Works of the US government are not copyrighted.
There is none, they're automatically public domain.
Mostly true. There are some restrictions on images containing NASA logos or NASA employees, for example. https://www.nasa.gov/nasa-brand-center/images-and-media/
I've been waiting for a new Earth Rise/Set shot (which is thankfully at least 5568x3712px from a NIKON D5).
Edit: https://www.nasa.gov/nasa-brand-center/images-and-media/
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