1959-06-03 USA Discoverer 3

Discoverer 3 was an American optical reconnaissance satellite launched on June 3, 1959, at 20:09:20 GMT. It was the third of three test flights in the Corona KH-1 spy satellite series and marked the first Discoverer mission to carry live animal passengers. Unfortunately, the mission was lost when its Agena-A booster crashed into the Pacific Ocean. Three black mice were launched from Vandenberg Air Force Base, California, aboard Discoverer 3 as part of this covert operation. They perished when the Atlas upper stage was intentionally fired downward into the Pacific.

Discoverer 3 "Goldcraft" launch cover postmarked at Vandenberg Air Force Base on June 3, 1959.
(Reference from Discoverer 3)