tallbear: "....However, the Cassin Tour will be altered in early 2006 and these opportunities may go away while others may appear.....

Uh... Say WHAT?....is this a branch to an entirely different series of encounters than the second half of the primary mission that they've been designing in (necessarily) obsessive-compulsive detail for the last decade? Did they have to retarget to make a close flyby of a Vogon Mothership? Or did they find a spinning blue telephone call-box floating in Saturn orbit? <grin!>

Maybe he's talking about the next phase in the mission where Cassini won't be in the ringplane anymore and hence not in a favorable position to do serendipous non-targeted flybys?

The Cassini tour is indeed being changed from 2006 onwards. This is due to the Titan flyby altitudes being raised -- Titan's atmosphere is thicker than expected at the flyby altitudes. The change is intented have have a minimal impact on the Science----> the new trajectory is as close as possible to the previous tour.

Hopefully these changes won't have any effects on the planned close encounter with Iapetus in september 2007.
Would be a big disappointment to change the flyby distance as well as the planned trajectory above the until now mostly unknown parts of this moon.
After Titan and Enceladus, Iapetus seems for me to be the most interesting satellite of saturn.
Unfortunately there are only few occasions to look at him more closely.
I hope this will change on an expended mission after 2008.
Does anyone know, when decisions concerning this be will made ?

Yes, but what is the impact delta-V wise? While I'm at it, why don't the Cassini weekly status reports indicate any hints of remaining consumables, whether they be in kilograms, percent, deltaV, ANYTHING?
It would really be nice, I'm often wondering how long an extended mission Cassini might pull off one day since I keep hearing about these OTMs and unscheduled tweaks to the tour that expend several meters/s and somehow I get the feeling Cassini doesn't have that much fuel left onboard as is?

Many of those Cassini weekly reports include information on how much delta-V was consumed in an OTM. I've had it in my head to go back through those reports at some point and count up which OTMs were cancelled and which used at how much delta-V cost but haven't had time. Anybody else want to take a crack at it?

--Emily

this change of the planned Tour is a health/safety issue .... so it is a 'must do' sort of tweak ... delta_V is a factor, preserving planned flybys and planned science is another ....minimiinng the disruption of flybys and science is the goal... but delta_V is costs may force some revisions here and there.....and all the trade offs are being worked ... the new Tour is scheduled to become reality in late Feb 2006....

Extended Mission discussions are in their *infancy* ... but 2 years or more of extended mission is the goal...

I've moved all posts on the new tour, caused by the raising of all Titan flybys at 950 km, to this thread

I guess I'll add my $0.02.

Basically, because the upper atmosphere of Titan is thicker (or at least provides more torque on the spacecraft) than expected, all flybys with a close approach altitude of 950 km will be raised, and other, more distant encounters, lowered. This will effect the tour in subtle ways, nothing too drastic, but it will be a bit of a headache because the changes do cause some differences in the timing of encounters. It is also not yet clear how such tweaks will effect non-targeted encounters (it is my understanding that the Enceladus March '08 encounter will be frozen at 100 km and the Iapetus encounter will only change by a marginal amount). This tour change is still being evaluated. the first 950 km encounter is in July '06 (T16), so there is still some time to work on these changes.

I chime in and make the pot $0.04

I note the Project has "tentatively" selected the T18 and T19 flyby altitudes as 960 km and 980 km, respectively, while the T20a ORS/T20b RADAR thru T25 altitudes get raised back up above 1000 km. And the next 950 km flyby after T16 is T32a INMS/T32b solar occultation.

The new flyby altitudes have been selected, the decision made. Two remain at 950, the rest vary between 960 and 1030 km, based on a number of complicated factors. These include what latitude the closest approach is at (our new atmospheric model varies with latitude), what activities (that use control authority) the spacecraft is performing, it's attitude (which affects the torque the spacecraft receives), etc. Our thrust level also goes down with time as we use hydrazine. We also chose to be more optimistic later in the tour.

The new tour - without trying to preserve the original timing and geometry of *everything* - actually saves delta-vee. But that's likely to be spent trying to restore timing and geometry of the most critical opportunities.

I think it's likely that the new tour will not only preserve just about all of the existing science opportunities, but have manageable timing and geometry shifts for replanning. But we'll see. In any case I wouldn't worry about Enceladus or Iapetus. (This may actually serve as an opportunity to - briefly - reoptimize those encounters based on what we now know.)

