Now that Cassini’s extended mission is established (though we’re still waiting for final budget approval), the planning phases for what to do with Cassini after the extended mission (XM) are not too far away. Science requirements will be needed in early 2008 for the extended-extended mission (XXM). The exciting discovery of geysers spewing water vapor from Enceladus’s south pole made that small moon a prime target for the XM. Recent observations show that others of Saturn’s large family of icy satellites are also getting into the act. If Enceladus’s geysers are the “steaming gun” (as UVIS co-investigator Candice Hansen phrased it) responsible for Saturn’s E ring, recent measurements by the Cassini Plasma Spectrometer (CAPS) have identified Tethys and Dione as radiating guns (okay, I’m probably pushing the smoking gun metaphor way too far here), spewing electrons into Saturn’s magnetosphere.
The difficulty with getting follow-up observations of the moons is that, for all intents and purposes, moons are points in space while the rings, for example, are a plane. That makes it much harder to get good observation time of a moon. Cassini’s orbit has to be tuned to provide a “targeted flyby” and even then the prime observing period is less than a day long. Most of these flybys are best achieved from equatorial orbits. That contrasts completely with observations of the rings which require inclined orbits to make the razor-thin rings visible, though once the orbit is inclined useful observations of the rings can be had virtually continuously. (Saturn’s faint E and G rings, and for some purposes its bizarre F ring, can be best observed from un-inclined equatorial orbits, however.) A conflict looming for XXM planning will be the inclination of Cassini’s orbit. Equatorial orbits favor observations of Saturn’s atmosphere (at least at low to mid latitudes) and satellite flybys, while inclined orbits favor ring observations, observations of Saturn’s polar regions and aurorae, and are generally more favorable for magnetospheric observations. In the meantime, we are going through the incredible amount of excellent data returned by Cassini’s dozen instruments to find the next smoking gun. There must be one hidden in the rings somewhere….
Too bad it\’s not the good ol\’ days when NASA sent two probes per mission. Wouldn\’t it be possible to set Cassini into an inclined orbit with some resonance with the orbit of Dione or Tethys? Near periapsis there would be two crossings of the orbital plane.
If memory serves, there\’s only one 16000km pass of Tethys left, and only one for Dione in the extended mission.
It is indeed possible to visit moons while on an inclined orbit. One problems is that inclined orbits generally require more spacecraft fuel to keep them stable than equatorial orbits, although there is a particular inclination where most perturbations to the orbit cancel.
Cassini should be aerobraked into orbit around Titan to radar-map the surface. That\’s where the real action is.
You can still observe Saturn\\\\\\\\\\\\\\\’s atmosphere from an inclined orbit, even low latitude atmosphere.
I don’t think going into orbit around Titan is energetically feasible. There are also what are called “planetary protection” concerns: Cassini was not built to be a biologically sterile spacecraft, so there may be concerns about putting it in an orbit where it might someday crash onto either Titan or Enceladus. dfk: I’m with you on that one. The only aspect of Saturn studies that can be truly decimated is study of the rings by going into an equatorial orbit. Inclined orbits offer something for everyone. We’ll see how it plays out over the next year or two.
Yeah, mission manager Bob Mitchell told me explicitly some time ago that aerobraking into orbitr around Titan is just totally impossible for Cassini — not only would it require far more remaining fuel than it will have, but its magnetometer boom would throw it into a dangerous tumble every time it brushed through Titan’s atmosphere. A pity; scientifically, that would be the perfect finish-up for this mission.
Let me recommend that you take a look at the most recent analyses of the best follow-up missions to Titan and Enceladus at
http://www.lpi.usra.edu/opag/TitanEnceladusBillionDollarBox.pdf
http://www.lpi.usra.edu/opag/may_07_meeting/presentations/titan_flagship_studya.pdf
http://www.lpi.usra.edu/opag/may_07_meeting/presentations/titan_flagship_studya.pdf
http://www.lpi.usra.edu/pss/presentations/niebur_flagship.pdf
Correction: that third URL should be
http://www.lpi.usra.edu/opag/may_07_meeting/presentations/titan_flagship_study_b.pdf