Primary Target For Extended Mission Options

[JRehling]

What about Iapetus?

Only one close approach on Sep 10, 2007 and four or five far-encounters between 120.000-900.000 km until 2008.
Bad show so far...
There should be at least 2 more fly-bys imaging the region of the 'Snowman' and the giant southern bassin in prime light,
a far from what will be found out by the Sep 10 fly-by next year.

The crucial problem will be shifting the direction of Cassini's periapsis; then it would not be too hard to get it to intersect Iapetus's orbit in two different locations than it does now; then we could see different longitudes. So long as Cassini visits the same part of Iapetus's orbit, we'll keep seeing about the same parts of Iapetus over and over. A similar thing happens with Titan; the inner moons present less of a problem, because the distances across the inner system are shorter.

I don't think Iapetus merits such a change on its own, but a campaign to visit some other Titanian longitudes in the extended mission might be synergistic with Iapetus followup. One caveat is that Iapetus's orbital inclination means that we have to work in a third dimension to get a very close flyby except where Iapetus happens to cross the ringplane. And that could make it hard to get back to Titan, maybe setting up a phase of mission where non-Iapetus flybys are rare. On the other hand, if the mission length is dictated primarily by thruster fuel, then maybe taking a relative "lull" in the extended mission might not be such a bad thing: we'll still get to have the same total sum number of encounters before thruster fuel runs out, but stretching them over time would let us see more seasonal progression (esp. interesting with Titan).

The other caveat is that the best Iapetus flyby comes near the end of the primary mission, and it may just solve most of the Iapetus mysteries. Then the market value of Iapetus followups would plummet, and we might wish we'd spent more time playing hummingbird with Enceladus or RADAR-mapping Titan.

[Guest_AlexBlackwell_*]

According to the Wikipedia entry (taken, I believe, originally from the Cassini website anyway), the maximum delta-V for a gravity assist maneuver is twice the orbital speed of the assisting body. In the case of Titan, the orbital velocity is 5.5 km/s. Obviously you're only going to approach this value in very specific circumstances, but it does provide an upper bound.

I'm still not sure where you're getting 11 km/s. It that Saturn-relative or solar-relative? ΔV is directly related to how close to Titan's center you fly; in other words, it's a function of flyby altitude. A flyby that requires passage through Titan's atmosphere or lithosphere is, shall we say, not practicable. In any event, one of the primary "givens" that Cassini Mission Planning and Cassini Navigation operate under for Titan gravity assists is: "...a Titan flyby at an altitude of 950 km [the previously accepted minimum flyby altitude] imparts an equivalent ΔV of about 800 m/s to the spacecraft. Since the orbiter has a total ΔV capability after SOI of less than 500 m/s, missing almost any planned Titan flyby would have serious consequences."

This post has been edited by AlexBlackwell: Mar 23 2006, 06:17 PM

[Bart]

For all purposes, that upper bound is probably more than an order of magnitude higher than what's realistically doable, especially taking into account the pretty large radius the flybys need to be done -- a consequence of Titan's small density and very extensive atmosphere.

"...a Titan flyby at an altitude of 950 km [the previously accepted minimum flyby altitude] imparts an equivalent ΔV of about 800 m/s to the spacecraft. Since the orbiter has a total ΔV capability after SOI of less than 500 m/s, missing almost any planned Titan flyby would have serious consequences."

Thanks! That's the kind of useful feedback I was looking for.

Maybe I'm missing the point you're trying to make, which is distinctly possible, but the fact remains that onboard propulsive delta V (both deterministic and statistical) is needed to "circularize [Cassini's] orbit" to make it "Titan-like." And practice has shown that Cassini loses 10 m/s from its propellant budget on a typical targeted Titan flyby.

And that's the final nail in the coffin. Obviously if you've only got 16 or 17 Titan flybys left for the extended mission (I'm basing this on the above & Alex's post #43), then I heartily endorse doing something more interesting than just circularizing the orbit.

Bart

[Guest_AlexBlackwell_*]

It would be hard to predict just where [the main engine cover] would hit, wouldn't it?

