John Spencer: Cassini's proposed extended-extended mission tour
Posted by John Spencer
24-02-2009 14:58 CST
I asked John Spencer to write something on the proposed plans for Cassini's second mission extension. The mission has just selected an orbital tour for the extension -- the plan for where Cassini will travel, when -- and is now seeking approval from NASA Headquarters for the funding necessary to keep the Cassini mission going through the middle of 2017. Thanks so much to John for this! --ES
EDIT: Check out also this neat graphic summarizing the moon flybys throughout the Cassini mission to Saturn. --ESL
It seems like no time since we selected Cassini's extended mission tour of the Saturn system, in early 2007. Now we're flying that tour, which extends Cassini's original four years in Saturn orbit for another 27 months, until September 2010. So now we're looking into the future- far into the future. If NASA approves the mission plan that was recommended by the Cassini project in late January, and if the spacecraft remains healthy, we hope to continue Cassini's exploration of Saturn for another seven years, until 2017. The ungainly working title of this planned new mission phase is the extended-extended mission, or XXM.
The year 2017 is special because May 2017 marks northern midsummer in the Saturn system, and the more elegant alternative name for the XXM is the Solstice Mission. A Saturn year is 29.4 Earth years, and the considerable 27 degree tilt of Saturn's pole (which, incidentally and bizzarely, can probably be blamed on the planet Neptune), means that the slow progression of the seasons has major effects, especially on Saturn itself and on its giant moon Titan.
We arrived at Saturn in July 2004, in the depths of northern winter, equivalent to January 15th on a terrestrial calendar. The southern poles of Saturn and its entourage of large satellites were brightly illuminated by sunlight, and the northern poles were in darkness. The shadow of Saturn's rings was draped dramatically across most of the planet's northern hemisphere, robbing even mid-latitudes of sunlight. The entire northern hemisphere of Saturn had the winter blues, literally -- the yellowish haze that normally gives Saturn its golden color was missing, leaving a clear blue sky that made a striking contrast with the yellow hues of the southern hemisphere.
Meanwhile, on Titan, the constant sunlight was powering summer methane storms at the south pole. The northern winter pole was shrouded in a thick blanket of haze overlying a frigid polar atmospheric vortex where exotic chemicals were accumulating. Titan's vortex is similar in many ways to the polar vortex that traps pollutants over Antarctica and creates the winter ozone hole on Earth. Cassini's radar and cameras later revealed a vast complex of methane lakes and seas near the north pole, in striking contrast to the drier south pole.
Now the seasons are changing, and the northern spring equinox will finally bring light to the many north poles of the Saturn system in September 2009. The shadow of Saturn's rings has retreated almost to the planet's equator, and the northern blues are fading. Titan's south polar storms have abated, though the northern vortex persists. Even the rings are showing seasonal changes- the ghostly ring "spokes" discovered in early northern spring by Voyager 1 in 1980 were absent early in Cassini's mission but have reappeared as the sun slants at ever more grazing angles across the ring system, revealing that sunlight probably plays a key role in producing these odd features. The few days centered on August 11, 2009, when the sun's illumination switches from the south side of the rings to the north side, should be one of the highlights of the current extended mission, as the sunlight skimming across the rings throws any irregularities in the ring plane into sharp relief.
We want to learn more about all these seasonal changes, and other long-term changes, by continuing our sojourn in the Saturn system from northern spring equinox all the way through the 2017 northern summer solstice. We have so many questions! When and how will Titan's northern winter vortex break up, and when and how will its southern counterpart be established (if at all...)? Will any of the northern methane lakes dry up, and will new ones form in the south? Does the Titanian north have its own summer storm season, and how will the storms interact with the northern lakes and seas? Will Saturn's hazy southern skies clear as winter approaches? How do the seasons affect Saturn's weather patterns? Will the ring spokes fade away with the advancement of northern Spring?
There are also other kinds of questions we hope to answer by extending Cassini's mission deep into the next decade. How will the magnetosphere, and Saturn's system of aurorae, react to the increased buffeting of the solar wind as we approach solar maximum around 2012? Are there long-term changes in Enceladus' geysers and internal heat radiation, and if so, what's the effect on the E ring and magnetosphere? Are there long-term changes in the structure of Saturn's ring system? Is Enceladus' geological activity unique among the mid-sized satellites, or is Dione, which has shown subtle hints of low-level activity in Cassini data taken so far, also active now or in the relatively recent geological past?
So there are lots of reasons to keep exploring, but there's a catch (well, several catches, but here's the biggest one from a technical standpoint). By the end of the current extended mission we will have expended about 80% of the propellant that Cassini originally had available for its orbital maneuvers. We use close flybys of Titan to bend Cassini's trajectory and fling it wherever we need it to go in the Saturn system, but it takes propellant to set up those flybys just the way we want them, and to nudge the spacecraft towards its other targets, such as Enceladus. Now we want to fly the spacecraft for seven more years using only a quarter as much fuel as we'll have used in the first six years in orbit.