I'd like to know the ballpark figures... a document predating SOI mentioned a very long life as being possible for Cassini... it will be 20 years from now before Cassini will be as old as Voyager 2 is today. If extended mission(s) lasted that long, then the original primary mission could one day be remembered as a small portion of the total cruise.

The history of NASA spacecraft is that most craft that don't fail early don't fail soon thereafter, either, and that lifetimes greatly beyond the posted "shelf life" as common. Presumably, the main issue here is the consumption of fuel, and I note that the Voyagers have great reserves left over: three-axis stabilization itself, in deep space, anyway, does not consume very much (which makes sense). Note that course modification and attitude control employ *two different* subsystems, so that this story should not impact the lifetime of Cassini in any direct way. Except that avoiding the torque of a close Titan flyby will certainly help!

Burning course modifying fuel of course, limits our ability to keep tweaking the trajectory, but Cassini will stay in Saturn orbit regardless. The hazard would be that the trajectory would eventually crash into something (Titan, if probability were to have its way) and we'd lack the delta-v to avoid it. But a long, semi-aimless life in an orbit that intersects that of Enceladus and that of Titan would give us good science indefinitely far into the future.

I have found at this link

http://www.ssd.rl.ac.uk/news/cassini/mission/ops.html

that Cassini is expected to burn "less than half" of its attitude control fuel in the primary mission. Well, 1% is "less than half", but if they mean less than and close to half, then we might expect only about 4-5 more years of life after the primary mission. On the other hand, it seems that it is pointing, not mere maintenance, that eats the fuel up -- my so-rusty-it's-dead rotational mechanics background says that you could run these operations with less fuel consumption if you were willing to limit the ability of the craft to swivel quickly on close flybys, and to pick and choose observational opportunities so that you got fewer observations per flyby, but a substantially higher number of flybys. I would certainly prefer to see a full RADAR map of Titan trickle in over 25 years than to map half of it in six years, then have Cassini be utterly dead. Not only to reap the most spatial coverage, but in recognition of the fact that Enceladus may have time-varying phenomena of interest (although maybe the fountains are constant in their output over long spans of time?), and Titan's weather is sure to change over the very long seasons: it would be great to get even sporadic observations through 2034 to get to see a whole Titan year take place. To sum up, I'd like to see a strategy of conservation that gives us long-term climate observations and steady progress in RADAR mapping of Titan, along with intermittent observations of Enceladus. Encounters with the other icy moons would continue to happen if only by accident.

I understand that Cassini Mission Planning just recently released an updated consumables report, though I haven't seen the exact figures. However, I seem to recall an updated propellant usage report from late October 2005 that showed:

309.6 kg of bipropellant oxidizer remaining (vs. 1869.000 kg loaded)
189.2 kg of bipropellant fuel remaining (vs. 1131.000 kg loaded)
104.9 kg of monopropellant remaining (vs. 132.000 kg loaded)

You *recall* those figures, Alex? Good memory!

Of course the bipropellant has mainly been used because of, if nothing else, SOI. The monopropellant is about 80% intact after 1.5 years of operations at Saturn? Naively, one might pencil in a lifetime of 6 more years. As I noted, there should be ways to budget.

I've posted updated Titan flyby altitudes in my Cassini Tour Page. I left the old altitudes struck through so people can see what changed. I'll take the strikethroughs out in a month or two.

--Emily

Not sure what station you hold in the grand scheme of things in the mission, but it was my understanding that a tour has been designed (I have the same number you have), but the tour is right now in a review process with the instrument teams. But you bring up a puzzle that I noticed when I saw the new C/A altitudes, that in fact two were still at 950. Based on everything I understood about the problem, 950 is still a major problem. Are those two INMS passes by any chance?

Those can be at 950 mostly because they are over the polar regions where the atmosphere is much thinner at that altitude. Also interesting is that they're exploring the possibility of flying through or near the plumes of Enceladus.

I would presume that another thing they'll want to add to their previous observation list is long-distance temporal observations of Enceladus' plumes, for changes in activity. (I also wonder whether they may make more intense observations of the very complex temporal changes in the F Ring.)

BTW, I've been meaning to ask someone this and you seem like a good candidate: Was it ever determined (by TAMWG or anyone else) why INMS needed a 3.1 multiplier to get their density data to fit AACS?

If I remember correctly, some of the early(ish) EOM options for Cassini involved radical changes such as orbital 'pumping' to such a degree that (unnamed) further outer planetary encounters would be possible. Others discussed included insertion into orbit around Titan.

