Todd J. Barber, Cassini lead propulsion engineer
Dr. Murray obtained his B.S. in Applied Mathematics with Astrophysics from Queen Mary University of London in 1977, followed by a Ph.D. in 1979 from the same school. His doctoral work concerned the evolution of small particles in the solar system, with some work on planetary rings (recall at this time the Jovian rings had just been discovered). His postdoc work at Cornell included studying Kirkwood gaps in the asteroid belt, orbital locations with a paucity of asteroids due to orbital resonances with Jupiter. I think I was most shocked to learn that Kirkwood himself was inspired to look at asteroid belt gaps after studying the Cassini Division at Saturn, another gap caused by orbital resonance! I would have bet heavily that this inspiration would have occurred the other way around. After leaving Cornell, he returned to England to work on chaos in the solar system, one of my favorite topics (in fact, it nearly derailed my attempts to learn more about the F ring!). During the Voyager 2 encounter at Uranus (early 1986), Dr. Murray was lecturing back at his alma mater, but for Neptune encounter in 1989, he obtained press credentials and came to JPL to cover Voyager 2’s planetary swan song for “Physics World”! Anyway, it’s not hard to see from where Dr. Murray’s passion for the F ring comes. This perplexing, beautiful Saturnian ring seems to me the perfect marriage of his vast and diverse academic and professional career.
I asked Dr. Murray about recent observations, as well as plans for the future. He told me last year was historic, for there is an orbital precession with a 19-year period that “peaked” last year, when the apochrone of Prometheus’ elliptical orbit aligned with the perichrone of the F ring. Readers familiar with the terms “apogee” and “aphelion” for the most distant portions of Earth and solar orbits, respectively, can likely deduce that “apochrone” is the farthest point in a given orbit to Saturn (“Chronos” being the Greek name for Saturn). The same holds true for “perichrone” as an analogue to “perigee” and “perihelion”—the closest points in these respective orbits. Since Prometheus orbits inside the F ring, this means last year Prometheus made its deepest possible “dip” into the F ring region, causing gravitational mischief not seen in two decades! He also told me his current paper includes a discussion of bright F ring objects that were found to cast shadows during last year’s Saturnian equinox. Outside of his area of expertise, I asked about the composition and age of the F ring, but I may have to save that for another column (particularly since some results may not have been published yet).
We closed our wonderful chat by talking about his plans for studying the F ring during the Cassini Solstice Mission. He said it’s very difficult to locate objects that collide with the F ring, but he will keep looking for them. He will also try to determine the lifetime of F ring features, and he reminded me that the mass of the F-ring remains an important unknown. Finally, Dr. Murray patiently awaits Cassini’s “F-ring orbits” in November of 2016, inclined orbital passes within just a few thousand kilometers of the F ring. Dr. Murray, I along with the rest of the world await what mysteries the F-ring will reveal and how you and your team will explain them to us. Thank you very much for your time.