http://www.time.com/time/photogallery/0,29307,2044829_2230933,00.html
The Challenger Shuttle 25 Years Later
Time said:
[b]Prototype[/b] Challenger began its life as STA-099, a test vehicle, above, designed to explore the stress limits of a lighter-weight frame that engineers at NASA and its contractor Rockwell had designed.
Time said:
[b]Roll Out[/b] In 1979, NASA commissioned Rockwell to convert STA-099 to a space-rated orbiter. The process required the disassembly and replacement of many components, including the crew module, which had only been simulated in the craft's test version.