[X] My Assistant

[imran]

Whether this is a two-year extension or longer I'd still have to go with Titan. There is so much of this world which we don't understand and have only begun to unveil that it begs us to focus our attention on this dynamic world. Some important questions that may only be answered with additional flybys:

- What is the source of methane in the atmosphere?
- Are there any bodies of liquids on the surface?
- Is there active precipitation?
- Is Titan geologically active, i.e. cryvolcanism, tectonism, etc?
- What is the extent of the organics present?
- Is there an ammonia/water mixture under the surface?

[Guest_BruceMoomaw_*]

Digging on the Web has revealed two limited fragments of publicly available information about the planning for an extended mission.

(1) The Cassini Plasma Spectrometer team has its team meetings available on the Web ( http://caps.space.swri.edu/caps/TeamMeetings/teammeet.html ), and the last two (#31 and 32) have some interesting tidbits. First, we have confirmation that the main limiting factors in the planning are the fact that will be 165+ 35 meters/sec of main engine delta-V left for the E.M., and at an average of 10 meters/sec for each targeted encounter that means 13-20 encounters (presumably including other moons as well as Titan). Of course, these can -- and very well may -- all be stretched out in timing for a very long period, just as Cassini can go on observing the Saturn system long after it loses its ability to set up any more flybys. Second, there are the four momentum wheels, of which one is currently off due to "bearing cage oscillation" (but can still be reactivated as a substitute if one of the others fails). When we lose two, Cassini will have to start falling back on its attitude fuel during all three-axis stabilized observation periods.

Second, the final tour selection will come next February -- but the first 4 orbits of the extended mission have already been chosen to ease transition planning (just as the early orbits of the primary mission, through mid-2005, were also chosen well ahead of the rest). No clue, however, as to what they involve.

Third, all plans call for Cassini to be returned to a ring-plane orbit; but there's a dispute over how long to wait before doing so. Ring and aurora scientists want to leave Cassini highly inclined for up to 9 more orbits before starting down; others feel differently. Whenever they start the transition, just getting back to the ring plane will itself take about 8 Titan encounters -- half the number of moon encounters that Cassini will have left in it.

Fourth, the document says that nine sample tours have been generated by the nav team, mostly to show the consequences of basic choices (reducing inclination fast or slowly, moving apoapsis to the nightside clockwise or counterclockwise; using the "180-degrees apart" Titan flyby strategy a second time or not). These, however, being just crude models, lack many of the features of the real Extended Mission -- for instance, they include no specific icy moon flybys or designed occultations. Not do they include any "special things" -- and the CAPS team specifies two such special things that might possibly be put into the Extended Mission. Cassini might be put into an orbit with a period that's a multiple of Enceladus' period, so tht it can make repeated flybys of that moon; and/or it might be put into a modestly inclined orbit with its periapsis very greatly lowered so that the craft can actually fly "over" much of the A Ring for detailed cross-sectional observations (as it did during its initial orbit insertion).

(2) The Magnetometer team have a document that provides more data on those nine sample tours:
http://www.sp.ph.ic.ac.uk/~nach/EM/Extende..._Encounters.xls

Three of them start out by actually further increasing Cassini's inclination at the end of its primary mission (75.4 degrees) up to about 80 degrees before lowering it again to the ring plane. Another, after spending some time in the ring plane, raises the craft's inclination again during equinox. Two of them return to the magnetotail; and one "attempts to maximize the number of close non-targeted small satellite flybys, with particular emphasis on Enceladus." That one uses the maximum delta-C (159 m/sec); some of the others use as little as 82.