New images today, taken on October 28. On one of them, there is a feature that looks like a landslide (white arrows):
Click to view attachment
Is the configuration of this gouge compatible with the gravity gradient at that spot?

This jumped into my eyes, too, when I saw this image for the first time (in the blog).
If it's actually a landslide, the dust layer needs to be very very soft, and barely adhering.
Taking the very low gravity, there may be nothing, which would compactify the dust.
On the other hand: If it's a landslide, why did it stop there?

So it might also be kind of an impact crater or a scratch formed by the collision with a boulder, which bounced back afterwards.

There's a 'slumpy'-looking feature at the extreme bottom of this image as well.

I suppose that it's conceivable that these might be caused by gas escapement from under the dust layer as well. It's a dynamic surface in comparison to other types of bodies we've examined this closely, so there may be subtle mechanisms at work with no common Earth analogs.

Wondering if these smooth 'snow fall' looking patches might be areas where the nucleus is accreting material that has previously sublimed or been ejected. Kinda falling though its own debris field. Is there a coorelation between these smooth areas and the direction that faces forward to its rotation?

I think so. They could be thought of as 'coma fallout'. Their undisturbed appearance may be indicative of relatively recent deposition. Otherwise, they should have a hummocky, pockmarked texture after eons of impacting.



That would be interesting to determine. Also, I wonder if the particular directions of the jets may favor some portions of the surface over others?

" Is there a correlation between these smooth areas and the direction that faces forward to its rotation?"

I don't think so, as far as I can tell so far.

Phil

Darn!... thanks Phil.

Just cannot help but think that re-depositing of materials plays some part in what we are seeing.

What a marvelous landscape to try and interpret!

The idea that fine grains on up to boulders might shake loose and float off the surface long enough to be noticed by Rosetta and/or the lander just has me stocking up on popcorn, eager for the side-show!

Yep; I know I'll be surprised if the first post landing images don't remind me of that footage from ocean submersibles. As soon as some robot arm or turbine disturbs the sediments on the bottom, big mud clouds fly up and take forever to disperse.

Water's a lot more viscous than vacuum, though. Even if Philae stirs up much dust (I don't know why it would), it won't stay up for long.

Yes, it will just go in a parabolic trajectory back down; there hopefully won't be anything 'hanging' .
T-10 days....

The behaviour in such low gravity is pretty unintuitive. The landing speed is supposed to be of order 0.5-1 m/s. If the landing kicks up particles with similar speed, their parabolic paths will keep them up for of order hours, and take them up to heights of order kms. This assumes comet mass of order 10^13 kg and radius 2km, for a surface gravity of order 10^-4 m/s^2. So although they'll stay up for a long time, they'll get extremely thinly dispersed.

Of course there may be a large range of velocities for particles kicked up by landing - faster ones would stay up longer and go higher, slower ones shorter and lower.

Looking back at some closeup pictures after reading this and other messages makes me "see" different things on this weird terrain: doesn't it look like a snow layer "draped" over boulders and craters and rims?

Click to view attachment

I don't have suitable SW, but I guess it would give a better appearence of snow if I could colour in white or light-blue the "sandy" area.

I think Philae is going to discover (or sink into...) a new kind of material, for which we have not yet a name: it's a composite of dust grains, ice grains and forzen-CO2 grains, all mixed together in a fluffy grey snow.
Maybe it has same "solidity" of volcanic ashes just fallen. But unlike on Earth, where gravity and rain eventually compact the ashes into mud and eventually into rock-solid ashes, it remains fluffy forever on a no-rain low-gravity world.


Click to view attachment

Interesting page on volcanic ashes properties:
http://geology.com/articles/volcanic-ash.shtml

An additional weird thing of this small world is that it probably has an... Escher gravity! :-)

So in no way we can figure out only by images (especially if just 2d) why a boulder or landslide/avalanche stops in one place rather than in another

Click to view attachment

Though smooth-looking, the material is very dark and so not just new-fallen snow. It must have dust/pebbles/organics in it at significant concentrations. It could have enough structural strength to hold together pretty tightly--I imagine aeons of vacuum cementing, plus solar UV flux creating chains of hydrocarbons, resulting in a material that could be reasonably solid at low temperatures.

(I sure hope Philae doesn't go "blub-blub-blub" and sink through fluff!)

Would the ejection velocity be higher for lighter particles than heavier ones? If so, the 'snow' (assuming it is 'fallout') could be dominated by larger particles that are more likely to settle back to the surface. Perhaps the 'snow' is sand and gravel, with relatively little finer material. I think of it like cinders falling back around a volcanic vent (though we're dealing with escape velocities in a vacuum rather than convection).

Also, the best images seem to hint at a granular texture of the 'snow', as well as slumping structures remaining in place. Could this be due to some 'tacky' properties of the particles? If so, I think this would bode well for the landing conditions.

Add to the list of landing complications we hope to avoid:
-landing upside down.
-missing the target.
-failing to attach to the surface.
-insufficient electric power.

-and now, disappearing beneath the dust/snow.

Due both to the lack of control after separation and the awkward characteristics of the target body, we already know this is going to be very tough.
The possible difficulties are already numerous enough that adding more imaginative failure modes probably doesn't affect the overall odds very much.
We are just going to have to keep the peanuts handy and keep our fingers crossed.

