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Posted by: AlexBlackwell Feb 24 2006, 09:12 PM

Excerpt from Cassini Significant Events for 02/16/06 - 02/22/06:

"As mentioned in previous weeks, the project has been working on adopting a new reference trajectory in order to raise the minimum Titan flyby altitude for various encounters. Today the project reached a decision to proceed with the 'optocc2' trajectory. Additional work is still to be performed before delivery of the final files. This will include minor tweaks that have been analyzed in other trajectories, adjusting orbit 68 timing, and capture of an Enceladus plume occultation on orbit 28."

For the record, the new reference trajectory will result in an even more spectacular Enceladus-3 flyby [61EN (t) E3] on March 12, 2008.

Posted by: BruceMoomaw Feb 25 2006, 04:03 AM

It was casually confirmed in one of the new EGU abstracts that Cassini's extended mission will include an attempt to determine Enceladus' quadrupole gravitational field -- which should tell us vastly more about its internal mass distribution, and thus about what really IS going on in its innards (which seems at this point to be the single biggest mystery unearthed by Cassini):

http://www.cosis.net/abstracts/EGU06/05029/EGU06-J-05029.pdf

Posted by: volcanopele Feb 25 2006, 04:12 AM

Even though the encounter distance is closer, don't expect too much better imaging. We come in at high-phase, C/A is over the night side, and Enceladus goes into eclipse 3 min. after C/A.

Posted by: AlexBlackwell Feb 27 2006, 04:13 PM

Just out of curiosity, and assuming the geometry was favorable, how much pixel smear would ISS expect at the new 25 km C/A altitude?

Posted by: Holder of the Two Leashes Feb 27 2006, 04:40 PM

Any chance at all that they can snap off some pictures of the south pole before Enceladus goes into shadow?

Posted by: ljk4-1 Mar 9 2006, 06:42 PM

Check this out:

http://drudgereport.com/flash8na.htm

Posted by: volcanopele Mar 9 2006, 07:37 PM

Unfortunately, the distance for E3 quoted is the distance to the center of enceladus for the old flyby altitude. The old altitude was 100 km, now it is much lower.

Posted by: AlexBlackwell Mar 9 2006, 07:43 PM

I guess no one has leaked Drudge the new reference trajectory recommendations

Posted by: BruceMoomaw Mar 9 2006, 09:36 PM

TWENTY-FIVE KM?!! You guys are really taking this place seriously, aren't you? I hope the navigation is better than that on MCO...

In that connection, one catastrophe that has yet to strike the US space program is the failure of an long, expensive outer Solar System mission just before its arrival. What will the public and Congressional reaction be when that finally happens? It looked as though it might happen with Galileo, but JPL succeeded (to their unquestionable credit) in pulling enough chestnuts out of the fire to remove that impression in that case.

At the risk of setting off the dog (aka Alex) again: I mentioned this point to Spilker's subgroup on Europa lander design at the Europa Focus Group Workshop, and got an enthusiastic response from JPL's Karla Clark -- we do NOT want something as expensive and long-term as a Europa Astrobiology Lander to fail at the very moment it's trying to land on Europa, and for that reason the best role for a little piggyback lander attached to Europa Orbiter may well be to check out the engineering characteristics of Europa's surface rather than carrying out any specifically scientific studies.

Posted by: ugordan Mar 9 2006, 09:42 PM

I'm wondering about this figure as well, I noticed volcanopele edited it out of his post. Does that figure really hold in the new updated trajectory?
That has got to be the fastest and closest approach by any mission so far!
I hope Enceladus doesn't grow any really tall mountains in the meantime

What exactly is the scientific justification of such a dangerously close pass?

Posted by: AlexBlackwell Mar 9 2006, 09:47 PM

Now that's a visionary statement.


"Officially," the new reference trajectory is still in review at the Project/Program level; a decision should be forthcoming very shortly. I won't speak for Jason, but unofficially, the 25 km figure is the new Enceladus-3 flyby C/A altitude under all options that were being considered.


