In this historical photo from the U.S. space agency, a collection of NASA's research aircraft sit on the ramp at the Dryden Flight Research Center in July 1997: X-31, F-15 ACTIVE, SR-71, F-106, F-16XL Ship #2, X-38, and X-36.
Each weekday, SPACE.com looks back at the history of spaceflight through photos (archive).