NASA / SCIENCE PHOTO LIBRARY NASA / SCIENCE PHOTO LIBRARY
Pioneer plaque. Coloured version of a pictorial plaque attached to the exterior of the Pioneer 10 and 11 spacecrafts, indicating their origin in case an intelligent alien species one day examines it. 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 - cosmic sources of radio energy. Human figures, the man's hand raised in goodwill, are set against the spacecraft to give 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, relative to that of the Hydrogen atom with its "1" unity symbol.
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