NASA says it's conducting diagnostic tests on its Cassini spacecraft orbiting Saturn after its signal was not detected during a tracking pass in late December.

The spacecraft has been communicating with Earth using a backup radio system, the space agency said in a release Thursday.

The problem centers on the ultra-stable oscillator, used for radio science experiments and also as a means of sending data to Earth.

Cassini mission engineers say the spacecraft has shifted to an auxiliary oscillator, whose frequency stability is adequate for transmitting data from the spacecraft to Earth.

Tests this month will help mission managers decide whether it will be possible to bring the ultra-stable oscillator back into service, NASA said.

Age may by a reason for the problem, NASA said, noting Cassini has been in orbit around Saturn since 2004 and has had its mission extended twice.

Cassini was launched in 1997.