Are there any updates on this?

Bob Shaw

Nope.. I think the ball is in the INMS court right now as independent analysis from Langley verified the numbers AACS was getting. But it also doesn't really matter in the end because AACS measures directly what causes tumbling, and so their numbers are what should be used for picking the altitudes.

Cassini could escape from Saturn, but not with enough energy to reach another planet... and getting into orbit around Titan is not possible.

You could probably find this stuff out from AIAA conference papers from the Cassini Nav team. I seem to remember something like this from the AIAA Astrodynamics conference this summer in Lake Tahoe.

I don't think the cancelled manuevers have had much of an impact on the XM delta-v budget one way or the other. I think its something like 150-200 m/s and they're estimating 10 m/s a Titan flyby on average.

A basic note regarding extended mission options. Something I read a ?couple? years ago on extended mission options is that how long an extended mission lasts basically depends on how vigorously you keep yanking the orbit around and trimming before and after critical flybys to optimize high-value close encounters. If Cassini hot-rods around the system, it will burn fuel like crazy, if it goes on a Sunday-drive, it will last a decade or two.

What the science opportunities and trade-offs of tours that are high-fuel versus low-fuel are, I have utterly no idea. There may have been some papers on things like that in aerospace geek journals like Journal of Spacecraft and Rockets, or Acta Astronatica, or some of the astronautics conference proceedings like the hardbound series on Guidance and Navigation. (damn, who puts those out?)

Robert Mitchell told me flatly that insertion into Titan orbit is simply not possible -- the aerobraking sequence involved would be intolerably long and complex. (Now, aeroCAPTURE into Titan orbit in one installment -- using a spacecraft designed for it -- is a completely different matter. Aerocapture into Saturn orbit using a Titan flyby is of course equally possible.)

The problem with aerocapture through repeated aerobraking is that there's too much velocity, and you'd have to lose it in very brief segments at closest approach on "bent-stick" hyperbolic flyby trajectories. Aerobraking like Magellan first demonstrated and is now operational at Mars, is done over very extended periapsis passes in eccentric, then more and more circular elliptical orbits.

By the time Cassini could finish aerobraking into Titan orbit in 30 or so years, a Titan orbiter mission would have been flown and be over, almost!

Is flying through the plumes really wise? Even a small frozen drop of water is nasty at planetary speeds.

tty

The VIMS team recently stated the average particle size is about 10 microns. That shouldn't pose a problem to Cassini and its micrometeoroid shielding, but if there were any macroscopically-sized ice particles the outcome could be nasty...
The odds at that are probably very small, though.

A few comments on recent posts.

Propellant and extended mission: we expect to have nearly 60% of our hydrazine left at the end of the nominal mission. This, along with the quoted biprop numbers, could be used to support an extended mission for 1 year or 10; it all depends on how we use it. We've received positive messages from HQ that we'll get at least 2 years of funding. Funding aside, I think Cassini could last many years. Even when the prop gets low, we can continue to observe Saturn quite well (its orbit will precess with J2 and move w.r.t. the Sun anyway). As other posters have pointed out, maintaining attitude control is cheap. If our wheels are healthy, there are even some creative ways to manage their speeds without using prop (solar and RTG pressure - slowly).

Delta-vee in OTMs: it all depends on the OTM. Many have fixed components (apoapsis maneuvers in particular), others are just statistical cleanup maneuvers. We are doing better than we expected in the latter category to date.

Aerocapture to Titan & escape: I agree there's no conceivable way to get to another planet; we just don't have the fuel to do anything but barely escape Saturn in a near-Saturn orbit. As to Titan capture, I'm not completely convinced this is impossible; there may be some tricks the tour designers can play. But we won't consider it before an extended-extended mission I expect.

Plume dangers: We are sensitive to particles > 1 mm in size. We certainly need to consider the likelihood of these near Enceladus, but at this point there's a lot of evidence that the plume and E ring is dominated by non-hazardous stuff.

I should point out that Cassini already flew through the plume in July, and survived.

NASA Considering Extending Cassini Mission Through 2010
By Jefferson Morris
Aerospace Daily & Defense Report
12/27/2005 08:57:09 AM

It's nice to hear an official word on an extended mission, but I'm wondering:


How does that cope with this:


That's gotta be some eventful extended mission if they intend to spend all their hydrazine on the short end of the above estimate, 2 years...

