Ascension island, British dependency of St. Helena (which also includes St. Helena and the Tristan da Cunha group), in the South Atlantic Ocean, about midway between Africa and South America and 700 miles northwest of St. Helena. The small, volcanic island was sighted by the Portuguese in 1501, but it was uninhabited until the exile of Napoleon to St. Helena in 1815, when the British established a garrison here to guard against his escape. In 1922 the island was annexed to St. Helena. It was a refueling base for U.S. aircraft in World War II. The United States built a station here in 1957 as a post for observing guided missiles. Huge numbers of sea turtles and sooty terns breed on the island. Georgetown is the main settlement.