Note: This only applies to works of the Federal Government and not to the work of any individual U.S. state, territory, commonwealth, county, municipality, or any other subdivision.
Use of NASA logos (which include the current "meatball" logo, the old "worm" logo, and the seal) is restricted.
The NASA website hosts a large number of images from the Soviet/Russian space agency, and other non-American space agencies. These are not in the public domain.