From NASA's website Brand Guidelines

NASA Graphic Standards

The goal of the NASA Graphics Standards Manual is to establish a clear and consistent visual identity in keeping with general brand standards. Issued under the authority of 14 Code of Federal Regulations (CFR) 1221. The following (comprehensive) guidelines are extracted from the Graphics Standards Manual and are intended to provide basic guidelines for Insignia, Logotype and supporting graphics use.

NOTE:
The NASA Insignia, Logotype, identifiers, and imagery are not in the public domain. The use of the Insignia, Logotype and NASA identifiers is protected by law, and imagery is made available for use consistent with Media Usage Guidelines.