(covers information from several alternate timelines)
The spacedock in a synchronous orbit of Earth, commonly known as Spacedock, was a Federation Spacedock-type space station in service with Starfleet in the late 23rd century. This facility was constructed prior to 2285.
In 2285, the USS Enterprise returned to Spacedock after serving as a cadet training vessel on an ill-fated mission in which Khan's plan to steal the Genesis Device and kill Admiral James T. Kirk for revenge was thwarted. At Spacedock, the Enterprise was to be decommissioned, rather than repaired. Kirk – with the help of Captain Scott as well as Commanders Uhura, Sulu and Chekov – stole the Enterprise to rescue Spock's body from the Genesis Planet. The USS Excelsior attempted to give chase, but had been sabotaged by Captain Scott, and "stalled" when it attempted to jump to transwarp just outside the station. (Star Trek III: The Search for Spock)
In 2286, an unknown probe entirely disabled Spacedock upon reaching Earth. The shutdown happened before any vessels within, including the Excelsior, could be launched, resulting in the equally powerless vessels being trapped behind the space doors. When Kirk and his crew had resolved the crisis, power was restored as the probe left the planet. Kirk was demoted to captain, and offered command of a starship – namely the USS Enterprise-A, which was docked within Spacedock. (Star Trek IV: The Voyage Home)
After a brief shakedown cruise, the Enterprise-A returned to Spacedock while Captain Scott worked to resolve design flaws made apparent by the ship's maiden voyage. (Star Trek V: The Final Frontier)
The Enterprise-A was kept at Spacedock prior to an ill-fated trip to escort Klingon Chancellor Gorkon to a peace conference on Earth in 2293. Kirk ordered a departure from the facility at impulse speed, despite the fact that regulations specified thrusters only while inside the structure. Although Lieutenant Valeris (the ship's helmsman) initially questioned the captain's order, she successfully performed the departure and later admitted she had "always wanted to try that."
Background information Edit
In script sources, this facility is most commonly known simply as "Spacedock", with an emphasis on the term being capitalized.
Spacedock was imagined as having a total height of over three miles. At that size, the facility could easily be visible with the naked eye from the surface of Earth. (text commentary, Star Trek III: The Search for Spock (Special Edition), DVD)
Although windows providing a view of Spacedock's interior docking bay appear only in Star Trek III: The Search for Spock, they were also originally intended to be seen in Star Trek IV: The Voyage Home, with cheering audience members gathered by them. (The Making of the Trek Films, 3rd ed., p. 64)
It was not made clear on-screen whether the facility featured in "Non Sequitur" was intended to be the same spacedock as seen in the films, as the Star Trek Encyclopedia speculates. The only view of the station in the Star Trek: Voyager episode is of a set of doors, themselves stock footage of the Dyson sphere doors from "Relics".
It is probable, though unconfirmed, that this prime universe facility is either identical to, or a part of, the never-mentioned but all-but-mandated-to-exist Starbase 1.
For the month of December in the 2010 Star Trek: Ships of the Line calendar, Spacedock is shown being constructed as the USS Enterprise's refit is nearing completion.
In the game Star Trek Online, this spacedock was replaced with another sometime before the year 2409. However, the new spacedock's design was not popular amongst players, and there were many calls for it to be replaced with the classic design. For STO's first anniversary on 2 February 2011, the developers replaced the new Spacedock with the original facility, explaining it as being the work of Q. Though the external model (as visible from space) appeared to be the classic spacedock, the interior area (which served as a major player hub) remained based on the original, launch design. With the release of the game's Season 9 update, Earth Spacedock received a second revamp to correct the inconsistency, and the interior area received a major redesign to be more appropriate to the scale and presumed layout of Earth spacedock. The change was justified in the story via mission "Surface Tension", in which the Undine caused severe damage to the station during their attack on Earth, necessitating extensive repairs and relocation of player activities to a different area of the dock.