Explorer III with generic Swanson cachet

Space event covers, in contrast to First Day or commemorative envelopes, are used by astrophilatelists to tell the story of the exploration of the heavens with postage stamps. Unlike thematic or topical collecting, it is the postmark with the correct date and the nearby post office location adjacent to the controlling event, which tells the story. NASA unmanned programs included; Ranger, Discoverer, Explorer, Lunar Orbiter, Surveyor, Vanguard, Viking, Mariner, Pioneer, and Voyager.