From Stranmillis to Saturn

By Prof. Carl D. Murray
Professor of Mathematics and Astronomy
Queen Mary, University of London

I am not quite sure how I got here. It seems to me that one minute I was trudging up a cold Stranmillis Road in Belfast to attend an Irish Astronomical Association evening lecture at the Ashby Institute and now, more than twenty years later, I am heavily involved in the most exciting planetary mission of the millennium – the Cassini-Huygens mission to Saturn. The lectures at the IAA meetings inspired me and today I feel as excited about astronomy as I did all those years ago. The only real difference is that back then I could only dream about my ideal career whereas now I am lucky enough to be living it.

As a schoolboy in Belfast (on the Antrim side of the Lagan!), I devoured information about the Apollo missions to the moon. To be honest, I was not interested in astronomy as such; what captured my imagination was space travel! In those days the IAA gave free public showings of the NASA films about the Apollo moon landings. That was the hook that got me to my first IAA meeting and from then on I knew that I wanted to do “research” in astronomy. I wasn’t quite sure what that entailed but if it involved learning more about astronomy then that was good enough for me. I did A-levels in Maths, Physics and Chemistry. Realising that I was reasonably good at mathematics I decided to combine this with my interest in astronomy by doing a degree course in Applied Mathematics with Astrophysics at Queen Mary College, part of the University of London. Various people had warned me about specialising in astronomy too early and I figured that if there were no jobs in astronomy I could always emphasise the mathematical content of my degree to gain useful employment.

I did well enough in my BSc to obtain a funded studentship to do a PhD in Astronomy, also at Queen Mary. By that stage my research interest had focused on planetary science. I had realised that the upcoming Voyager missions to the outer solar system would revolutionise the subject and that, if I was good enough, I could play my part in understanding our local area of the universe. More importantly, the questions posed by the results from Voyager could be answered in my lifetime. My main thesis topic was meteor streams and I was one of the first people to undertake computer simulations of meteoroids evolving under the gravitational effect of the planets. The direction of my research changed to planetary rings when my supervisor arranged for Stan Dermott (then at Cornell University in the USA) to visit Queen Mary. At the time Stan was working on a new dynamical model to explain narrow planetary rings. We started working together on the “horseshoe orbit” model whereby a single moon embedded in a ring maintains a narrow ring against the spreading effect of various processes. Stan invited me to work with him at Cornell University when I completed my PhD. So it was that on 31 st December 1979 I arrived in freezing upstate New York.

Saturn's F ring with shepherd moon PrometheusCornell was a wonderful place to work. Several members of the Voyager imaging team were based there, including Carl Sagan, and there was always a steady stream of visitors telling us about the latest results from on-going planetary missions. I worked with Stan on a variety of problems, most connected with understanding the dynamics of the asteroid belt between Mars and Jupiter. One of the highlights was when the Voyager spacecraft passed Saturn in November 1980 (Voyager 1) and August 1981 (Voyager 2). I think that is when my fascination with Saturn’s F ring began. Seeing the Voyager 1 images of this bizarre, twisted, braided narrow ring orbiting just beyond Saturn’s main rings, posed enormous puzzles for me and all ring dynamicists. What could be causing such strange behaviour? Could we ever understand the physical processes that were at work? Explaining objects such as the F ring is fundamental because they make us question all of our assumptions and test our understanding to the extreme. We have to assume that we understand the basic laws of physics and so the task is to work out how they are operating in this particular environment – if we succeed then we are making real progress.

I left Cornell in the summer of 1982 and returned to Queen Mary to do research in problems connected with long-term orbital stability in the solar system. To remain in academia a researcher has to obtain a faculty position. While this provides some form of job security it also means less time for research. I began a lectureship at Queen Mary in 1985 and when Voyager 2 passed Uranus in January 1986 I was too busy lecturing to get out to the Jet Propulsion Lab in California to witness the event. However, I did manage to be at JPL and see the final planetary flyby when the spacecraft encountered Neptune in August 1989. As the Voyager era drew to a close the Cassini era was about to begin.

By the end of 1989 NASA and ESA had issued a joint “announcement of opportunity”, inviting interested scientists to propose instruments or participate in the Cassini-Huygens mission. By that stage I had become interested in image processing and had done some work with Voyager images of rings and so I applied for membership of the Imaging Science Subsystem team on Cassini. My application described the sort of work I would want to do with the cameras on Cassini, the NASA-built spacecraft that was designed to orbit Saturn. Not surprisingly my application mentioned the F ring and other narrow rings, as well as the work on orbits of small satellites that I wanted to undertake. It was almost a year later that I heard from NASA Headquarters that my application was successful and that I was now a member of the Imaging Team on Cassini. In fact, UK scientists were remarkably successful in the selection process both for Cassini and the ESA-built Huygens probe that is designed to land on Titan, Saturn’s largest and most mysterious moon.

F ring with streaming particlesIt has been an incredible privilege for me to me involved in this mission. The selected scientists, many of them veterans from the Voyager project, had to work for seven years before the spacecraft was even launched and have had to wait another seven years before it finally reached the ringed planet in July of 2004. That was a definite highlight for me and my colleagues from Queen Mary. We were all at JPL to witness the Saturn Orbit Insertion and analyse the close-up images of Saturn’s rings as they were returned. I had already had a foretaste of what the F ring might have to offer when I analysed images taken on approach to Saturn. These showed that the F ring was up to its usual tricks and had its fair share of unusual features. However, the images taken soon after the main engine burn had been completed showed us the inner part of the F ring being disrupted by the moon Prometheus as it passed by. We had long suspected that this small moon had played a role in the dynamics of the F ring but here was proof positive. Subsequently we have had incredible images showing streams of material being dragged out of the F ring by Prometheus, confirming numerical models that I had worked on with one of my research students several years ago. I was lucky enough to be the first person to have detected a small object, previously unseen, that was orbiting just beyond the outer edge of the F ring. Because the F ring changes all the time with features coming and going, there is no way of knowing whether this new object is a permanent member of the system or just some temporary clump of material that may vanish one day. Time will tell.

Saturn by Cassini

Titan is dominating everyone’s thoughts at the moment, with the successful Huygens probe separation which occurred in the early hours of Christmas Day and then, if all goes according to plan, landing on the surface of Titan on 14th January 2005. The images of Titan’s surface returned from our cameras show bright and dark features but nothing that look like craters. Lots of different instruments on the spacecraft are scanning Titan but the real ground truth will come when Huygens breaks through the cloud and haze layers surrounding Titan and sends back images of the exotic surface of this alien-like world. Whatever happens with Huygens, the Cassini spacecraft will carry on orbiting Saturn until at least June 2008. We will have ample opportunity to study this wonderful planet, its array of moons and spectacular rings. I am sure that we will produce enough data to keep future generations of planetary scientists busy for many years to come. I still cannot quite believe that this is all happening but the adventure is not over yet and I am thrilled to be playing my part in it.



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