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

The Saturn-bound Cassini spacecraft suspended its observations of Jupiter on Wednesday because of troubles with its pointing system, ending the much-anticipated picture-taking and research as the probe heads to a close encounter with the giant gas planet next week.

On December 17 one of three primary reaction wheels in the two-story tall probe stopped operating normally. That caused Cassini to automatically switch its maneuvering system from the electrically driven wheels to the alternate method of pointing -- firing small hydrazine thrusters.

Worried the thrusters will guzzle too much of the precious fuel while pointing the spacecraft's instruments at Jupiter instead of saving the supply for the $3.4 billion mission's main goal to study Saturn, ground controllers on Wednesday commanded a halt to the current observations.

Tests are being performed on the suspect reaction wheel to gather more information on the problem. NASA says the wheel was needing more than the normal amount of force to turn it, possibly due to a piece of debris.

"We are responding cautiously while we test the systems," Cassini Program Manager Bob Mitchell said. "This might turn out to have no long-term consequences, but we want to better understand what happened before we proceed with using the wheels more."

It is possible the problem could be resolved in a few days, allowing the Jupiter observations to resume in about a week.

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.

"The spacecraft did what it was designed to do: It shut off the reaction wheels and began using thrusters instead," Mitchell said.

In a diagnostic test on December 18, reaction wheel number two still had higher than normal torque, the amount of force needed to turn it, when it was accelerating to a speed of 50 revolutions per minute. But it spun freely at speeds between 50 and 300 rpm, the space agency said, giving hope that if there was a small obstruction, it had been cleared.

Launched in October 1997 aboard a Titan 4B rocket from Cape Canaveral, 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.

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.