As it turns out, converting our SUV into a Prius is easier than you might think. Mostly it's a matter of being patient. In the old paradigm, we would spend fuel to cram in as many flybys, occultations, and other goodies as we could into our limited time in orbit. In the future, with more time to play with, we plan to take a more mellow attitude, waiting for those opportunities to come to us instead of spending fuel to make them happen.
With the realization that a long mission extension is technically feasible, and with our science goals enumerated, we had to choose an actual orbital tour to meet those goals. This process has taken most of the past year, and the bulk of the work has fallen on Cassini's two tour designers, Brent Buffington and John Smith. The process of sorting through the innumerable potential options to meet all the competing demands of the science teams is fiendishly complicated, but John and Brent did a magnificent job. As an example, I had several conversations with them as they tried to arrange at least one Enceladus flyby that would provide a good look at the active south pole from a range of a few thousand kilometers. In early versions of the tour, we had no such flybys (they are difficult to arrange with a limited fuel budget), but by the time they had finished working their magic, we had two of them, along with 10 other Enceladus flybys with a variety of geometries, allowing us to map the moon's gravity, penetrate even more deeply into its geysers, and get a good look at its northern hemisphere too.
Alas, some miracles are beyond even John and Brent. They tried mightily to set up a close flyby of distant, enigmatic, Iapetus to follow up on discoveries made during Cassini's only close flyby so far, in September 2007, but they just couldn't make it happen without blowing our fuel budget.
The final XXM tour includes those 12 Enceladus flybys, three very close Dione flybys, 56 flybys of Titan, and many other goodies, such as another chance to observe the amazing tendrils of Enceladus from within Saturn's shadow. But we plan to save the best for last. Once Cassini's propellant runs out, we will no longer be able to control its trajectory. We need to dispose of the spacecraft before that happens, so it doesn't run into Enceladus or Titan (or other moons), and contaminate them with any terrestrial bugs that might have survived all those years in space. So Brent and John came up with a plan (still tentative, as it requires approval by the NASA Planetary Protection Office) to safely dispose of Cassini in Saturn's atmosphere, and gather some amazing science along the way.
If the current plan is approved, around Thanksgiving 2016 Cassini will be in an orbit which brings it as close as 3.63 Saturn radii from Saturn's center, between the orbits of Mimas and Enceladus. Then, on November 29, 2016 the spacecraft will use its penultimate Titan flyby to alter its orbit so that Saturn closest approach drops to 2.51 Saturn radii, just 10,000 kilometers beyond the narrow F ring, and not far from the outer edge of the main rings.
Cassini will execute 20 of these close "F-ring" orbits before setting up for a final close Titan flyby on April 22, 2017. This flyby will do something astonishing: it will perturb the orbit so that Saturn closest approach jumps, in a single leap, from just outside the main ring system into the narrow zone of safety between the inner edge of the innermost ring (the D ring) and the planet itself, just 3,800 kilometers above Saturn's cloud tops. Cassini will continue to thread this needle for 23 orbits (called, with some understatement, the "proximal" orbits) until a final distant nudge from Titan on September 11, 2017 delivers the death blow, altering the orbit just enough to drop Cassini into Saturn on September 15.
These final orbits will be an entirely new mission for Cassini, similar in many ways to the Juno mission, which will be executing a series of cloud-skimming orbits of Jupiter at about the same time. By coming so close to Saturn, we can map its gravity and magnetic field in exquisite detail, probing the planet's deep interior. By flying between the planet and the rings we can separate the gravitational effects of the rings from Saturn itself, providing for the first time a reliable estimate of the total mass of Saturn's ring system. Currently we don't know the mass of the rings, which is crucial for understanding their age and evolution, to better than a factor of ten. We should even be able to analyze the composition of Saturn's atmosphere directly with Cassini's mass spectrometer, as we have done in the past for the atmosphere of Titan and the plumes of Enceladus. This will be a fantastic way to end the mission.
There remains one major hurdle before we can put these plans in motion: money. Cassini will need another seven years of funding, and NASA has many other claims on its budget. To sweeten the deal the Cassini project has found ways to simplify operations by concentrating efforts on the highest-priority science, and it looks like we can run each year of the XXM on considerably less money than we need to maintain the more hectic pace of the prime and extended missions. A couple of weeks ago I participated in a "Senior Review" at JPL, where we made the scientific case for the XXM to an external panel of scientists. The panel will pass on their recommendations to NASA headquarters, and we will wait more-or-less patiently for the final decision. In the meantime, we have lots of preparatory work to do to flesh out the details of the XXM. If and when we get the go-ahead from headquarters, we will be ready.
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