U.S. space agency scientists say the Cassini spacecraft has successfully switched to a backup set of propulsion thrusters as it orbits Saturn.

The National Aeronautics and Space Administration said the late Wednesday swap was performed because of degradation in the performance of the primary thrusters, which had been in use since Cassini's launch in 1997.

The space agency said the switch marked only the second time in Cassini's 11 years of flight that its engineering teams have gone to a backup system.

"The thrusters are used for making small corrections to the spacecraft's course, for some attitude control functions, and for making angular momentum adjustments in the reaction wheels, which also are used for attitude control," NASA said, noting nearly all Cassini engineering subsystems have redundant backup capabilities.

Cassini, which has successfully completed its original four-year planned tour of Saturn and is now in extended mission operations, is a cooperative project of NASA and the European and Italian space agencies.

The Cassini mission is managed by NASA's Jet Propulsion Laboratory in Pasadena, Calif.