No, I never needed an appointment. This is a very small museum you can see everything within an hour.
No. It's a museum kinda. No appointment needed. It's kinda small and might be crowded with school kids though.
No, the visitor center is open to the general public. As others have mentioned, you can check out everything in about an hour. You may want to consider visiting when there's another event or program going on, though: https://www.nasa.gov/content/goddard-visitor-center-events-and-programs
No, there is a visitor center. Like the other answers say it is quite small and can be seen quite quickly.
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