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Frederick Jackson Turner, "Significance of Frontier in American History" (1893), in American Historical Association, Annual Report for the Year 1893 (Washington, D.C.: Government Printing Office, 1894), 199-227.
The Great Plains
Launius, R. D. (1998). Frontiers of space exploration. Westport, Connecticut: Greenwood Press.
The Space Race
Heppenheimer, T.A. (1997). Countdown: A history of space exploration. New York: John Wiley and Sons, Inc.
The Space Race
NASA. Official Website. "The vision for space exploration." [2]http://www.nasa.gov/externalflash/Anniversary_VisMar/index_noaccess.html. [17 Mar 2005].
Humans upon Mars
Spudis, Paul D. "Humans Should Explore Space." Part of Scientific American Debate on Who Should Explore Space? Robots vs. Humans. April 18, 1999. Reproduced in Encyclopedia Encarta, CD ROM Version, 2003
History of the U.S. Space Program
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