Cassini Extended Mission Tour
This week the Cassini Project Science Group (PSG) is meeting at JPL to decide on a two-year “tour” (the trajectory to be flown by the Cassini spacecraft) for the extended mission, affectionately known as the “XM”. The task is not small because the Cassini XM is planned to operate in the same mode as the nominal mission, which means that all of Cassini’s five main science objectives (Saturn, Titan, icy satellites, rings, and the magnetosphere) continue to get equal weight for the XM. Thus there are five competing interests for the geometry of Cassini’s XM tour. While the interests of one group are not necessarily in conflict with those of others, in general it is difficult to accommodate all of the top priorities of each group. Time taken for close flybys of an icy moon, for example, means less time to get the spacecraft in position to observe the effects of the changing season on Saturn’s rings. The JPL tour designers have done a terrific job of coming up with a dozen or so excellent tour candidates, so I’m confident that the various science groups will be satisfied with the tour we select by the end of the week.
In my science discipline (rings) one of the key goals for the XM is to see how the rings behave at Saturn’s equinox, in mid-2009. Saturn’s seasons are about 7 years long, so one might expect very gradual changes in response to a change in season. However, since the rings are a plane, there is a sudden transition from one season (Sun shining on the south face of the rings, currently) to the next season (Sun shining on the north face of the rings). The CIRS instrument measures the temperatures of the ring particles, so this one change-of-season event is an opportunity to see the temperature change of ring particles that may have spent years with only one face in sunlight. These observations in the extended mission will tell us about the rotation of the particles and how they move relative to their neighbors in the rings. That’s just one of many examples of unique science opportunities in the Cassini extended mission.
March 26th, 2007 at 10:14 am
The equinox view of the rings sounds exciting. Do you have optimal distances and view angles for those studies, or is it best to have a variety? They probably won’t leave Cassini in the high inclination ‘08 orbit for a whole year, would they? Or did the mission planners cross their fingers on a possible extension and have Cassini right where they wanted it?
Will the icy satellite people get any more flybys of Mimas, Hyperion, and/or Iapetus? I know the latter two are inclined to and beyond Titan’s orbit, but they seem like such interesting targets. No other irregular moon in the solar system is as large as Hyperion. The empty spaces within that body…aside from the curiosity factor, I can imagine 23rd century spelunkers itching to delve into that moon.
Will we have to wait for NASA/gov approval before the XM plan is published? Can you pass along any other hints aside from what’s been gossiped about on other sites? What if Cassini works like Spirit and Opportunity at a pace of multiples past warranty? Would it be a disappointment if Cassini operating for another few decades delays another mission out there?
March 28th, 2007 at 10:25 pm
Wow, a lot of great questions. Thanks for your interest. There are indeed optimum view angles and distances for studying the response of the ring particles to equinox, but we are constrained by the fact that we’re just orbiting Saturn and can’t position ourselves at the best spot and stay there. There is also the pesky matter of those other scientists who are interested in things other than the rings (incredible as that may seem). So it’s a matter of compromise. The selected tour for the extended mission does a good job of satisfying multiple requirements. The inclination profile (how the inclination varies with time) peaks at the end of the nominal mission, stays high for a little while, and then gradually comes down to the equator, and pops up to small inclinations before the end of the extended mission. The reason we have the high inclination sequence at the end of the nominal mission rather than at the beginning is that there were strong geometric reasons for meeting some of the low-inclination goals early in the mission (for example: delivering the Huygens probe, getting radio occultations of the rings, and also arriving at Saturn near the equatorial plane).
There will be more good views of Mimas, but not Iapetus in the extended mission, and I think we’re done with Hyperion as well. There is another look at Iapetus in the prime mission, however. Hyperion is definitely an interesting beast.
I’m not sure about a formal publication of the XM plan. John Spencer had a good summary at the Planetary Society blog a few weeks ago. We’re all hoping Cassini gets more than one extension, but decades is probably out of the question because maneuvering fuel will run out. We’re using more than half of our remaining budget of fuel in the first two-year XM, so any subsequent extensions will have to be run in a more conservative manner. And that usually means cheaper.