Pictorial plaque attached to the exterior of the Pioneer 10 and 11 spacecraft, indicating their origin in case they encounter an intelligent alien species. The 'dumb-bell' symbol at top left represents a hydrogen atom, the most common element in the universe. Below, 14 lines radiate from our Sun to the relative positions of 14 pulsars (sources of radio energy acting here as cosmic signposts). Human figures, the man's hand raised in goodwill, are set against the spacecraft for scale. At bottom is a plan of the Solar System, showing the route of a Pioneer spacecraft from Earth. Pioneer 10 and 11 left Earth in 1972 and 1973 respectively. The frequency of each pulsar at the time of launch is quoted in binary (dashed lines) relative to that of the hydrogen atom with its '1' unity symbol.