PASADENA, Calif. (AP) - The U.S.-European Cassini spacecraft has been listening to natural radio signals from Saturn, and what it's hearing raises questions about the length of day on the ringed planet. Cassini found the rhythm of natural radio signals, regarded as the most reliable indicator of the length of day, shows a complete rotation takes 10 hours, 45 minutes and 45 seconds, plus or minus 36 seconds, NASA said in a statement Monday. That's about six minutes longer than the radio rotational period measured by the Voyager 1 and 2 spacecraft that flew by Saturn in 1980 and 1981....