Probably, since the main engine cover wasn't designed for impact experiments. Assuming, though, that the ejection dynamics could be accurately modelled, I wouldn't see any showstoppers for such an experiment. Just place Cassini on an intercept trajectory with, say, Rhea or Tethys and let 'er rip. Then perform an orbiter deflection maneuver to set up the flyby geometry. Hey, that technique sounds familiar, doesn't it?

This post has been edited by AlexBlackwell: Mar 23 2006, 06:29 PM

[JRehling]

Probably, since the main engine cover wasn't designed for impact experiments. Assuming, though, that the ejection dynamics could be accurately modelled, I wouldn't see any showstoppers for such an experiment. Just place Cassini on an intercept trajectory with, say, Rhea or Tethys and let 'er rip. Then perform an orbiter deflection maneuver to set up the flyby geometry. Hey, that technique sounds familiar, doesn't it?

If such a daredevil thing were attempted, I think Iapetus would be the strong favorite, since that might probe the thickness of the dark stuff. Dione might be a distant third, since it has some kinda dark stuff. Enceladus would obviously be verboten; attempt no crashing there.

[Guest_AlexBlackwell_*]

The crucial problem will be shifting the direction of Cassini's periapsis; then it would not be too hard to get it to intersect Iapetus's orbit in two different locations than it does now; then we could see different longitudes. So long as Cassini visits the same part of Iapetus's orbit, we'll keep seeing about the same parts of Iapetus over and over. A similar thing happens with Titan; the inner moons present less of a problem, because the distances across the inner system are shorter.

At the start of EM (following T44) Cassini will be at 75 degrees inclination, periapsis of 2.7 Rs, and an orbital period of 7 days (4:9 resonance). For the scenario you outline above, a non-resonant series of flybys would be needed. I'm not sure if, for example, the P-series or Q-series of pi-transfer descent class EM tours allow this before the ring plane is reached, when, of course, the opportunity for low-inclination Iapetus flyby(s) arises.

One other note: the propellant budget, in addition to statistical ΔV and deterministic ΔV, also includes disposal ΔV and tweak ΔV. All four must be budgeted for during onboard ΔV estimates.

And that's the final nail in the coffin. Obviously if you've only got 16 or 17 Titan flybys left for the extended mission (I'm basing this on the above & Alex's post #43), then I heartily endorse doing something more interesting than just circularizing the orbit.

I guess the upshot, at least for me, is that I don't think "circularization" is even possible, even if it was attempted right now.

[dvandorn]

If such a daredevil thing ((using the main engine cover as an impactor - ed.)) were attempted, I think Iapetus would be the strong favorite, since that might probe the thickness of the dark stuff. Dione might be a distant third, since it has some kinda dark stuff. Enceladus would obviously be verboten; attempt no crashing there.

Actually, I would think Hyperion would be a better target for such a small, low-energy impactor. We might find out someting about the reasons for the dark flooring of those craters by putting a small scratch into Hyperion -- more than we would find out by making a tiny little crater that we might not even be able to resolve on Mimas, Rhea or Dione, or even on Iapetus.

-the other Doug

[TritonAntares]

Ok, I try on advertising for more Iapetus encounters...

The map below shows the planned image coverage of the Sep 10, 2007 close fly-by:

I'm not sure - are these the only regions that will be researched?
Some very interesting structures like the 'white mountains' of the equatorial ridge, some dark floor craters or the eastern CR borderzone of the only bad known - as far as resolution concerns - side will be in prime focus,
but what about the rest?
The 'snowman' seems to be in shadow - maybe observable in saturnshine - unknown southern regions like the large bassin are probably below horizon for Cassini.

If this is 'all' the encounter will show, there must be additional passages.

The other caveat is that the best Iapetus flyby comes near the end of the primary mission, and it may just solve most of the Iapetus mysteries.

Maybe this mysteries can be solved, but unlikely...
Another aim is to achive a whole coverage of the Iapetus globe under higher resolution as so far.
If you compare the image resolution quality of Rhea, Dione, Tethys and Enceladus with the one of Iapetus, he is quite off.

I don't think Iapetus merits such a change on its own, but a campaign to visit some other Titanian longitudes in the extended mission might be synergistic with Iapetus followup. One caveat is that Iapetus's orbital inclination means that we have to work in a third dimension to get a very close flyby except where Iapetus happens to cross the ringplane. And that could make it hard to get back to Titan, maybe setting up a phase of mission where non-Iapetus flybys are rare.

