(covers information from several alternate timelines)
|Pre-warp ships Enterprise|
|Sail: HMS Enterprize • Enterprise • USS Enterprise|
|Powered: USS Enterprise CV-6 • USS Enterprise CVN-65|
Enterprise was a prototype/test vehicle for Earth's first reusable spacecraft, dubbed Space Shuttle. She was used primarily to verify the performance of the Space Shuttle Orbiter design during final approach and landing and never flew in space. She was displayed in the Steven F. Udvar-Hazy Center of the Smithsonian Institution's National Air and Space Museum from December 2003 until April 2012, and now makes its home at the Intrepid Sea, Air and Space Museum in New York City; Leonard Nimoy, who befittingly came full circle, as he was also present at the unveil of the orbiter thirty-six years earlier, attended and spoke at the arrival ceremony.
The first space shuttle was originally supposed to be named Constitution, in honor of the United States' bicentennial celebrations in 1976. However, a massive fan letter campaign organized by Bjo Trimble produced over 200,000 letters asking President Gerald Ford to name the shuttle Enterprise after the Star Trek starship (which, coincidentally, is Constitution-class) instead. When the Enterprise was unveiled on 17 September, 1976, at Rockwell's plant at Palmdale, California, Star Trek creator Gene Roddenberry and most of the principal cast of the Star Trek: The Original Series were on hand at the highly publicized dedication ceremony as guests of honor in recognition of its fictional namesake. (Starlog, issue 3, p. 7: Star Trek: The Magazine Volume 1, Issue 9, pp. 84-88, et al.) Not invited for the event was the original designer of the fictional ship, Matt Jefferies, but he was present, invited by NASA as guest of honor, at the first free-flight test of the shuttle on 12 August 1977. (Beyond the Clouds, pp. 285-286) As serendipity would have it, Jefferies, who was working on Star Trek: Phase II at the time, received a memo less than a month later, on 9 September 1977, from Roddenberry who, inspired by a letter he had received from a fan, wrote, "Some fans have suggested that our new Enterprise should carry a plaque somewhere which commemorates the fact it was named after the first space shuttle launched from Earth in the 1970's. This is an intriguing idea. It also has publicity advantages if properly released at the right time. It won't hurt NASA's feelings either. I'll leave it to you where you want it on the vessel and who should design it." Jefferies , the accomplished aviation artist, needed no further enticement to create the artwork for the historical vessels named USS Enterprise, that served as the source for the backlit transparencies, including that of the OV-101, ultimately seen on the wall of the recreation deck in Phase II's successor, Star Trek: The Motion Picture. (The Making of Star Trek: The Motion Picture, p. 94)
In the real world, Enterprise (OV-101) was not built to be spaceworthy, and was meant only for atmospheric tests, although NASA intended to upgrade the vehicle for space flight after the test program ended. However, changes to the final design and advances in materials technology in the interim made this too costly; an existing structural test article was used instead to build the shuttle Challenger (OV-099). Enterprise was again considered for modification after Challenger was lost, but it was cheaper and easier to build Endeavour (OV-105) out of structural spares. 
A model of a space shuttle docked with the International Space Station was given to the Star Trek: Deep Space Nine series as a gift by Majel Barrett-Roddenberry, where it served as a desktop model in Benjamin Sisko's office. The art department staff made one minor alteration on the model; changing the name of the shuttle to bear that of the Enterprise, even though, as stated, that orbiter never made it into into space in real life. (Star Trek Encyclopedia (4th ed., vol. 1, p. 372))
Footage of the shuttle Enterprise included in the montage of historic images shown during the opening credits for Star Trek: Enterprise is actually one of the later shuttles with the name Enterprise digitally inserted. The footage shows the orbiter's name on the forward fuselage under the cockpit windows, where the operational shuttles had their names painted; Enterprise had her name painted on the payload bay doors just above the hinge and behind the crew module, where it remains to this day.
The Artisan prop and model shop of Quantum Mechanix, QMx FX Cinema Arts, was asked to illustrate the history of space flight with models for the film. They constructed fourteen models in total. On their website, along with a picture of the model of the Enterprise, there was this caption, "It's all well and good to send people up in rockets for brief periods, but to make Human habitation of space a reality, we needed a way to ferry crew and supplies to permanent orbital stations. The U.S. Space Shuttle supplied the solution to that challenge for more than two decades. It also has the distinction of having the first spaceship named Enterprise, which ironically never actually flew in space." 
The mission patch for the Enterprise, along with other symbols, is displayed in the Star Trek Encyclopedia (4th ed., vol. 2, p. 350). The patch was for the Approach and Landing Test. The names of the astronauts involved in this mission - Fred W. Haise,Jr., C. Gordon Fullerton, Joe H. Engle, and Richard H. Truly - were identified on the patch by their last names.
The novel A Flag Full of Stars states that she instead was named for the World War Two aircraft carrier because the brother of one of the engineers who worked on her died serving aboard said ship, and that she was the last surviving space shuttle. She was refitted with impulse engines so she could participate in a parade of antique spacecraft celebrating the 300th anniversary of Apollo 11.
- Enterprise at Wikipedia
- "What Shuttle Should Have Been: The October 1977 Flight Manifest", David S. F. Portree, 24 March 2012 at Wired.com - projected uses of the space shuttles, including the Enterprise.