Scientists who figured they knew how long a day lasted on Saturn (search) are having second thoughts.

The Cassini spacecraft has been listening to natural radio signals from Saturn, the most reliable method of determining a day's length. Cassini's transmissions show a complete rotation takes 10 hours, 45 minutes and 45 seconds, plus or minus 36 seconds, NASA (search) said in a statement Monday.

That's about six minutes longer than measurements performed by the Voyager 1 and 2 spacecraft (search) that flew by Saturn in 1980 and 1981. Observations made in France in 1997 also differed from the Voyager findings.

Cassini, due to enter Saturn's orbit Wednesday night, gathered radio data from April 29, 2003, to June 10, 2004.

"We all agree that the radio rotation period of Saturn is longer today than it was during the Voyager flyby in 1980," said Michael D. Desch, a member of the Cassini team and a scientist at NASA's Goddard Space Flight Center.

But scientists doubt the planet is rotating more slowly. They are looking instead for something deep inside Saturn that would cause variability in the radio pulse.

"I don't think any of us could conceive of any process that would cause the rotation of the entire planet to actually slow down," said Don Gurnett, a Cassini scientist who works at the University of Iowa.

Gurnett said there appears to be "some kind of slippage between the deep interior of the planet and the magnetic field, which controls the charged particles responsible for the radio emission."