My guess is that you won't experience the same exact sustained G-Forces astronauts do when they lift off on an actual space mission. My guess is they will do about 2 G's max to push you into your seat a bit (the most a modern roller coaster will do is about 5 and that gets people close to blacking out) The bottom line is that this is Disney and they arent going to put anything in their park that is too intense that people arent going to ride it. If people want thrill rides, they go to IOA, if they want a family fun time, they go to Disney. Even ToT and RnR are tame compared to most theme park rides out there.
I wouldnt worry about the ride. It will most likely be a very fun experience and not very intense. Just a hunch.
Cheers,
Bob Hansen (an avid Roller Coaster enthusiast who thinks that even if Mission Space is tame it is still going to rock!)