military was an early user of the Space Shuttle. By 1990 eight classified
satellites were deployed from the Shuttle for electronic intelligence,
photographic and radar reconnaissance, and defense communications.
Since then, classified military satellites have been launched by
expendable rockets. The Department of Defense also used the Shuttle
occasionally for unclassified research payloads when a crew was
Force Payload #675, an unclassified early warning satellite, was
launched on STS-39 in 1991.