WASHINGTON (AP) _ The Magellan spacecraft settled lower in its orbit of Venus Wednesday and project managers said an effort to put the science probe into a circular orbit is going as planned.

''It's been just as we scripted it so far,'' Douglas Griffith, the Magellan project manager, said at a news conference. ''There's been no difficulty to this point.

Spacecraft controllers commanded Magellan to fire its thrusters on Tuesday to start an 80-day maneuver that would put the craft into a near-circular orbit of Venus. A second firing on Wednesday continued the process.

Magellan had been in an orbit of about 105 miles by 5,282 miles above the Venusian surface. With a series of small rocket thrusts, the orbit will be changed so that the spacecraft dips shallowly into the Venus atmosphere. This will increase resistance on the craft and cause it to settle toward a more circular orbit.

The technique, called ''aerobraking,'' uses the dragging effect of the atmosphere to reshape the spacecraft orbit.

Once at the lower orbit, scientists will use measured speed changes on the craft to map the gravity fields of Venus. Just as on Earth, gravity strength varies from point to point on the planet. By measuring the gravity changes, scientists can draw conclusions about the density of unseen structures beneath the surface.

Controllers are able to measure Megallan's velocity changes to within a third of an inch per second. Gravitational highs, caused by dense material on Venus, will cause the craft to accelerate slightly.

The gravitational measurements are a bonus for the $800 million mission.

Magellan was launched from Earth four years ago and arrived at Venus orbit on Aug. 10, 1990. During an eight-month cycle, the spacecraft used radar signals to probe the surface of Venus. An altimeter gathered measurements of the planet's highs and lows. Engineers used these data to create the most detailed map yet of the mountains and valleys and volcanic peaks on Venus.

Griffith said the aerobraking was designed to squeeze even more data from the mission and he admitted there was some risk that Magellan could be destroyed.

''The craft was never intended to do aerobraking,'' he said. But Magellan is near the end of its fuel supply and the aerobraking manuevering is the only way the orbit could be lowered, the scientist said.

If Magellan dropped too swiftly and encountered the atmosphere at too high of a speed, it could burn up. For this reason, Griffith said the aerobraking manuevers are planned as small steps over 80 days that will keep temperatures on the spacecraft about 100 degrees cooler than dangerous levels.

Eventually, engineers hope the craft will achieve an orbit of about 124 by 372 miles.

In any case, the series of manuevers may be the last activity for Magellan, one of NASA's most successful space probes. The mission is scheduled to end by August and officials said there is little likelihood funds will be found to extend the program.

When the program ends, engineers will stop monitoring the spacecraft. Griffith said that without periodic instructions from the ground, Magellan will drift out of control within two months or so and then eventually burn up in the Venusian atmosphere.