As one might expect, the EM options have varying degrees of propellant intensivity. There are Titan-only tours, tours that have targeted icy satellite encounters from high orbital inclination, etc. It all depends on the EM option that is selected and the propellant margin level (and level of risk) the Project decides to accept. Bob Mitchell and "canis minor" may be referring to two entirely different options, which would not be too surprising at this juncture.

Unless something completely surprising crops up in the near future and throws a curve-ball at us once again, I'd expect the priority targets for an extended mission would be (in order of decreasing importance): Titan, Enceladus, Iapetus.
Iapetus is an expensive target to reach, orbiting quite far away from Saturn so I don't think there'd be more than one (if any!) additional flybys in the extended mission.
Personally, I'd set up a mission phase where Cassini's orbital period would be in resonance with Enceladus' and get several repeated flybys and gravity runs to get a better understanding of this enigmatic moon. Any extended time periods spent near the rings and in the E ring itself would probably be slightly risky so I'd set up that phase at the end of the extended mission. The obvious trouble would be the resonance - you couldn't have a low one, such as 3:1, that would imply lowering your apoapse below Titan's orbit, a big no-no.
Then there's Titan - obviously even if we had 100 more flybys, we'd still have unmapped territories and competing observations to be done.
So much to do... so little fuel...

All in all, I'm looking forward to seeing what the mission folks will devise.

A few things I should note: first, in order to accomplish the icy satellite flybys, we have to descend from the high inclination orbit we have at the end of the nominal mission to Saturn's equatorial plane, and that will take lots o' hydrazine, especially if we want to do it quickly. We could stay up to continue to monitor the rings, but that would also require hydrazine, as we would naturally decrease in inclination over time.

I do know that one of the tour trades Cassini MP revisited during the EM brainstorming process was: "Targeted icy satellite opportunities available during inclined orbits." Another one was: "How to obtain low latitude diametric Titan RSS occultations." I know the former is propellant-intensive; I'm not sure about the latter, though, if I remember correctly, RSS did lose a Titan occultation (equatorial?) earlier in the tour due to a trajectory tweak.

Of course, concentrating on Enceladus flybys (from a low inclination orbit) is popular for an extended mission, which is similar to the Galileo Europa Mission. It'll be interesting to see what happens.

One of the serendipitous aspects of even the current Cassini orbital campaign has been the 'unplanned' encounters - are there any 'interesting' extended missions? I'm thinking, perhaps, of Enceladus/Titan opportunities, odd things which might happen on the flip-side...

Bob Shaw

I hope one option being considered for the extended mission is to get in one more good pass at Iapetus in addition to the solitary scheduled close encounter. Hopefully, Titan could be used to de-pump the spacecraft back into an inner icy moon sequence thereafter with minimal propellant usage.

Iapetus really needs at least medium-resolution global coverage (if that's not planned already; don't know the geometry of the 2007 pass). Who wants to sit through another Mariner 10/what's-on-the-other-side-of-Caloris-Basin situation for who knows how long? I want to see what's on the opposite side of that ridge!

I pretty much take for granted that an extended mission will include lots more Titan flybys, several of Enceladus, a second one of Iapetus, and probably one of Mimas. Beyond that, it's anybody's guess -- and I imagine it will depend on what interesting phenomena they see during their long-term observations of Saturn, its rings and its magnetosphere, as well as what we see on the remainder of Titan.

This is good, because it will nail down whether or not the anomally is truly connected with the molecular head count...Good that is, if the margin is great enough to counter the torque without compromising the mission.

If I am reading Emily's table correctly, the first 950km flyby will be T16 on July 22, 2006.

I'm not smart enough to do the math on this so if impractical or unmeritorious, mea culpa.

Iapetus is not so far off of being in a 1 to 5 resonance with Titan, and is one of the larger satellites of Saturn and is very distant to boot.

What if....

Cassini is put in a ~79.3 day orbit of Saturn and makes a couple of (fairly identical trajectory wise) succesive flybys of Iapetus. One pass could be devoted to photographic scrutiny, and the other could have the radar instrument examine the same ground track as the photo series. (nice if the path crossed the edge of Cassini Regio and over the equatorial ridge structure)

My thoughts are that bringing all the crafts sensors to bear on the dark stain and the ridge structure might yield enough data to verify one or some (or none!) of the many theories as to what happened to this amazing moon.

