I can give you the details. It's my favorite attraction and I'm afraid of heights and can get a little sick reading in the car.
You will be assigned (by the ever good looking Gary Sinise) to groups of four to take a test mission to Mars. You'll be assigned to positions and "expected" to complete pushing buttons for a successful mission. During all this, there will be repeated warning about motion sickness and if you want to leave, you can certainly do so after the preshow. Once you are done with the preshow, you will be lined up for your "ships". You can still bail out at any time.
The ships (or I like to call them pods) will be next. Four people in each, all lined up in one row. This is where people have the problems. It's a small space. Getting into the pod is not a problem. And you can still bail out if necessary. There will be barf bags in front of you. You store everything you have into the front containers. You will pull down the overhead constraints. Then, before take off (and you can still bail out), the front will close up and the controls will be right in front of you. You will now be launched to go to Mars.
The takeoff simulates some G forces. And it does it quite well. The first time I went on this, I felt a little weird after but I think it was just because I didn't know what to expect. You'll get a little feeling of weightlessness but that only last a few seconds.
Words to the wise and they tell you this in the preshow. Look straight ahead - do NOT look to the sides. Keep your arms down unless you are getting into your role and pushing the buttons when Mr. Sinise tells you do. Even if you don't, not a problem, you'll still get to Mars. And do NOT shut your eyes.
For kelmac284 - my 6-1/2 year old granddaughter was a concern for all of us. She got out of that pod with the biggest smile on her face and yelled "we gotta do that again!"
For paladin - Rock N Roller Coaster gives me a headache too. I think it's because it jerks around so much. Mission:Space doesn't do that.
For kim35 - I think what scares most people is that they will never experience anything like this. How many times do you get to say you know what it's like to take off in a space shuttle?
For everyone, I'd say, at least go through the preshow and see the pods. Then if you don't want to go on it, bail out. The Cast Members are great and it's very simple to do.