File:Kuiper belt - Oort cloud-en.svg is a vector version of this file. It has been marked as inferior to this JPG file. (reason: It was obviously derived from this .jpg version, not the other way around. It is not nearly as clear.)
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]