Illustration of Cassini during its flyby of Jupiter. Photo: NASA/JPL

A glitch with the pointing system aboard NASA's Cassini space probe appeared to be resolved on Thursday, giving scientists optimism the craft could resume observations of the planet Jupiter during next Saturday's flyby. Cassini is on a 2.2-billion mile, seven-year interplanetary trek to Saturn.

Ground controllers were planning to switch the satellite's control system back to three electrically driven reaction wheels Thursday night. Cassini automatically abandoned the wheels needed for pointing last weekend because of an apparent obstruction in one of the units.

The spacecraft began using its hydrazine-fueled thrusters for orientation control as a substitute for the reaction wheels, allowing Cassini's instruments to continue picture-taking and the ongoing studies of Jupiter.

But officials decided to halt the observations in a bid to save the hydrazine for Cassini's main mission to explore the ringed planet Saturn and its moons starting in 2004.

Tests on the reaction wheel this week showed the suspect No. 2 wheel was spinning freely again with the mysterious obstruction suddenly gone.

"The results were all normal," Cassini Program Manager Bob Mitchell said Thursday. "If we had just seen results from these tests and nothing earlier, we wouldn't have any concern. It's encouraging, but we need to proceed cautiously."

An initial test on Sunday showed high friction on the wheel as it turned at slow speeds but not at higher speeds. Subsequent tests on Tuesday and Wednesday found no evidence of high torque on the wheel.

Speculation of what caused the problem centers around the idea that a small bit of material, perhaps from one of the motor's magnets, worked its way to a position that caused friction in the motor.

"If that's what happened, maybe centrifugal force threw it out or the motor ground it up," Mitchell said. "It doesn't seem to be there now."

Another possible cause may be reduced lubrication in the bearings due to prolonged operation at low rotation speed. If this is the cause, then the higher speeds used in the tests appear to have restored the lubrication, and new operating restrictions may need to be implemented about low-speed operation.

Cassini has four reaction wheel assemblies. Three are mounted mutually perpendicular to each other, and one is a spare. When an electric motor accelerates a wheel, the spacecraft rotates slowly in the opposite direction, obeying the physics law of each action having an opposite reaction. Moving the three wheels in various combinations can point the spacecraft in any desired direction.

Cassini won't start taking more pictures of Jupiter for at least a few more days while engineers watch how the reaction wheels perform. The craft is being positioned so its main antenna points towards Earth.

Scientific studies of the Jupiter system that do not require the spacecraft to use the wheels or thrusters for pointing, such as measurements of magnetic fields, are able to continue without interruption.

The plutonium-powered Cassini remains on track to pass Jupiter at a distance of about 6 million miles on December 30, receiving a sling-shot gravity boost from the planet to reach Saturn in July 2004. The craft's velocity will be over 30,000 mph following the gravity assist, an increase of 2,500 mph.

The space probe received gravity assists from Venus in 1998 and 1999. A close flyby with Earth occurred in 1999, too.

Cassini began studying Jupiter in October in collaboration with NASA's Galileo spacecraft, which has been orbiting Jupiter since 1995. Cassini has already returned thousands of images and measurements of Jupiter and its surrounding environment.