[X] My Assistant

[edstrick]

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!>

[volcanopele]

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.)

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?

[EccentricAnomaly]

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.

[Guest_BruceMoomaw_*]

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.)

[Guest_AlexBlackwell_*]

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.

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?

[Bob Shaw]

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

[EccentricAnomaly]

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?

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.

[EccentricAnomaly]

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

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

[EccentricAnomaly]

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

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.

[edstrick]

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?)

[Guest_BruceMoomaw_*]

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.)

[edstrick]

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!

[tty]

Also interesting is that they're exploring the possibility of flying through or near the plumes of Enceladus.

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

tty

[ugordan]

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

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.

[canis_minor]

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.

[volcanopele]

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