[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?

[antoniseb]

I voted for Iapetus, but realistically we need to look at what sensing capability Cassini has, and also how much Hydrazine it has to do the work. I think it makes a lot of sense to keep Cassini in a high polar orbit so it can still be operational in July 2010 when the rings are next lined up with the Sun. I don't think that we'll learn all that much more about Titan by having another fifteen encounters with it. We might get some more detailed mapping. Possibly we could see some Titalogic feature change, but that's a long shot.

My thought would be to put Cassini into a near polar orbit via a series of encounters with Titan and Iapetus, and increase the perisaturn distance roughly the orbit of Iapetus so as to reduce the number of encounters with moons that they need to use Hydrazine to fine tune.

[ljk4-1]

How about sending it on to flybys of Uranus and Neptune? And Chiron?

[JRehling]

How about sending it on to flybys of Uranus and Neptune?  And Chiron?

A minimum-energy transfer from Saturn to Uranus would take about 50 years, during which time the probability of craft failure would only climb. Neptune would be far worse. During the cruise, no close flybys would take place. Even if Saturn-escape were feasible, this would be a way to minimize Cassini's science return.

Really, the priorities have to be (1) Titan, (2) Enceladus, (3) the rings, (4) Iapetus. Titan needs greater spatial coverage (it seems that even the maximal extended mission will result in gaps in the RADAR mapping). Enceladus needs greater temporal coverage (do the jets vary in their activity over mission timescales?), and will be sufficiently mapped at medium, but not high, resolutions in the primary mission. The rings offer vast spans of territory that is not entirely isotropic, and lots of temporal and spatial coverage could be a plus. Iapetus is a world of perhaps as few as one mystery, and it might be resolved with as few as three (or less) choice images that could be snapped in the remaining primary mission flybys if we're lucky.

If Iapetus is not figured out by the end of the primary mission, it should earn just one more well-targeted flyby at most. Titan, though, merits as many flybys as possible.

[dvandorn]

...If Iapetus is not figured out by the end of the primary mission, it should earn just one more well-targeted flyby at most. Titan, though, merits as many flybys as possible.

I agree. Also, you have to understand the situation with Iapetus -- that moon may embody a mystery, but it's a *frozen* mystery, frozen into Iapetus ever since the events that resulted in its current configuration and appearance. Whereas Titan and Enceladus are both exhibiting dynamic changes that can be studied during the course of the mission, Iapetus' secrets will stay safe and sound for many missions to come.

During the remaining time Cassini has to explore the Saturn system, I'd rather see it explore the dynamic systems available for obervation -- primarily, Titan and Enceladus, although Saturn and the rings also qualify.

If we can't unlock Iapetus' secrets in another two or three good, close flybys, I think we really ought to leave that mystery for later missions. That particular mystery isn't going anywhere. But it's always possible to see something unique in a dynamic system that will unlock a greater set of mysteries, either within the Cassini dataset or in combination with additional datasets collected, before, during and after Cassini.

-the other Doug