The science of space travel. It was formerly subsumed under aeronautics.

Both subjects include hypersonics, the study of airflows and forces at speeds above five times that of sound (Mach 5), as experienced by space rockets and guided missiles.

http://solarsystem.nasa.gov/basics/ Illustrated tutorials covering essential aspects of space flight, including the space environment, flight projects, and operations. The site has a useful glossary of technical terms and abbreviations.