Discoverer (Program, Satellite) was the first U.S. Military satellite program. It
was an unmanned program to place a relatively large satellite into a low-earth (90
nm) orbit. Conceived about the time of the USSR’s Sputnik, it was considerably larger
that this basketball-size “first” in space. The first Discoverer, an Agena built
by Lockheed, was launched on a Douglas Thor rocket in February, 28 1959 and was about
the size of a school bus.
[Wikipedia]