For those of you who came here looking for the term's idiomatic meaning, that's a Cloud Cuckoo Lander.
As for actual space cadets:
A Discredited Trope from the early Pulp Magazine days of media science fiction (peaked in the 1950's in movie serials and TV shows), involving a hero who was part of an organization that handles law and order in outer space, much like in a Western... in space. Frequently the titular Space Cadet was a child or teen who was a new recruit or a sidekick to an older hero. May be the equivalent of Walking the Earth with isolated outposts and frontier planets; especially strange when the Kid Hero seems not to have a family at home. His training place would be the Space Cadet Academy.
- Commander Cody.
- Buck Rogers, especially in the comic strip and movie serial, where the kid sidekick Buddy was added.
- Dickie in The Adventures of Captain Bucky and his Space Marshals, in Outer Space.
Dickie: Gee golly, Captain — I would have never thought of that!Captain Bucky: Well Dickie, that's why you're not The Captain.
- The Last Starfighter is about an Earth teen who gets recruited by aliens for this sort of organization.
- What Jim becomes in Treasure Planet.
- Space Cadet: Robert A. Heinlein novel, best known for its opening pages being so prophetic as to not read like Science Fiction today.
- His earlier story Misfit also fits the trope, and makes the inspiration from the Depression-era Civilian Conservation Corps more explicit.
- Tom Corbett, Space Cadet (based on the novel)
- The Lucky Starr series of books by Isaac Asimov, which started with David Starr, Space Ranger.
- Jeff Wells of Isaac and Janet Asimov's Norby series is also technically a space cadet, although most of his adventures are unofficial ones.
- Various midshipmen throughout David Weber's Honor Harrington stories, including Harrington herself in the novella "Ms. Midshipwoman Harrington".
- The Scott Saunders Space Adventure series by Patrick Moore.
- Space Cadet Happy from Space Patrol (1950's radio and TV)
- The Video Ranger from Captain Video and his Video Rangers
- Winky and Tagalong Kid Bobby from Rocky Jones, Space Ranger
- Tom Corbett: Space Cadet, as well as his companions Astro and Roger Manning, all cadets on a training ship. Rod Brown of the Rocket Rangers had a similar premise.
- Wesley Crusher represents the best example in the Star Trek universe. As a child prodigy, he represented an Audience Surrogate for a young viewer and could be considered a "sidekick" to Picard. Later in the series, Wesley became a literal cadet in Starfleet. Harry Kim is the closest example in Star Trek: Voyager, while "Red Squad" in Deep Space Nine is something of a Deconstruction of the trope.
- 3-D Pinball: Space Cadet is named after this.