Interesting point to read, not all upcoming encounters in such an extented mission need to be that very close.
Concerning Iapetus's orbital inclination, passages - like the one of newyear 2005 - crossing the right longitude even only over Iapetus poles could also be instructive. The effects on Cassinis orbit shouldn't be such enormous...
Getting a series of such events even with a longitude shift would be fantastic.

Bye.

[Guest_BruceMoomaw_*]

Just out of curiosity, have forward-contamination protocols been established for Titan and Enceladus? I'd sure hate for Cassini to accidentally 'slime' them, however remote in the future that might be...

As far as I can tell, Titan is still "Category 2", just as it was for Cassini/Huygens. http://planetaryprotection.nasa.gov/pp/about/categories.htm : "Category II includes all types of missions to those target bodies where there is significant interest relative to the process of chemical evolution and the origin of life, but where there is only a remote chance that contamination carried by a spacecraft could jeopardize future exploration. The requirements are only for simple documentation. This documentation includes a short planetary protection plan is required for these missions, primarily to outline intended or potential impact targets; brief pre-launch and post-launch analyses detailing impact strategies; and a post encounter and end-of-mission report providing the location of inadvertent impact, if such an event occurs."

HOWEVER, this could very easily change before the end of Cassini's primary mission. http://www.cosis.net/abstracts/COSPAR04/00...R04-A-00545.pdf :

"Galileo, a Jupiter orbiter, and Cassini, a Saturn orbiter, are category II missions under United States National Aeronautics and Space Administration planetary protection regulations. Such missions are obligated to produce planetary protection documents, but otherwise have no real requirements (e.g., avoidance of accidental impact of the target planet). This characterization is based on neither planet nor its satellites at the time of the approval of the planetary protection plan being of significant biological interest.

"However, for Galileo, there was the possibility that one or more of its satellites would be upgraded in biological interest by the scientific investigations of Galileo itself. Therefore the PP plan included an agreement that if Galileo made such discoveries, then the project would modify the end of mission planning to reflect the new circumstances. Europa was on the list of solar system objects that might be made more important to the search for life. This was a unique approach to protecting Europa, basically a flexible PP plan. Of course, from the science data returned by Galileo, it is now generally accepted that Europa has a liquid water ocean under an ice layer, and interest is increasing in Callisto and Ganymede. Thus the decision was made in discussions between the NASA Planetary Protection Officer and the Galileo Project
to send Galileo into Jupiter to protect Europa (and the other satellites). The Galileo Project’s mission, after an extremely successful operation, ended in an intentional entry into Jupiter’s atmosphere in Sept. 21, 2003.

"Cassini will arrive at Saturn in July 2004 (just before this conference) and send the European Space Agency’s Huygens probe into Titan atmosphere and a possible landing in January 2005. There is an interesting parallel with Galileo in that the interest in avoiding an accidental impact of Titan by the Cassini spacecraft is increasing. Titan’s atmosphere includes organic compounds; while this is not as compelling as evidence for liquid water, it may well be of biological interest. Whether Titan should be upgraded in importance to biological investigations depends on the Huygens probe findings. The probe is also category II, but the science to be obtained compensates for the risk of contaminating Titan. There is no science gain from an accidental impact by the orbiter. Thus Cassini near its end of useful life might be faced with intentionally ending the mission to avoid Titan."

Now, of course, Enceladus has complicated things still further. As far as I can tell, no group has ever tried to categorize it at all, protection-wise. I presume some meetings will have to be held very soon by COSPAR and/or the Space Studies Board to decide on the new status of these two moons.

[The Messenger]

The Rings, Saturn, and Titan Orbiter Science Team (TOST) groups have held
meetings this past week to continue their evaluation of possible variations in the extended mission tour. For the TOST meeting, participants were
requested to come armed with criteria that could be used to rank the various tours under evaluation, and to consider how long an extended mission should
be in order to observe seasonal changes in the Saturnian system. Overviews of eight variations of the tour were presented along with a description of a
new tour analysis tool, and discussion of tour products that are currently available.

