I was the Imagineer that pitched the idea of what became Mission:Space. (ducks as tomatoes fly!) I quit WDI before it was completed so it evolved alot after I left. Horizons was probably the best of the "show" attractions in Future World. I liked it alot. It had the optimistic soul of EPCOT as it's message. It was the most forward looking thing out there.
Truth be told, it got old. GE wanted out in part because the ride was getting low guest ratings and had lost much of it's ridership popularity. I saw the comment and turnstile reports and walked on it several times in it's last year. So WDW was looking at a thrill attraction for that area as kids fought their parents going to EPCOT. That slot was to be that "thrill" attraction to add some variety to the mix. In the beginning, MS had the "capsules" indoors on a coaster train in the original Horizon's building. I later found out Universal was developing exactly the same thing as Apollo 13!
MS did not "replace" Horizons, as Horizons was already off the table in their minds. It was not in lieu of some big Space Pavilion with guests on their own flying MMU, etc. That was way over anyone's budget and was already dead on arrival. The die was cast. Everything failed because it did not meet the thrill criteria.
That laid the groundwork for new thinking. It was not to be a "pavilion", as no sponsor could step up with that much investment. It was hard enough to keep the sponsors they had. We pitched the idea of sustained G Forces in a real capsule where you press the buttons. Trying to make it a science fact thrill attraction where you feel as many of the real sensations of space travel as possible was the EPCOT of doing a thrill ride was what we pitched. Make it as real as you can. Many things could have been done better, bigger, longer and it could have been a pavilion. All true. It was a miracle that the team even got it funded by Compaq/HP. We worked very hard to stay true to the EPCOT mantra of futurism. No SciFi. We flew the real shuttle simulator at NASA. Interviewed astronauts as to what things felt like, even rode the centrifuge they ride to train,etc. MS simulates the dream that anyone may really go to space in their lifetime, and with Branson launching tourist rockets, it may come true. It was a conscious decision to say that it's not for everyone as most thrill rides aren't. It's tight inside and filled with controls and the forces are unique too. We rolled the dice and in some cases we broke new ground, but in any event you learn something. The content and message of Horizons belongs in EPCOT, so now maybe it's time to take things even further. Here's an article that has some ideas for how that optimism and future living could be accomplished. I think theres room for both!
http://imagineeringdisney.blogspot.com/2009/05/wwed-armchair-imagineering-with-eddie.html
So it's a love/hate kind of project. That's fine too. Just thought you'd like the inside story.