Final Magellan mission completes a revolution in spaceflight

IN a few months, we will say goodbye to one of the outstanding scientific explorers of our time - the Magellan Venus-mapping spacecraft.

Its radar has revealed 98 percent of the surface of that cloud-shrouded planet in stunning detail. The spacecraft now is helping scientists build a similarly detailed gravity map. But in October, controllers will sacrifice Magellan. It will dip far enough into the planet's atmosphere to burn up as it tests the aerodynamics of various maneuvers.

Magellan will, of course, be remembered primarily for its Venus portrait. However, it will end its service in a manner that highlights a unique aspect of its mission that also is worth celebrating. For the first time in the development of spaceflight, engineers have dared to use the drag of a planet's atmosphere to change a spacecraft's orbit.

Such aerobraking can save a lot of rocket fuel. It is also dangerous. Small errors in navigation or misjudgments of the atmosphere's density and makeup could send a spacecraft off course. It might even drop too low and burn up. Spacecraft navigators have wondered for decades if aerobraking really is practical. Last year, they finally tried it with Magellan, and it worked.

When Magellan reached Venus on Aug. 10, 1990, it entered an orbit whose closest approach to the planet was 186 miles while its farthest distance was 5,270 miles. It did all of its mapping and made detailed gravity measurements in equatorial regions from the highly elliptical orbit.

To get detailed gravity readings over the rest of the planet, the project's scientific team wanted to move Magellan into a tight circular orbit close to the planet.

It would have taken nine times more fuel than Magellan had on board to do this using rocket thrusters alone. So on May 25 last year, controllers at the California Institute of Technology's Jet Propulsion Laboratory (JPL) in Pasadena, began dropping the orbit's low point to 87 miles. Once every orbit, Magellan skimmed Venus's thin outer atmosphere. The friction circularized the orbit. Gravity mapping began last August and continues.

The distribution of subsurface mass on Venus causes slight variations in its gravity field that show up as changes in Magellan's speed. These speed changes, in turn, make slight frequency shifts in the craft's radio signal. JPL's tracking stations can detect Magellan speed changes as small as 0.1 millimeters a second and do it every two seconds.

The insight the gravity data give is already paying off. Last week, JPL reported that the gravity data show evidence of active volcanic processes associated with some surface features. Geophysicist Suzanne Smrekar explained that the distribution of subsurface mass indicates several hot spots where molten material rises up in regions known as Alta Regio, Beta Regio, and Western Eistla. This causes the kind of volcanic action seen in Hawaii.

Dr. Smrekar explained that both top loading and bottom loading effects show up in the gravity data. In top loading, a large mass such as a mountain pushes down on the planet's crust. In bottom loading, an underlying mass indicates the welling up of hotter material beneath the surface.

Thanks to the ``magic'' of aerobraking, scientists now have gravity data to enrich their understanding of Magellan's radar maps. However, JPL has given the National Aeronautics and Space Administration (NASA), for whom it manages the Magellan project, more than a scientific bonus. It has given mission planners at that agency - and at every other space agency - confidence that they can use the cost-effective, fuel-saving techniques of aerobraking whenever they would be helpful.