Explorer 27 was a satellite, launched in 1965, designed to conduct scientific research in the ionosphere. It was powered by 4 solar panels. One goal or the mission was to study in detail the shape of the Earth by way of investigating variations in its gravitational field. It was the third and last of the Beacons in the Explorers program. The satellite was shut off in 1973 because its transmission band was going to be used by higher-priority spacecraft.