More on that later; however, sooner than 2008

Posted by: volcanopele Mar 9 2006, 10:12 PM

Technically, that number is official (well, it isn't going to change). As the Sig. events report suggest, a few additional tweaks are planned, but nothing that will effect Enceladus-3.

Posted by: AlexBlackwell Mar 9 2006, 10:17 PM

I meant officially released. As you note, though, it's not going to change, especially since that C/A altitude was the same under all options. And I am heartened that the new reference trajectory helps out RSS, too

Posted by: The Messenger Mar 9 2006, 10:23 PM

I agree - 25km is much too close. I suggest that they take a good hard look at what happens in the 950km pass of Titan in July, before cutting the number in stone. There is clearly an atmospheric hazard that we cannot be at all certain about.

Posted by: ugordan Mar 9 2006, 10:34 PM

You can't compare apples and oranges. What happens at Titan hardly suggests anything about Enceladus. Besides, the C/A distance will probably not be above the south pole where the plumes originate.
Personally, I'm not afraid of the plumes as much as I am of delivery errors. I do want to see an extended mission!

Then again, the navigation team probably knows what they're doing by now.

Posted by: Richard Trigaux Mar 9 2006, 11:05 PM

Passing at 25 kms of Enceladus would be very interesting for measuring its gravitationnal field, allowing to detect its inner structure. Why 25kms and not 30 or 10? I think it is a matter of accuracy of navigation: not to take a risk to crash on Enceladus (a double catastrophe, if we contaminate a world where our bacterias can undoubtly live) but passing as close at possible to have a better gravitation measurement.


To take images? Wow fantastic images from such an altitude, not much more than an airliner... But probably motion blurred beyond recognition. Unless they develop a special imaging software coupled with a variable tilt of the ship...


To pass into the plume? Not necessary for gravitation measurement. But a bit difficult to avoid, as it takes much place.

Dangerous to pass into the plume? I don't know. Certainly it is not benign, that depends on the size of the snow particules, that we don't know. Measures could tell us that there are many micron-sized particules, but not show a minority of larger ones, for instance soil bits expelled by the vents. Only one snow flake, and it is the end of the mission... In any case, Cassini should pass through the plume antenna ahead, as during the insertion.

The ideal would be to pass just besides the plume, and take images of the vents...

Posted by: DDAVIS Mar 9 2006, 11:38 PM

To take images? Wow fantastic images from such an altitude, not much more than an airliner... But probably motion blurred beyond recognition. Unless they develop a special imaging software coupled with a variable tilt of the ship...


I think they have pretty short shutter speeds available, and if they point the camera either forward or backwards along the spacecraft path the blurring would be minimized, and radial in character. The lighting would be probably better for photography looking one way rather than the other, and a rapid sequence including the closest approach would make a hellova animation.

Don

Posted by: Sunspot Mar 9 2006, 11:49 PM

I think this is the highest resolution image taken of Enceladus so far:

NAC: http://saturn.jpl.nasa.gov/multimedia/images/raw/raw-images-details.cfm?feiImageID=45681

WAC: http://saturn.jpl.nasa.gov/multimedia/images/raw/raw-images-details.cfm?feiImageID=45711

As you can see the narrow angle image seems to have motion blur - from an altitude of 545km.

Posted by: JRehling Mar 10 2006, 12:04 AM

According to the Photojournal page, the surface imaged here was actually just 319 km distant, a side-looking shot taken while Cassini was 208 km above the surface.

http://photojournal.jpl.nasa.gov/catalog/PIA06252

This is 4 m /pixel, indicating that were it not for blur, imagery from a range of 25 km (which isn't going to happen if C/A is on the nightside, among other reasons), would have a resolution of 31 cm (one foot for Standard fans).

Posted by: EccentricAnomaly Mar 10 2006, 04:43 AM

what I heard is that the altitude is not set in stone, but could change with another new trajectory... I'm guessing is that there will be more studies made to make sure it is safe. At that low altitude, I just hope they model Enceladus as an ellipsoid rather than as a sphere... the radius differs at the pole and the equator by something like 10 km, right? ...actually, I wonder how well they really know the radius anyway.

