Enceladus-3 (March 12, 2008) Options

[Guest_Richard Trigaux_*]

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...

[DDAVIS]

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

[Guest_Sunspot_*]

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

NAC: http://saturn.jpl.nasa.gov/multimedia/imag...eiImageID=45681

WAC: http://saturn.jpl.nasa.gov/multimedia/imag...eiImageID=45711

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

[JRehling]

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

NAC: http://saturn.jpl.nasa.gov/multimedia/imag...eiImageID=45681

WAC: http://saturn.jpl.nasa.gov/multimedia/imag...eiImageID=45711

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

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).

[EccentricAnomaly]

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.

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.

[Guest_BruceMoomaw_*]

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.

Now that's a visionary statement.

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".

[The Messenger]

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.

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.

[Guest_Richard Trigaux_*]

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".

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.

[Guest_AlexBlackwell_*]

So I'll settle for "important" rather than "visionary".

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

[Guest_AlexBlackwell_*]

what I heard is that the altitude is not set in stone, but could change with another new trajectory...

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.

[JRehling]

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.

[EccentricAnomaly]

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

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...

[Guest_AlexBlackwell_*]

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.

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?

[Guest_RGClark_*]

...
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.
...

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


- Bob Clark

[gpurcell]

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?