People mention the expensive parking. The expo line of the metro system has a stop across the street from the center. So if you use metro, this is good way to get there. I also should mention that they charge something like $2 to be able to make reservations online. The site is a little misleading because it looks like you have to pay a $2 fee for each place you need an appointment (i.e. special exhibits, the Endeavour, etc.), but it's just one flat fee of$2 to make however many appointments you need.
Parking is $12 I believe. Maybe $15? Entrance is free. Movies are a separate fee.
Entrance free you have to pay parking and extra on specials exhibits
Most part of the ScienCenter is free. If you want to see the Endeavour, or if you want to visit the Natural History Museum, you pay a few bucks, and it is always worthy. Of course, if you consider what you pay for parking, is always best if you spend more than a couple of hours there.
Admission is free for everything but special exhibits and the space shuttle. Parking is not free though.
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