What ride at DISNEYLAND is most like Disney World's Space Mountain? An easy answer, right? It must be Space Mountain. But that would be incorrect- the real answer is the Matterhorn!
After the Matterhorn was a huge success at DISNEYLAND, Walt Disney decided that the park needed a second roller coaster. His new coaster would blast guests into near total darkness and take place in a fanciful, futuristic structure, with multiple "rockets" embarking on multiple trips to space and back at the same time.
To do all this would require a sophisticated ride system. Walt Disney asked his best imagineers to come up with a workable system that would allow him to fully realize his ideas. Unfortunately it was not possible with 1960's technology, so someone suggested merely replicating the Matterhorn setup indoors. This was not good enough for Walt, so he put the idea on hold until technology caught up with his dreams.
Years later, the idea of building the Matterhorn indoors was revived when Walt Disney World's "Magic Kingdom" needed a major attraction. Since Disney officials doubted that an outdoor roller coaster would work in the humid Florida weather, building the Matterhorn indoors was seen as a viable option. So instead of building the Space Mountain of Walt's dreams, Florida's "Magic Kingdom" got the Matterhorn copy Walt had originally rejected.
However, advances in technology eventually made it possible to fully realize Walt Disney's vision. So when Space Mountain opened at DISNEYLAND, it was exactly as Walt had imagined.