Posted by: BruceMoomaw Mar 10 2006, 05:01 AM

Unfortunately, as Jason noted back on his own (sorely missed) Titan blog, whenever Cassini flies by Enceladus it's also flying through the densest part of the E Ring, and has to keep its high-gain dish pointed in the direction of travel as a shield against any unusually large particles -- which means that they can't use image-motion compensation to deblur the craft's closeup photos of Enceladus. (This will certainly be the case if they're flying through the plume at such low altitude.) Thus such incredibly close-range photos would be so badly blurred that their overall resolution wouldn't be much better than the much wider-field photos taken several hundred km out. It's yet another loss resulting from the cancellation of the scan platform, although I don't think anyone could possibly have foreseen back then that Enceladus would turn out to be THIS interesting.

Am I correct in guessing that one of the main purposes of this flyby is to analyze the denser low-altitude portion of the plume, and thus get a better analysis of its trace components? At any rate -- just as we had to do with Europa (and Io) two decades ago -- we must now unexpectedly add another world to our high-priority list of Solar System targets, and start seriously thinking about followup missions. If this eruption has been going on for 100 million years or more, there is a genuine chance that it might be an environment in which life could evolve.

DOWN, boy! You can't call it "visionary"; but certainly when Ms. Clark and I raised it, it produced a considerable stir in Spilker's little subgroup, suggesting that they hadn't fully considered the consequences of such an Astrobiology Lander failure and thus the full extent of the need to try to make advance observations to minimize its chances of occurring. So I'll settle for "important" rather than "visionary".

Posted by: The Messenger Mar 10 2006, 06:37 AM

That depends upon whether the uncertainty is systemic, navigational, or ah, related to unknown rules or force gradients. There is much to learn, and we have learned precious little from crashing perfectly good planetary probes in the past.

Posted by: Richard Trigaux Mar 10 2006, 08:44 AM

To maximise our chances of finding life (or at least active pro-life chemistry) I think an Enceladus probe should land very close to the vents where steam gets out, to have some chances of finding life particules. Any other place on Enceladus is covered with 100kms of pure ice... So I imagine the following scenario:

1) a probe satellises at a low altitude around Enceladus (It must be possible, but orbits may be unstable, so the mission has to be lead at a quick pace)

2) the probe examines the vent region to find holes, deposits, etc

3) a small lander is sent at an appropriate location. (Eventually we may consider two or several such landers, piggy-backed or sent separately)

4) How to get samples depends on how the vents look like. If there is a deposit cone around, the lander just have to land on it. But if we have to fiddle in a crater or hole, the landers should be able to get in.

5) make analysis, take microscopic images, see if the shapes reproduce...

6) fire the SAMPLE RETURN rocket.

Actually it may be easier to get samples in an Enceladus vent than on Europa (even assuming that life chemistry could be present on the surface). And at least we are sure that Enceladus water is not some ammonia eutectic like on Titan, or filled with sulphuric acid, like maybe Europa: it is plain water, relatively pure, propicious for life.

Posted by: AlexBlackwell Mar 10 2006, 03:52 PM

Looks like you missed my sarcasm. A better description would be "blindlingly obvious."

Posted by: AlexBlackwell Mar 10 2006, 04:26 PM

That's a good point: any trajectory is subject to change, and I wouldn't be surprised if 61EN (t) C/A ends up being tweaked upwards slightly. Still, unless the new reference trajectory resulting from optocc2 (or even those that would have resulted from 28EN_occ3, 28EN_opt3b, or Option 3b) changes dramatically, Enceladus-3 is going to a barnstorming flyby, regardless.

And there is an important non-targeted encounter coming up later this year, September 9, 21EN (nt), which will allow for occultation data on Enceladus' plumes.

P.S. The significant events report I started out with in this thread referred to "optocc2." Is this different from optocc3? I've seen the latter referred to extensively by MP during this latest trajectory re-design but not the former. I assume both are in the same "class" but have different tweaks.

