In an earlier post about “flybys” I used a brand new image of Pluto from NASA. Did you know that nearly everything in NASA’s archive of images, videos, audio, animations (and more!) are freely available for use, completely royalty free?
This is an incredible resource for science, space, or sci-fi related productions, like a performance of Holst’s The Planets (as an obvious example).
A great place to start is NASA ArtSpace for links to various collections, including those from other agencies such as the European Space Agency, the National Oceanic and Atmospheric Administration, and the U.S. Fish and Wildlife Service.
Other than acknowledging the source and not using their logos or seal, there are virtually no requirements but you can check out the complete media usage guidelines here.