Tricky Maneuver in Space

WITH their primary mapping mission successfully completed, controllers of the Magellan Venus orbiting satellite are trying something no spacecraft handlers dared do before. They are letting Magellan pass through the outer fringes of Venus's atmosphere during its closest approach to the planet, using the aerodynamic drag to change the craft's orbit from elliptical to nearly circular.

This aerobraking is a tremendous fuel saver. It would have taken nine times more fuel than Magellan had on board to circularize the orbit using thruster power alone. But aerobraking also is potentially dangerous. Small errors in navigation or misjudgments of atmospheric density and makeup could send a spacecraft off course. It might even drop too low and burn up from atmospheric friction.

Magellan has been doing this tricky maneuvering since May 25. Its latest aerobraking "drag pass" was scheduled for today, according to spokesman James Doyle at the California Institute of Technology's Jet Propulsion Laboratory (JPL) in Pasadena, Calif., which manages the program for the National Aeronautics and Space Administration.

The maneuvers have "gone better than anybody ever expected," says Magellan project manager Douglas Griffith. Temperatures from aerodynamic heating are running about 20 degrees C less than anticipated. Although Magellan was not designed with aerodynamics in mind, it is self-stabilizing during its atmospheric passes. By orienting the craft with its solar panels set 90 degrees to the wind and its main dish antenna trailing behind, controllers use 25 grams of fuel per pass for attitude control versus the 50 grams allocated.

Mr. Griffith calls this "a good surprise." It shows that proper aerodynamic design is the key to making aerobraking work. This should help cut costs of many future planetary missions. As Griffith explains: "People have sort of shied away from it [aerobraking].... I think we have shown there's not that much to fear about it." Mission designers should feel free to use fuel-saving aerobraking to put craft in orbits around planets, he says.

BY circularizing the spacecraft orbit, the Magellan team is going after the icing on its already substantial cake. It has fully met all of its mission goals in mapping 98 percent of the planet and gathering related data. These data include variations in the planet's gravity that reflect the distribution of subsurface mass.

These gravity variations cause slight changes in Magellan's speed, which show up as slight variations in the frequency of the radio signal by which ground stations track the spacecraft. JPL's Deep Space Network of ground stations can detect Magellan speed changes as small as 0.1 millimeters a second and do it every two seconds.

This can produce a detailed map of Venus's gravity field - and, hence, of subsurface structure. The degree of detail depends on the height at which Magellan passes over the planet's surface. For its main mission, Magellan was in an elliptical orbit 8,500 by 170 kilometers (5,300 by 100 miles). That yielded a gravity map with a resolution of about 200 km (125 miles) in equatorial regions. But it is less detailed over polar regions where the orbit swung wide of the planet.

Magellan scientists would like to get the spacecraft into a tighter, more circular orbit to produce a more detailed gravity map. They are aiming for an orbit of 200 by 600 kilometers (125 by 375 miles). That's about the best they can get with the aerobraking within a reasonable time.

Gordon Pettengill, a radar astronomer at the Massachusetts Institute of Technology, says, "I think it's fantastic that we've done all that we promised to and then some [with the main mission]." The detailed gravity data will help scientists make the most of their new information.

Dr. Pettengill - who is principal investigator for the Magellan radar program - notes that the team was able to obtain stereo images of about a third of the planet. When combined with altimeter data, "we can get really beautiful detailed topography," he says. By matching topographic features with subsurface structure from gravity data, geologists should be able to gain better insight into what shapes the Venusian surface.