Posted by: JRehling Mar 10 2006, 05:32 PM

Various notes:

However we might wring our hands over missing out on 30cm/pixel images, note that we did obtain only slightly blurred 4m/pixel images which are certainly sharp when desampled to 8m/pix. The question is how many such images Cassini could snap in E3, and if an imaging strategy could guarantee a supercloseup of a Tiger Stripe near or at a vent.

Dangerwise, I start to imagine the possible endgame of the mission, setting a trajectory that points straight ahead and gets some low-blur images of the vents without the use of a shield on a flyby where we no longer care if Cassini is destroyed. This would have to be combined with a free trajectory into its destruction somewhere besides Enceladus or Titan, and my top candidate would be the B ring. Maybe a low-inclination orbit that slices into the B ring could return some "downlooking" shots of the rings at incredible resolution before sure death when the orbit hits the ringplane. (If light doesn't go through them, neither will Cassini.) Thus, two high-value images could be obtained on the final death orbit, whenever that would be.

Astrobiologically speaking, I wonder if one of the problems for the Enceladan reservoirs might be if the water is too pure. Undoubtedly, there's no way to get life out of 100% H2O if there were no other compounds there. Nature seems to abhor purity in most cases, so hopefully that will be the case with Enceladan H2O as well.

Explorationwise, I fear we might have a reservoir under pressure with no way to get into it. Imagine trying to stick your digital camera into an open fire hydrant. There's a reason why those jets are spraying so far up, even in that super-light gravity. Even if we drill our own hole, that might just uncap a new vent.

Sarcasmwise, Alex, Bruce isn't going to get your putdowns unless you do the rhetorical equivalent of hitting him on the head with an anvil that says "2000 LBS" on the side. He's too hyped up to come down.

Posted by: EccentricAnomaly Mar 10 2006, 05:57 PM

I don't think that is true for every Enceladus flyby. I'm pretty sure for E3, INMS will be pointed in the direction of motion...

Posted by: AlexBlackwell Mar 10 2006, 06:07 PM

Some veterans of my planetary_sciences Yahoo! Group may recall this exchange on July 21, 2005, which occurred at the tail end of a typical Moomaw anti-Shuttle fusilade. Of course, no one will be surprised that this message was preceded by several warnings/requests/pleas to Bruce to respect the topic of the discussion group

===================

--- In planetary_sciences@yahoogroups.com, <rmoomaw@s...> wrote:

> OK. You want me to drop this particular topic completely, I'll
> drop it.

Bruce, whatever gave you that idea? ;-)

You know, this is starting to remind me of the classic SNL sketch of
the movie mogul (John Lovitz) and the washed up movie star (Phil
Hartman) - paraphrasing, since I don't remember the exact dialog:

Lovitz: You're through, you hear me! You're all washed up!

Hartman: Don't sugar coat it, tell it to me straight.

Lovitz: Everyone hates your movies!

Hartman: What's the word on the street?

Posted by: RGClark Mar 10 2006, 06:26 PM

True but I wonder what would be the percentage of H2O of a sample from a fresh water lake?


- Bob Clark

Posted by: gpurcell Mar 10 2006, 07:13 PM

If the E-Ring is caused by the venting, why not try a Stardust-style sample return from THAT material, rather than dealing with all the issues required for a surface sample return?

Posted by: tty Mar 10 2006, 07:24 PM

Isn’t landing on Enceladus an unnecessarily complicated way to investigate the possible existance of life there? Here we have a watery planet that is obliging enough to blow its water and whatever is in it off into space and thereby preserving it at cryogenic temperatures where you can get at it without having to get down into the gravity well.
What is needed is a probe that gets into an orbit that is nearly, but not quite co-orbital with the E-ring and deploys a scoop to collect ice/water for analysis. It takes quite a lot of energy to heat ice from -200 and melt it, so you can use a fairly high relative speed (certainly a couple of hundred meters per second) and so sweep a fairly large volume in a short time.

