stainless steel disk.

Deployment of the WSF using the Space Shuttle robotic arm

The WSF was deployed using the Space Shuttle's robotic arm.[1] The WSF then used nitrogen gas thrusters to position itself about 55 kilometers (34 mi) behind the Space Shuttle which was at an orbital altitude of over 300 kilometers (190 mi), within the thermosphere, where the atmosphere is exceedingly tenuous.[1] 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.[2] The WSF thus created an ultrahigh vacuum in its wake.[2] 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.[1] 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.[1]

The WSF was flown into space three times, aboard 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.[3] These included gallium arsenide (GaAs) and aluminum gallium arsenide (AlGaAs) depositions. These experiments have been used to develop better photocells and thin films.[4] 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).[5]

The WSF was sponsored by the Space Processing Division in NASA's Office of Life and Microgravity Sciences and Applications. It was designed, built and operated by the Space Vacuum Epitaxy Center (since renamed the Center for Advanced Materials) at the University of Houston—a NASA Commercial Space Center—in conjunction with its industrial partner, Space Industries, Inc., also in Houston.

As of 2012 the Wake Shield Facility spacecraft is being preserved at the Center for Advanced Materials.[2]

See also

References
  1. Kiernan, Vincent (5 August 1995). "Making semiconductors out of thin air". New Scientist.
  2. "Vacuum in the Wake". Science First Hand. 31. 10 April 2012.
  3. Ignatiev, Alex (January 2001). "Advanced thin-film materials processing in the ultra-vacuum of space". Acta Astronautica. 48 (2–3): 115–120. Bibcode:2001AcAau..48..115I. doi:10.1016/S0094-5765(00)00148-X.
  4. Freundlich, A.; et al. (February 2000). "Photoluminescence of GaAs grown by metallorganic molecular beam epitaxy in space ultra-vacuum". Journal of Crystal Growth. 209 (2–3): 435–439. Bibcode:2000JCrGr.209..435F. doi:10.1016/S0022-0248(99)00586-2.
  5. Strozier, J. A.; et al. (November 2001). "Wake vacuum measurement and analysis for the wake shield facility free flying platform". Vacuum. 64 (2): 119–144. Bibcode:2001Vacuu..64..119S. doi:10.1016/S0042-207X(01)00383-9.
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