On July 7, 1961, the U.S. Air Force launched a satellite called Discoverer 26 into orbit with a classified payload.
It lifted off from Vandenberg Air Force Base in California at 4:29 p.m. local time (2329 GMT).
Discoverer 26 was part of a series of reconnaissance or spy satellites whose missions were kept top secret by the American government until 1992.
While other Discoverer satellites were spying on Russia, China and the Middle East, Discoverer 26 was used to conduct experiments and new test spacecraft engineering techniques in orbit.
The mission also evaluated the Agena-B second-stage rocket booster.
This part of the launch vehicle contained instruments that could measure how ions and micrometeoroids in low Earth orbit affected the spacecraft.
Source: https://www.space.com/39251-on-this-day-in-space.html