A mission to send a spacecraft to map the surface of Venus with imaging radar is underway at NASA's Jet Propulsion Laboratory. The project is called Magellan.   The spacecraft's basic science instrument is its synthetic aperture radar, or SAR, which can look through the thick clouds that perpetually shield the surface of Venus. Magellan will orbit the planet for 243 Earth days, one Venus day, on its primary mission. During that time it will map from 70 to 90 percent of the Venus surface.   Magellan was launched aboard the space shuttle Atlantis on May 4, 1989. Atlantis carried the spacecraft into low Earth orbit and an inertial upper stage, or IUS, sent Magellan on its way to Venus. The cruise will take 15 months because the Magellan spacecraft will make more than one and one-half revolutions around the Sun before it arrives at Venus.   Once at Venus, Magellan will be placed in an elliptical orbit that will take it as close as 250 kilometers (155 miles) and as far as 8,029 kilometers (4,889 miles) from the planet. The spacecraft will complete one orbit every three hours and nine minutes.   During its primary mission it will map most of the planet and if a further mission cycle is approved, the spacecraft will map areas it will have missed and perform gravity experiments.