You might also “trawl” the plumes themselves for fresher material but it would be difficult to stay in the plumes for any length of time without having to go into orbit around Enceladus.

tty

Posted by: BruceMoomaw Mar 10 2006, 07:24 PM

Oh, I got it. My whole point was that Spilker's group didn't seem to realize that this provided another good reason to orient the piggyback lander on EO toward engineering rather than scientific measurements until Karla Clark and I mentioned it. It hadn't been "blindingly obvious" to them -- or rather the political implications of a last-second EAL failure hadn't been blindingly obvious to them.

Posted by: volcanopele Mar 10 2006, 07:36 PM

a few notes:

E3 Imaging: Don't expect super high res images. Cassini will be approaching Enceladus while the moon is in high phase and closest approach will occur over the night side. A good amount of the blur in the high res images from July were because we were tracking gamma Orionis, not a point on the surface. If we were targeting the surface and had 5 ms exposures, it's possible the smear would be less, though as Bruce mentioned, you also have to keep the spacecraft point so that the HGA is in the RAM direction to protect the spacecraft from E-ring particles. Unfortunately, to see anything in saturn-shine, as would be possible at closest approach, we would need significantly more than 5 ms exposures to see anything. And that's assuming we had prime pointing. Which as of right now, we don't (INMS does I think). Then, three minutes after C/A, Enceladus goes into eclipse. Plenty of time for CIRS to look for even higher temps, but not good for ISS imaging.

Composition of reservoir: INMS observations of the plume suggest that it isn't pure H2O. Their composition measurements suggest 91% H2O, 3% CO2, 3.3% N2 (though CO is possible), and 1.6% CH4.

Posted by: Richard Trigaux Mar 10 2006, 07:41 PM

No, plumes are better than the E ring. Whatever material there is, it will qhickly burn with UVs, so that only the plume at low altitude is interesting. Anyway only one passge in the plume may collect more matter than scooping the E ring.

Posted by: AlexBlackwell Mar 10 2006, 07:44 PM

OK, Bruce, let me suggest this. Take a magic marker and write on your palm "EAL landing failure = bad; EAL landing success = good."

Hey, maybe it's me, but if the "political implications of a last-second EAL failure [isn't] blindingly obvious to them," then "the[y]" have been living in a cave if "they" do not realize the ramifications in a failure of a multi-billion dollar Flagship-class mission.

Posted by: JRehling Mar 10 2006, 08:12 PM

I've noticed that sometimes (ideally, in fact) people don't say blindingly obvious things; that doesn't mean that they don't recognize them. It also doesn't mean that they didn't recognize it if they don't tell a stranger that the stranger just said something obvious.</2000 LB weight>

Posted by: BruceMoomaw Mar 10 2006, 09:16 PM

OK. All I can report is what I saw: before Ms. Clark and I mentioned that, they'd been talking primarily about the scientific uses of an early Europa lander, and whether they were adequate to justify flying one. SHE certainly thought it was a worthwhile point for me to bring up. Since the cost-benefit lever for this whole squabble is beginning to resemble that of the arguments on Laputa, it might be wise for me to drop it at this point (acompanied by jeers about how I'm Admitting Defeat, which in this case I'm not).

Posted by: AlexBlackwell Mar 10 2006, 09:34 PM

Doesn't RADAR have a scatterometry sequence during this flyby that at some point precludes HGA shading from SC_RAM? (i.e., POS_Z to Sun, NEG_Z to Enceladus, NEG_X to North_Pole_Dir.).

If I remember correctly, after the hand off from ISS NAC, RADAR has control of both primary and secondary axes to obtain correct polarization.

Posted by: AlexBlackwell Mar 10 2006, 10:15 PM

"Admi[t] [d]efeat"? I would never expect that from the http://redwing.hutman.net/%7Emreed/warriorshtm/tirelessrebutter.htm.

Actually, Bruce, you're manufacturing a dispute. I was merely amazed that someone involved in architecting a Flagship-class mission, and no, I don't mean you, would have to state something as obvious as "we do NOT want something as expensive and long-term as a Europa Astrobiology Lander to fail at the very moment it's trying to land on Europa." Moreover, I was amazed that you found that such an important point. I guess I would have just said, "You think?"

