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I have long believed that Enterprise was just an empty craft, with only the instruments to operate in shorts flights in the atmosphere.
Basically a large glider with odd wings.

However, I have been seeing Enterprise referred to as not space-rated or space-ready, and sometimes as an 'orbiter'. (For example Space Shuttle Orbiter Approach and Landing Test: Final Evaluation Report that is linked in @OrganicMarble's answer calls it "Orbiter 101" a few times)

This makes it seem like Enterprise was actually a complete Space Shuttle, but missing some testing or without a few required space elements.

How much of a prototype actually was Enterprise?

It apparently did not have engines, but the other shuttles also needed to refurbish the engines after each flight.

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  • $\begingroup$ I think I've been guilty of referring to Enterprise as an orbiter when talking about the shuttles as a fleet; it's a bit awkward to say "STS orbiters-and-gliders". I don't think I realized before now how appropriate it was that the "pretend spaceship" was named after a "pretend spaceship!" (--and I say this as a lover of Star Trek). $\endgroup$ – Russell Borogove Jul 17 at 18:27
  • $\begingroup$ @RussellBorogove This is where I saw Enterprise being called an orbiter quora.com/… $\endgroup$ – Speedphoenix Jul 17 at 21:35
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Your belief was pretty correct.

Major systems missing on Enterprise:

  • orbital manuvering system
  • reaction control system
  • main propulsion system
  • space radiators
  • airlock
  • star trackers
  • thermal protection system (tiles)
  • cryogenic tankage for fuel cell reactants
  • payload bay payload mounting hardware
  • galley
  • aft and overhead windows
  • Ku-band antenna

Major systems different on Enterprise:

  • hydraulics
  • electrical
  • thermal control
  • cockpit controls and displays
  • structure
  • communications

Enterprise-only systems

  • nose-mounted air data probe
  • high pressure tankage for fuel cell reactants
  • flight test instrumentation
  • umbilical to 747 mothership
  • tracking beacons
  • ram air scoop

She was intended only for testing the final stages of aeroflight1 so had only systems required for that installed.

1 The original plan was to convert her into a spaceworthy orbiter by adding the missing systems but it ended up being too expensive, largely because of structural design changes to the wings, mid-fuselage, and thrust structure. Challenger was built out of the structural test article instead.

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References

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  • $\begingroup$ Are those lead ballasts in the first figure present in the space rated orbiters too or are they to simulate the weight of missing components? $\endgroup$ – Speedphoenix Jul 17 at 12:05
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    $\begingroup$ Definitely the latter, but the operational Orbiters had ballast too: space.stackexchange.com/a/43749/6944 Enterprise almost certainly had extra. $\endgroup$ – Organic Marble Jul 17 at 12:08
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    $\begingroup$ @Speedphoenix I looked for details on the life support system but none of the references really mentioned it :( That "ram air" thing probably took the place of some of it. I would guess you are correct on all counts but I try not to put stuff in answers unless it's backed up by references. $\endgroup$ – Organic Marble Jul 17 at 12:53
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    $\begingroup$ @luther Thank you for the concern! I was however only asking about how/why Enterprise couldn't go to space, not about its usage inside the atmosphere. Your comments do raise the topic of dropping regular Space Shuttles from the carrier aircraft though! I just posted this question about that space.stackexchange.com/q/45416/29286. $\endgroup$ – Speedphoenix Jul 17 at 20:59
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    $\begingroup$ @Speedphoenix sure, it could be mounted on the ET. It was used to fit-check the facilities. Here's a good artlcle with pictures showing it on the pad. nasa.gov/feature/… No main propulsion system plumbing though. $\endgroup$ – Organic Marble Jul 17 at 21:03

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