Find a local supporter; schl, clubs, work, etc, and use their membership for free admission.
Pay as you like / can /want to. But they recommend an entrance fee as shown in the entrance lobby or online. Theaters are charged. See lobby or online.
General admission to walk around museum and planetarium is "pay as you wish" (donation) based. If you would like to see the Dark Universe space show then it's a set price of $28.
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