Cassini Edging Toward Jupiter

December 29, 2000 / 7:14 PM EST / CBS/AP

Engineers suspect reduced lubrication caused a friction problem in a maneuvering system on the Cassini spacecraft and they will change the way it is operated, Jet Propulsion Laboratory said.

The spacecraft, launched in 1997 on an 11-year $3.4 billion mission to Saturn, experienced the problem with one of its reaction wheels on Dec. 17. Friction was indicated by a motor drawing extra electricity to make the wheel turn.

The durability of the reaction wheels is important because of the mission's length. Cassini is expected to study Saturn for four years after dropping a probe on the moon Titan and entering orbit around the ringed planet in July 2004.

Engineers determined that a probable cause was prolonged operation of the reaction wheel at relatively low speeds, which may have caused reduced lubrication in the bearings. The distribution of the lubricant may have been restored when the wheel was run at higher speeds in subsequent tests.

"That's our leading theory, but we may never know for sure," Cassini program manager Bob Mitchell said in a statement.

Engineeers initially speculated that some bit of material could have become caught in the motor mechanism and then was ground up or spit out.

Controllers will develop operational procedures for Cassini's reaction wheels to avoid low-speed operations of significant length, Mitchell said.

Cassini uses three reaction wheels to change its orientation. Spinning the wheels in various combinations points the spacecraft and its instruments in particular directions. The wheels don't change Cassini's course through space.

The spacecraft has a spare reaction wheel and it can also change its orientation with a thruster system, but the supply of thruster fuel is limited.

The reaction wheel system has been trouble-free since it was put back in operation on Dec. 21, JPL said.

Cassini, a project of NASA and the European and Italian space agencies, will pass Jupiter at a distance of 6 million miles early Saturday, picking up a boost from the big planet's gravity.

Thousands of images and scientific measurements of Jupiter acquired by Cassini between early October and mid-December are being processed.