Additionally (crossing my fingers here) in view of Iapetus' mass and remoteness from Saturn, would allow a sufficient gravity assist to get back to a close approach to Titan to resume the Titan resonant orbits as seen in the rest of mission?

(I'm thinking an orbit that synchronizes to Iapetus doesn't go back to Titan unless you 'do something', like utilize Iapetus, to get back to Titan's proximity to resume orbit mods there.)

{are you having fun watching me squirm at the narrow confines of my intellect? }

What the heck, might as well drop this one in too:

At the conclusion of the Cassini mission, have the craft crater in Cassini Regio.

Hopefully a future mission can photograph the crater (shades of Ranger 7!) and perhaps some more clues as to the dark stain can be gleaned?

Perhaps the dark coating is very thin, Cassini might blast quite a hole in it and reveal quite an acreage of white material beneath.

Maybe its' thick and the crater barely pierces the black stuff.

Maybe the coating is thin, and slowly reforms over time (years? centuries?).

Worth a shot?

I wouild assume a major science opportunity during an extended mission would be ring observations in 2009 when the spring equinox occurs and the sun passes directly through the orbital plane of the rings. The grazing illumination should better reveal vertical structure of the rings, including vertically extended dust hazes, out-of-plane wave structures, overall ring warp due to satellite tides, etc. I would assume the highest value observations would be from high inclinatin orbit, more or less over a pole at the extended moment the sun (3 arcmin diameter, approx) goes through the ring plane.

Volcanopele: What's the discussion on science merits of different observations during the solar ringplane crossing period, and effects on extended tour plans?

depends upon which sats have priority ... during revs 6 thru 14 the ring plance crossing occured righte close to 4.0 Rs which is great for Enceladus... and those orbits were inclinded in the 20'ish degree range to optimize for RSS OCCs. Studies are in place to see what sorts of orbits are required for what satellites and if any satellite flybys require
an EQ orbit. ... and there are lots of events that can not be viewed in EQ orbits so the pressure is on ..... all these EM studies and discussions are in their infancy .... so don't expect more than specualtion for ages.( Feb 2007 with a fair HINT at the naure of the Tour out in Aug 2006) ,....

Rings folk definitely want to be out of the ring plane for the equinox passage period and also for the Prometheus - F-Ring collision / interaction that falls late in 2009. To be honest... Rings folk have little desire to get into the Ring Plane <G> .... but these Geometric events are in the Rings High Priority listings.

Actually, rings folk really want to be in the ring plane... in the rings....

But that requires a Prometheus-Class nuclear/electric Ring Rendezvous Mission.

I have a xeroxed article, probably from Journal of Spacecraft and Rockets, maybe from 15 years ago, describing such a mission. Spiral inwards, reach the rings, tilt the ion-engines out of plane and levitate about 10 km above the rings, continuing to spiral inwards. Mission to end 1,000 km above Saturn's equator. There was various discussion on ways to lower probes directly into the ringplane without risking the main spacecraft.

I want... I want... <whimper>

That's my understanding too. I wouldn't be surprised if somekind of compromise tour is select, where we stay up reasonably high, a few Enceladus encounters (1-3) and plenty of non-targeteds. HOWEVER, such a plan would all but eliminate another Iapetus encounter, I would think.

BTW, thanks for explaining what observations the rings people could get from staying high.

As someone (Jason?) alluded to, targeting Iapetus while retaining other EM tour options is tough.

In any event, just to give a flavor of the EM science priorities (identified earlier this year at the PSG meeting in Florence, Italy) a couple of the teams are considering:

The CIRS folk really want to see the season change on the Rings as the Sun crosses into the Northern Hemisphere and sunlight hits the North side of the Rings. Right now the unlit side of the B-ring looks like most of it hardly knows about the sun .... they want to see how fast the lit side cools and how fast the dark side warms when the 'season' swap occurs.

Star Occs arund the Prometheus - F_ring collision will tell us about how this event affects the F_ring as well as the wakes in the A ring ... those star Occs will also tell us about how the next J-E orbital swap afftects those wakes.... and the speculation about the height of the wakes aldo is a driver for watching the EQ crossing to look at possible shadows of those wakes/waves... but we don't need to be at a super high inclination to watch any of these events....

BTW... there is a proposed Saturn Ring Mission that would orbit just above the Rings (on the lit side) .. requires lots of orbital maintenance maneuvers ... it's out there on the wish list of Missions that are being proposed....

Is that Tom Spilker's Saturn Ring Observer mission concept?