THE PLANETARY REPORT
The Planetary Society WeblogBy Emily Lakdawalla
Cassini's Extended Mission TourFeb. 5, 2007 | 14:28 PST | 22:28 UTC
Weblog Archiveby John Spencer
We have a Cassini extended mission tour! A metaphorical puff of white smoke emerged from Building 167 at JPL on Thursday afternoon as the Cassini science team leaders met in executive session and made their final decision. Tour PF6h9 it is. I wasn't there for the final decision (I'm not that high in the Cassini hierarchy), but I participated in a lot of the deliberations that led up to that moment and found the whole process fascinating.
Cassini is two and a half years into its planned four-year prime mission in Saturn orbit. The spacecraft is healthy and continues to send back amazing data. There's every expectation that 18 months from now we'll still have a healthy spacecraft with plenty of gas in the tank, with an even longer string of discoveries crying for follow-up. But before NASA considers providing the funds to extend Cassini's exploration of Saturn, we need a plan. The project has been working hard to come up with that plan for the past year, focusing on the first two years after the end of the prime mission, from July 2008 to June 2010. There have been lots of options to sift through. The spacecraft can fly pretty much anywhere it wants in the Saturn system (except close to the inner ring system, where the rings themselves get in the way) by carefully targeting its flybys of Saturn's one big moon, Titan, and using its gravity to bend the trajectory in the required direction. And there are so many great places to go and amazing things to see!
Titan itself is a compelling target, the most Earth-like world in the solar system in many ways. Much of its territory will remain unexplored at the end of the prime mission, and the coming of northern spring is likely to lead to changes in its atmosphere (a spectacular north polar cloud system is already coming into view as the Sun swings northward) and maybe on its surface too. While lots of Titan flybys are guaranteed, we want to fly by specific places and view at specific angles to get the most out of the encounters, so the wish-list of the Titan team is long and detailed.
The rings team, too, has a lot to look forward to. Seasons are important here too -- the Sun passes through the ring plane on August 11, 2009, so there will be a period of a few days around that time when sunlight barely grazes the surface of the rings, and subtle features will be thrown into relief. We are likely to see corrugations in the ring surface, or even the shadows of small satellites, but only if the spacecraft is in the right place at the right time. The rings folks are also anxious for more opportunities to pass radio signals through the rings between the spacecraft and Earth -- a powerful technique for understanding the ring structure -- and that of course also requires Cassini to fly a very specific trajectory.
Those of us on the team covering the icy satellites (i.e., all the smaller satellites, excluding Titan) have our own shopping list. Our job has been simplified, in a way, by the discovery of internal activity and active geysers on Enceladus -- this makes Enceladus our single prime focus for the extended mission, though the other satellites are still interesting. We'd love to go back to Iapetus, which has only one close flyby in the prime mission, but it's so far from Saturn that we realized some time ago that we might have to give up on that dream -- getting back to Iapetus might close out too many other options.
Saturn's magnetosphere has its own advocates, with a rich array of phenomena to study. The magnetosphere folks are trying to understand how plasma is created in the system, how it is lost, how the moons (especially Titan and Enceladus) interact with the plasma, and how plasma is fed to Saturn's poles to create its spectacular aurorae. These investigations, too, require the spacecraft to visit specific places in the magnetosphere.
Then there's the giant planet itself, often neglected like a mother with too many precocious children, but fascinating in her own right. For instance, recent infrared images have shown remarkable structure in the deeper atmosphere that requires follow-up observations, but Saturn is so vast that it takes a lot of spacecraft time to map the clouds in adequate detail frequently enough to determine wind speeds. Often, the main requirement of the Saturn folks is just to be left alone to get on with their observations, so they are happy when the spacecraft has time near Saturn without a lot of close satellite flybys to take attention away from the planet.
With so many competing interests, there was no way anyone was going get everything they wanted. Still, a team of three JPL tour designers, Nathan Strange, John Smith, and Brent Buffington, led by sometime Planetary Society blogger David Seal, set to work to satisfy as many desires as possible. They had powerful software at their disposal, developed while flying the prime mission, to help them fly their simulated spacecraft to all the places the scientists wanted it to go. By our June 2006 science team meeting, in Nantes, France, they had come up with 11 possible tours, which were evaluated by the various teams. The icy satellites team had one favorite, called PF2, which provided six close Enceladus flybys, but this tour was a terrible option for some of the other teams, so there was no way it would fly. Likewise, tours that worked for other teams didn't work for us. No problem -- there was plenty of time to improve things. As one of the tour designers said, "we grew our flowers, we found the pretty ones, and now we'll cross-pollinate them."
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Discussing Tour Options
Dave Seal leads the discussion of extended mission tour options at the Cassini science team meeting at JPL on February 1, 2007. Several tours had already been eliminated by this stage. Credit: John Spencer
Come the October science team meeting, back at JPL, the next generation of tours were ready for inspection. Nathan, John, and Brent had indeed managed to combine the best features of the Nantes tours, and this time, one tour, PF4, came up green on most people's lists. This one had an amazing total of eight close Enceladus flybys while also keeping everyone else happy, and was declared "flyable" by pretty much everyone. Dave and his team got a standing ovation from the science team for achieving what had seemed, after Nantes, to be an impossible task.
But they weren't done yet. The designers talked to each science group in turn about what further improvements might be possible, and by last week's JPL meeting PF4 had spawned a dozen variants and sub-variants. It seemed the tour designers were competing with each other, bending over backwards to meet everyone's science goals in the hopes that a tour they were designing would be the chosen one. You wanted a Rhea flyby to measure the dust impact flux into the system? No problem! A radio occultation over Titan's north pole? OK, sure. Still no Iapetus flyby, but that was always a long shot. So each team evaluated each of the new tours, giving them green, yellow, or red designations (or occasionally lime green or orange when we felt indecisive). At Thursday's plenary, Dave Seal talked us through a multicolored matrix showing every team's opinion of every tour. PF4 was still in the running, but our expectations were now so inflated that it was no longer anyone's favorite, and it had quite a few red or yellow marks against it. One by one, tours were eliminated, and it was down to three options, with the front-runners being PF9 and a hybrid of PF6 and PF9, called PF6h9.
At that point the plenary meeting adjourned and the science team leaders went into executive session, while I found a quiet corner to get some other work done. After an hour or so came the fateful e-mail: PF6h9 had been chosen. This one has only seven Enceladus flybys -- sacrificing one of PF4's eight flybys bought a lot of good stuff for other teams -- but it will do nicely for Enceladus and everyone else.
Now comes some fine-tuning of the trajectory (such as adjusting the exact altitude of satellite flybys), a period of "segmentation" where we decide which teams get control of the spacecraft during specific intervals, and then the detailed planning of every observation for those two years. We have eighteen months to get that job done before we start flying the tour
Edited by Barek Halfhand, 07 February 2007 - 02:31 PM.