John Glenn signature on Project Mercury first day cover
Project Mercury was the first human spaceflight program of the United States. It ran from 1959 through 1963 with the goal of putting a human in orbit around the Earth. The Mercury-Atlas 6 flight on February 20, 1962, was the first Mercury flight to achieve this goal.
The Post Office Department honored this first orbital flight of a United States astronaut on February 20, 1962, when it released the Project Mercury commemorative stamp, placed on sale throughout the country at the exact hour Colonel John Glenn's historic flight officially had returned to Earth safely.
In case the mission failed or was cancelled, the Bureau of Engraving and Printing kept word about the new press and the stamp issue's production a secret. The stamps, waiting at post offices around the U.S., were sealed and marked "Top Secret". Only after Glenn's trip were the postmasters allowed to open the packages and see what was inside.
Right after Glenn's safe return, the Post Office released the Project Mercury stamp. It became the first U.S. commemorative stamp issued nearly at the same time as the event occurred, and was released the exact hour the flight was officially completed.