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Magellan(mă-jell -ăn, -gel -) NASA's planetary probe to Venus that was launched from the space shuttle Atlantis May 4 1989 and reached the planet Aug. 10 1989. From three 8-month cycles of radar mapping (Sept. 1990–Sept. 1992), it produced a detailed radar map of almost 99% of the surface of Venus, revealing details as small as 100 meters across. It also provided a gravity map showing density variations in the surface features, and obtained an immense amount of data.
Magellan's mapping orbit carried it over the N pole and down the length of the planet almost to the S pole; it was close enough, during about 37 minutes of each orbit, for a 20–25-km swathe to be highlighted by the radar beam. In the remainder of the elliptical orbit Magellan transmitted data to Earth and fixed its position, before it resumed mapping over a new swathe of the slowly rotating planet. A topographical map was built up by radar altimetry, using signals from a small antenna pointing straight down. The larger antenna, pointing off to one side of the spacecraft, developed a detailed two-dimensional picture from the signals reflected off each object encountered; it also detected thermal emissions.
Impact craters, canyons, and other features were revealed on the planet's surface. Lava flows, fault zones, and volcanoes indicate volcanic activity and occur over about 85% of the surface of Venus, but no evidence was found for the type of plate tectonics found on Earth. There are relatively few impact craters, suggesting that Venus' surface is ‘geologically’ young. Craters are distributed randomly over the Venusian landscape, with roughly two-thirds showing no modification by volcanic activity. However, surface features include very long lava channels, vast lava plains, fields of small lava domes, shield volcanoes, and large ‘pancake’ domes that may have been formed from a type of lava produced by large-scale evolution of crustal rocks. Interpretation of these results in terms of past ‘geological’ activity is still under debate.
Gravity mapping of the surface commenced in Magellan's elliptical orbit. A successful aerobraking maneuver brought the craft into a low-altitude near-circular orbit in Aug. 1993, from which a detailed pole-to-pole gravity survey could be made. The Magellan mission ended Oct 11 1994 when the craft was sent plunging into Venus' dense atmosphere, sending back valuable data on its composition as it hurtled to its destruction.