The Source of the G Ring

Todd J. Barber, Cassini lead propulsion engineer

After a hiatus of two weeks, there is a myriad of newsworthy items from the realm of Saturn. Cassini continues to execute its orbital trysts with the ringed planet and its vast family of moons. Following the successful T34 encounter with smog-enshrouded Titan on July 19, 2007, the flight team exhaled a collective and happy sigh for the slight break that was to come. Cassini's current elliptical orbit of Saturn has a period of roughly 43 days, positively relaxed compared to the 16-day orbits that predominated between January and June of this year. One reason for this leisurely trip around the sixth planet is an impending solar conjunction, a purely geometric necessity that yearly places the sun between Earth and Saturn (roughly). It is again a happy coincidence that this relatively quiet period for the spacecraft team corresponds to summer vacations in the United States and particularly Europe. We look forward to this respite, courtesy one Isaac Newton.

These dog days of summer in the Northern Hemisphere are hardly a time to rest on prior accomplishments, however. The flight team has been busy taking and evaluating science data from a wide range of instruments, including multi-disciplinary investigations of Saturn's perplexing magnetosphere. We've also executed an engineering test of reaction wheel friction, a precious spacecraft resource for the prime tour and possible extended mission. Fortunately, we've noted little change in the friction characteristics of the reaction wheels since the last test in early May. Our radio science team also performed a boresight calibration for Cassini's high gain antenna. Even relatively quiet periods on Cassini are punctuated by a vast array of science and engineering activities!

One noteworthy science result was recently disseminated via press release. Our flagship spacecraft and its Earth-bound scientists may have determined the source of Saturn's intriguing G ring, one of the least understood of Saturn's rings. The source of this icy material appears to be relatively large particles within an arc of the icy ring's inner edge. Saturn's twin of the "Death Star," Mimas, apparently "babysits" these ring particles gravitationally and forms this arc. Micrometeroid hits in the arc and wafts of plasma within Saturn's magnetosphere conspire to spread out this icy material into the G ring. Named in order of discovery A through G, the rings of Saturn continue to astound us in their complexity and structure. Incidentally, in order of increasing distance from the planet, the rings are labeled D-C-B-A-F-G-E, a seven-letter collection that should be familiar to musicians. Various composers have actually used this ordering to concoct musical works based on this seven-letter sequence, truly "heavenly" music in a literal sense, as every science bit from Cassini is music to our eager ears on Earth.