We were there in September when you could walk up. Now you need reservations and they are full until late January. You may be able to get walk up but need to be there about 6-7 AM. Check on line.
Advanced tickets are made available several months in advanced first come first serve. Otherwise you can get same day tickets starting at 630a on the day of your visit by visiting https://nmaahc.si.edu/visit/passes
My daughter got our tickets. She said she called.
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