Posted by: RGClark Mar 10 2006, 10:48 PM

Wow that 1.65% methane figure itself has interesting astrobiological implications.
It could be due to hydrothermal activity or active microbes. Even if hydrothermal in origin it still raises the possibility of biology.


- Bob Clark

Posted by: BruceMoomaw Mar 10 2006, 11:20 PM

*sigh* The significance of my putting "NOT" in capitals, for special emphasis was something that YOU didn't catch. Of course everyone knew damn well that we don't want EAL to fail during landing. What struck me (to repeat what I've been saying from the start) was that until Clark and I opened our yaps, virtually all the discussion in the little group had been about whether a small Europa lander was justifiable on purely scientific grounds or not. There didn't seem to be much thought about the fact that minimizing the chances of an EAL landing failure was so important that it might well justify the small earlier lander even if it WASN'T justifiable on scientific grounds. Clark noted the same thing, and got downright shrill about it when she started talking.

Actually, I have the whole thing on tape, since I tape-recorded all the subgroup's deliberations (with their permission). At some point in the near future I'll review it -- and if it doesn't match up with my memories of what happened, I will not only admit it here; I will kill myself in the most disgusting manner conceivable. Now, back to Enceladus (I hope).

Posted by: AlexBlackwell Mar 10 2006, 11:51 PM

Bruce, you really crack me up. That's what I meant about manufacturing a dispute. I'm not questioning your veracity. Jeeez. I accepted what you said at face value. I was only amazed that something so obvious had to be stated. That's all.

Now, I'll await your inevitable (1,000 word?) response to this.

Actually, I'm beginning to feel like a sadist poking a stick at a caged tiger

Posted by: helvick Mar 11 2006, 01:54 AM

Bruce\Alex.
I'm not in a position to comment on history here but....

I value both of your perspectives on these issues and would like you both to keep to the topic. It's not my call so feel free to ignore this but far too much of this debate has been personalised.

You are both extremely well informed on the issues and can do better than this in a forum such as this. If you want to have a personal scrap I think you should do that elsewhere.

All of the above is clearly in the "ad hominem" space - I really hate feeling that I'm doing such a thing but I really would like you both to step back and think about what you post on this topic before hittting the post button.

[Doug: I'd be happy to see this post deleted but please do so in conjunction with a broad brush]

Posted by: AlexBlackwell Mar 11 2006, 02:00 AM

No, you're absolutely correct, helvick. Point taken.

Posted by: BruceMoomaw Mar 11 2006, 02:50 AM

Fine -- especially since the whole damn thing was a diversion from what this thread is supposed to be about anyway. (Admittedly I launched it, but I had no idea it was going to drag on THIS long.) Now back to Enceladus.

Posted by: BruceMoomaw Mar 11 2006, 11:19 AM

I found Jason's blog entry from almost a year ago ( http://volcanopele.blogspot.com/2005/03/enceladus-and-image-resolution.html ) quoting Zibi Turtle as saying that "we will probably not get better images than 25 m/pixel, despite coming within 100 km of Enceladus in March 2008 (which would yield sub-meter resolution images if ISS took images at C/A)." A pity, but we can't have everything.

Posted by: RGClark Mar 11 2006, 03:33 PM

Jason, does Cassini have the capability to do an isotopic measurement on that methane?


- Bob Clark

Posted by: EccentricAnomaly Mar 11 2006, 06:01 PM

why is that a pity? There are other instruments you know. And it seems like plume science is better done with in situ measurements than remote sensing. Remote sensing would be better for looking at the vents more closely... but E3's geometry doesn't allow that, so imaging on E3 would be a big waste INMNSHO.

Posted by: JRehling Mar 11 2006, 06:58 PM

One Enceladus mission I could see flying would be a sample return that flies through the plume (assuming it is "on" all the time -- what a horror if it weren't!) and brings the stuff back to Earth on a free return trajectory, or cruising there and back with NEP. Moreover, the same mission could grab some upper atmosphere stuff from Titan and/or the rings and/or Saturn (though surely not all four). Several add-ons could be imagined: an imaging impactor, etc.

