Radar-mapper Magellan orbits Venus.

Radar-mapper Magellan orbits Venus

The Magellan spacecraft climaxed its 15-month trip to Venus on Aug. 10 by entering a near-perfect orbit from which scientists expect to compile the most detailed maps yet of the cloud-covered planet's surface.

Magellan now orbits Venus every 3 hours and 15 minutes, following an elliptical orbit that ranges from 294 to 8,472 kilometers above the surface. It achieved an orbit so close to the one intended that engineers canceled a maneuver to "fine-tune" the satellite's flight track around Venus, says project manager Anthony J. Spear of NASA's Jet Propulsion Laboratory in Pasadena, Calif.

The craft's primary goal is to map the planet's surface, using synthetic-aperture radar and a radar altimeter that measures the ups and downs of Venus' topography. Magellan's orbit will carry it over the entire surface every 243 days, and scientists hope to get data from as many as five complete passes. The first pass should map 70 to 80 percent of the surface. Later passes should fill in the blanks. On these mappings, scientists will aim the radar at different angles to create stereographic, three-dimensional radar images and gain a more precise view of the planet's small-scale surface features.

Radar maps of Venus have also been made by the U.S. Pioneer Venus, mostly in 1979; by the Soviet Venera 15 and 16 craft in 1983; and by Earth-based instruments. Magellan scientists plan to check out its equipment and begin mapping about Sept. 1, Spear says.

An unexpected event occurred when Magellan jettisoned the solid-propellant rocket motor that had put it into orbit. Spear says the jolt apparently caused one set of gyros in Magellan's positioning system to switch off and another set to take over. The change posed no problem for the mission, he says, and the original gyros automatically switched back on and are available if needed.

A component failure in the memory of one of Magellan's computers should cause only limited problems, according to Spears. Engineers expect to radio Magellan instructions to work around the failed component, and Spear says the only loss will be 1,000 words of reserve capacity from the computer's 34,000-word memory.