No tix are required during low season, which I believe is until late March. But I would confirm by checking out their website.
There are no tickets but you do have to make a reservation by going through the website. Just as Tierra said but if you go now thru March you are good to just walk in because of the off season. My suggestion is to plan ahead as far as you can if you plan on going after March and go early because it's a lot to see, you can not see and enjoy everything all in one trip if you go later in the day.
Nmaahc.si.edu/visit
Go to their website
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