They issue same day passes (for entry before 1pm) starting at 6:30am each day. After 1pm no need for passes.
Jil H. Lesko has the answer. We went for a visit after 1:00 and they did not ask for a pass. The museum was free. Start on the bottom floor and work your way up. Everything closes at 5.
It's free, you just have to register for a pass on their site before going
Access free.
https://nmaahc.si.edu/visit/passes
The Access is free, without charge
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