Public domain & CC0 media
Definition
Images and footage free of copyright (public domain) or released with all rights waived (CC0), usable in videos without permission or fees.
Public domain works carry no copyright — in the US that includes most NASA media. CC0 works have their rights waived, as with the open-access collections of the Met, the Art Institute of Chicago and the Cleveland Museum of Art. Neither requires attribution, though crediting is good practice; CC BY does require it.
Watch for exceptions: NASA's insignia is not public domain, and archives also hold third-party items. SceneGen uses only CC0, public-domain and CC BY sources with strict filters and writes a Credits section automatically — see public domain footage for YouTube.