The NASA website hosts a large number of images from the Soviet/Russian space agency, and other non-American space agencies. These are not necessarily in the public domain.
The SOHO (ESA & NASA) joint project implies that all materials created by its probe are copyrighted and require permission for commercial non-educational use. [2]
Figure 5 from Quintana et al. "An Earth-sized Planet in the Habitable Zone of a Cool Star" NASA publication http://www.nasa.gov/sites/default/files/files/kepler186_main_final.pdf
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