After the start of the Space Age with the launch of Sputnik I on October 4, 1957, astrophilately (space-related stamp collecting) began. Nations such as the United States and USSR issued commemorative postage stamps depicting spacecraft and satellites. Astrophilately was most popular during the years of the Apollo program's Moon landings from 1969 to 1972. Collectors and dealers sought philatelic souvenirs related to the American space flight program, often through specially-designed envelopes (known as covers). Cancelling covers submitted by the public became a major duty of the employees of the Kennedy Space Center (KSC) post office on space mission launch days.