[Guest_BruceMoomaw_*]

Looks like it's time for me to ask Bob Mitchell again about their possible alternative plans. While I was scrounging through my printed records last night (as one of my periodic spasmodic ofice cleanups), I ran across Mitchell's Jan. 29, 2001 reply to my question on the possible goals of an extended mission:

"[The most desirable goal for an extended mission] depends on which of the scientists you talk to...However, the prime target that seems to be least satisfactorily covered in the current prime tour by the investigators is Titan. On the other hand, by the time we're halfway through the prime mission, we may have made discoveries that will completely change what would be considered the prime targets."

Speak of the Devil. We're about halfway through the prime mission, and Enceladus has indeed upset the applecart. Beyond that, though, the main new conclusion seems to be that Titan's surface is even more varied, hard to understand, and therefore worthy of extensive mapping than had been expected. (I wonder if they'll emphasize using the VIMS more for that than the ISS cameras during the Extended Mission?) Beyond those two moons, the only target I can think of whose relative importance MIGHT possibly be increased by Cassini's observations so far is Hyperion, with its very odd surface.

[volcanopele]

Speak of the Devil. We're about halfway through the prime mission, and Enceladus has indeed upset the applecart. Beyond that, though, the main new conclusion seems to be that Titan's surface is even more varied, hard to understand, and therefore worthy of extensive mapping than had been expected. (I wonder if they'll emphasize using the VIMS more for that than the ISS cameras during the Extended Mission?) Beyond those two moons, the only target I can think of whose relative importance MIGHT possibly be increased by Cassini's observations so far is Hyperion, with its very odd surface.

I don't think the discoveries at Enceladus really change the extended mission strategy all that much. Yes, we will want to include more Enceladus flybys, at least a couple right over the south polar region, one to cover the still poorly mapped leading hemisphere, and at least a couple to do gravity measurements. But the driver for any tour will still be Titan. The question will be whether we want a longer extended mission in order to observe seasonal changes, which will mean fewer RADAR passes and perhaps fewer targeted Titan flybys in general to conserve fuel, or a shorter extended mission with more RADAR passes at the expense of observing seasonal changes.

I prefer meeting these camps halfway, a two year extended mission with a little bit of both Titan camps to allow enough time for at least 5 Enceladus passes and an additional Iapetus pass. This is smack in the middle of the hyperactive, one-year extended mission that RADAR would prefer, and the slower-paced, 4-year extended mission that the ORS folks may want.

In terms of ISS vs. VIMS, once again, we have already done that. Closer in, when ORS has the reins, VIMS is driving. Farther out, ISS is driving because it can get much of the visible surface at only a little lower resolution than VIMS up close.

[Guest_BruceMoomaw_*]

Makes sense. Anything more that you or Alex can tell us about all this without getting arrested would be welcome.

[ugordan]

I prefer meeting these camps halfway, a two year extended mission with a little bit of both Titan camps to allow enough time for at least 5 Enceladus passes and an additional Iapetus pass.

Didn't you say once before an additional Iapetus flyby is not likely, given the probability we'll have all the answers to its puzzle after the 2007 encounter? Iapetus is pretty hard to reach and all that...

Farther out, ISS is driving because it can get much of the visible surface at only a little lower resolution than VIMS up close.

So, you're giving up on the highest resolution ISS surface mosaics (what was it -- 1 km effective resolution?)? I was kinda looking forward to those. VIMS may give better spectral and only slightly better spatial resolution (but only at C/A!), but its imaging footprint is just too small to provide wide coverage.
Which brings me to this question: we know how long it takes ISS to take one image, exactly how fast can VIMS readout the image cubes?

[Guest_BruceMoomaw_*]

VIMS' spatial resolution in its 2-micron band is MUCH better than ISS at 1 micron -- compare their views of the famous "Snail" (which is absolutely unrecognizable in ISS views). Any future Titan craft will certainly take all its surface images at 2 microns.

A few more questions for Jason and/or Alex:

(1) When you refer to observing "seasonal changes", are you referring just to Titan or (as as I expect) to Saturn and the rings too?

(2) Are you considering any additional flybys of other smaller moons besides Enceladus and Iapetus?

(3) Are you planning to leave Cassini in a high-inclination orbit, thereby allowing it to continue observing the rings?