Posted by: BruceMoomaw Mar 11 2006, 10:53 PM

The NEP idea -- which would orbit Saturn and fly through the Enceladan plume several times to collect a sample for return -- is what Oberg was talking about in his article. But it would obviously be expensive as hell.

The vastly cheaper idea of flying by Enceladus once and then returning to Earth is a longer range version of Chris McKay's "Europa Ice Clipper", and it might very well be worth doing even for one sample. It wouldn't even need to include the complex requirement, for Europa, of dropping an impactor on Europa and then making sure it flew through the impact-debris plume -- and it wouldn't necessarily need nuclear power; there's already been a proposal for a solar-powered Saturn entry probe flyby mission based on the "INSIDE Jupiter" design, but with 4 solar panels instead of two (and without the big JOI propulsion system). Basically, we would just be flying a duplicate of Stardust -- but with much bigger solar panels -- through the Saturn system and then back to Earth. We're definitely talking about a New Frontiers-class mission, and I have no doubt that people are already thinking about proposing it for the next New Frontiers AO. The catch might be whether Saturn's gravity would be strong enough to allow a hairpin-turn direct return to Earth -- but it might well be workable, instead, to arrange a gravity-assist flyby of Jupiter on the way back INTO the Solar System to finish bending the returning craft's orbit all the way back into the inner Solar System. (The craft would initially have been launched directly from Earth to Saturn, which our stable of ELVs could easily do.)

Parenthetically, there was still some interest at the Europa meeting in dropping an impactor onto Europa and either analyzing or actually collecting the resultant debris plume. C.A. Hibbitts gave a talk on a concept whereby Europa Orbiter would drop a large impactor onto Europa (maybe during one of its multiple orbit-trimming flybys of Europa in Jupiter orbit, before it finally brakes into orbit around Europa itself), take spectra of the resultant flash, and later photograph the fresh crater from Europa orbit -- that is, an Europan version of Deep Impact.

Afterwards I had a long and interesting talk with him about variations on this idea. His idea is to try to analyze material from below the radiation-modified layer -- but he agreed that it's open to question whether long-range IR spectra could properly analyze any organics dug up by the impact. It might be better to have the craft actually fly through the debris plume and analyze it directly with a dust-impact mass spectrometer. (In any case, his impactor would be a substitute for a small piggyback lander on Europa Orbiter -- but see more on that below.)

And it might be better still to have a separate mission that would fly through the Jupiter system without stopping, drop such a really big impactor onto Europa, fly through and collect some of the impact plume debris, and then double back to Earth with it -- that is, Europa Ice Clipper, but with a much bigger (and actively guidable) Deep Impact-type impactor. I've been thinking about this idea for some time, and it turned out that he has too. Again, this is a New Frontiers-class mission -- and if Europa Orbiter really does get hung up fiscally, it might be advisable to fly it first.

One thing we finally did agree on, though, is that one of the optional goals we'd both been thinking about for such an impactor -- using a descent camera on it to get those super-closeup views of Europan surface terrain which may be important to properly design the landing system for a big Europa Astrobiology Lander -- probably wouldn't be practical. In order to have time to return the last few really close descent images just before the craft hits the surface, you need a lander that can survive landing and play back the images after landing -- a crash-lander probably wouldn't be able to return any images of Europa's surface better than the final ones from the Ranger missions, whose resolution wasn't any better than the images we can get from orbit anyway using a HiRISE-type camera of the type that's already very likely to be carried on Europa Orbiter.

Posted by: BruceMoomaw Mar 12 2006, 11:30 PM

Oh, I'm most emphatically not saying that such very low-altitude flybys aren't very much worth doing for Cassini -- probably several times. Besides mass spectrometric analysis of the denser plume cloud and better gravity data, the CIRS could probably nail down with great accuracy both how wide the tiger-stripe vents really are and how warm the fluid coming out of them really is -- and, if it was warm enough, the NIMS could also provide more data on the composition of the erupting fluid when it first emerges. It's just a pity that we couldn't also get really high-res photos of the vent sites as well -- especially since, according to Scalbers in the "Stardust Mission to Saturn" thread below, the region will soon be in seasonal shadow until 2030.

