Pioneer 5 was an American Pioneer Program space probe that was in service with NASA in the mid-20th century. A record of Pioneer 5 was available in the Enterprise library computer. This data was flashed on a viewscreen when the Talosians scanned the Enterprise computer in 2254. (TOS: "The Cage")
Pioneer 5 was launched from Earth on March 11, 1960. This probe survyed the interplanetary void between Earth and Venus, and confirmed the existence of interplanetary magnetic fields.