In that connection, I was puzzled by the failure of Porco's "Science" article to mention at all one thing that positively jumps out at the viewer even in Cassini's first Enceladus close-ups: the moon's "freckles". Quoting Helfenstein ( http://www.aas.org/publications/baas/v37n3/dps2005/450.htm ): "Among the most mysterious newly-discovered features are small, sub-kilometer-sized dark spots and circular pits that sometimes cluster in a honeycomb like patterns near faults and scarps. Their origin is unknown, but perhaps the pits and dark spots identify sites of explosive venting of subsurface volatiles through fractures or volcanic conduits." But there's not a peep about these in Porco's otherwise very detailed "Science" article. The fact that they're dark makes it likely that we're looking at the dark products of methane and the other organics expelled from the vents and then radiation-processed by solar UV and Saturn's own radiation belts -- although it's possible that they might also contain some bits of rocky debris from Enceladus' interior, or salts dissolved from that rock into the water and then redrying on the surface after expulsion. Maybe they're mentioned in some of the other "Science" articles, since I only have Porco's and Spencer's?

Posted by: volcanopele Mar 13 2006, 02:46 AM

No, they are not mentioned. The dark leopard spots are found in Samarkand Sulci in the equatorial sub-Saturnian hemisphere. to keep the Science article manageable, it was limited to South Polar geology and plume analysis.

Posted by: ugordan Mar 13 2006, 08:38 AM

Are you referring to features seen in http://ciclops.org/view.php?id=937 and also http://ciclops.org/view.php?id=855 raw image?
If so, the statement that the features seen are indeed dark isn't backed up by strong evidence, IMHO. They really look like topographic shading effects on small dome-like or spike-like features in the grooves. Several of the larger ones, specifically in the second image I linked, at the top center clearly give the impression that we're looking at their shadowed regions. They otherwise don't look any darker than the surrounding ice to me.

Posted by: AlexBlackwell Mar 15 2006, 08:38 PM

Well, the Enceladus-3 flyby C/A distance is official in one sense: http://en.wikipedia.org/wiki/Enceladus_%28moon%29

Posted by: volcanopele Mar 15 2006, 10:06 PM

The planetary society Cassini tour page now has the new value. Emily says she got he new trajectory from Dave Seal on the navigation team, so this is about as official as it gets.

Posted by: AlexBlackwell Mar 15 2006, 10:19 PM

Yeah, it's been "official" for several weeks now; I got it from Seal's MP presentations, I believe, in late December 2005. In fact, the 25 km figure is popping up everywhere except on the Cassini public website

Posted by: AlexBlackwell Mar 28 2006, 09:26 PM

I just read the materials in the 060323 Reference Trajectory Update package that Cassini Mission Planning (MP) released today and the 25 km C/A altitude for the 61En targeted flyby does not appear to be set in stone (viz., "not finalized yet" according to MP).

Having said that, the new tour tables do show 25-31 km C/A figures but it looks as if MP may revisit this issue before 2008.

Posted by: BruceMoomaw Mar 29 2006, 01:11 AM

Can you tell us anything more about just what they plan to accomplish with such a low-altitude flyby?

Posted by: AlexBlackwell Mar 29 2006, 06:10 PM

The precise science observing schedule is still TBD, which is not surprising due to fact that 61En has brand new flyby geometry. As EccentricAnomaly http://www.unmannedspaceflight.com/index.php?s=&showtopic=2306&view=findpost&p=44995 earlier in this thread, one of the working assumptions is that INMS will be pointed to RAM near or during C/A, so I presume INMS plume data is going to be a high-level goal, assuming, of course, the 25 km figure holds up.

I'll only note in passing that the Rev-28 Enceladus RSS plume occultation is shaping up to be a very important observation.