I think some would benefit from reading the documentation regarding the design and testing of Philae....

http://www.simpack.com/fileadmin/simpack/d...lanck_hilch.pdf

http://elib.dlr.de/87802/1/Poster_DLR_Land...zeA4_IPPW10.pdf

http://www.lpi.usra.edu/meetings/lpsc2013/pdf/1392.pdf
from which this paragraph is particularly important....


And finally
http://issfd.org/ISSFD_2012/ISSFD23_GC_2.pdf

The "three sigma" simulation ellipse shown in the ISSID.org reference mentioned by Doug above is quite small, less than 200 m in its longest dimension. The circle designating landing site J seems to be several hundred meters in diameter based on Emily's September 15th blog post, which includes a 1 km reference square. The probability of landing within it should be at least 97%, or much better if the simulation assumptions are valid.

Click to view attachment

Click to view attachment

Click to view attachment

Site J is already known to have adequate sunlight. And after separation, the lander is designed to control its attitude but not its trajectory. So, if the trajectory is on target, solar energy should be sufficient at the site and the lander should be able to maintain an acceptable orientation with respect to the surface. That would leave the surface texture as a significant risk factor, while still suggesting good odds for a successful landing.

Be that as it may, if fate will guarantee an on-target trajectory, I will happily take my chances about Philae vanishing beneath the dust/snow. Please pass the peanuts.

A farewell NAVCAM image of site J released.

And, as of today, ESA will share all Rosetta/NAVCAM images under a Creative Commons Licence, officially allowing easy sharing, publication and manipulation!

The landing site has now been called Agilkia:

What I imagine instead is a rock traveling around the sun followed by a cloud of debris particles, most of which are too light to be able to "go back and land" due to continuous gas emission, so they keep orbiting around the nucleus like a cloud of (damned) midges around your head. ;-)

Once eventually the comet "turns off", the cloud/coma requires months to come to rest and the particles to slowly fall back to the surface.
If Philae has retro-rockets for landing (didn't study its design yet) it could encounter serious target-visibility issues.... or it could just spread away all debris and uncover underlying solid terrain, considering there's no air to keep the debris flying around close to ground (thinking of apollo landing).

Hard to say,
nice to see. :-)

That reference isn't dated, and skimming through it I see no sign that the calculations were made for the actual chosen landing site. It looks just like early simulations.

For a body like C-G, I could imagine the actual landing ellipse size would be sensitive to the landing location, so the size of that simulated ellipse may be very different from that of the actual landing site.

As far as I know, we haven't seen actual landing ellipses for the actual landing site yet...

No landing rockets. Philae is left in an orbit that will intersect the comet's surface at a relatively low speed. It has attitude control, but no retros. So, no target visibility issues due to rocket blasts.

-the other Doug (With my shield, not yet upon it)

Meanwhile, some new developments on the legal front:
http://blogs.esa.int/rosetta/2014/11/04/ro...ommons-licence/

So Wikipedia can finally illustrate them freely (along with everyone else).

T-8...

Thanks Doug for these helpful figures. I was trying to visualize this in comparison to snow on Earth to get a feel for Philae's design tolerances, this is what I came up with. Please correct me if I'm making any bad assumptions here.

The minimum surface compressive strength Philae was designed for is 2kPa. This is comparable to snow/ice that can support the weight of a cube made of steel with 2.5cm (1") length sides (on Earth!). Qualitatively this is something like lightly compacted snow or snow with a icy crust, but definitely not fresh snow. The "mission objectives will be compromised" limit for surface strength is 100Pa. This is like snow that can support a sugar-cube-sized cube of ice with 1cm sides, or a cube of balsa wood with 6cm sides. These would still sink at least a bit in the freshest of powder snow, but that's really very little pressure - as a former Coloradan, I'd estimate this to be like wet snow, or fresh powder after sitting in the sun for an hour.

What this doesn't take into account is the fact that the compressive strength of the dusty layer could be very dynamic if it is compacted. Think of packing a snowball, it's easy at first but the more you compact it, the more pressure it takes. If compaction does play a part, it seems like it certainly won't hurt, as it can (presumably?) only increase compressive strength. However, compaction could introduce other effects like slippage if Philae lands on a (relative) slope. Anyway, thanks again for posting these numbers, finally helped me understand a bit more of what Philae was designed to handle.

-d

1 kgf/cm^2 = 98.0665 kilopascals.
density of (balsa wood, ice, steel) = (.0016, .00093, .008)
(((2.5^3) cm^2 * 0.008 kg/cm^3) / (2.5^2) cm^2) * 98 kPa/(kgf/cm^2) = 1.96 kPa
(((1^3) cm^2 * .000934 kg/cm^3) / (1^2) cm^2) * 98 kPa/(kgf/cm^2) = .091 kPa
(((6^3) cm^2 * .00016 kg/cm^2) / (6^2) cm^2) * 98 kPa/(kgf/cm^2) = .094 kPa

A somewhat preposterous presentation, but it is about Rosetta. =)

saw that the other day
it is up there with the IBM "linux" TV commercial from 11 years ago
https://www.youtube.com/watch?v=s7dTjpvakmA

My thought is that there may be layers of tar-like crust sandwiched between less consolidated material. Since C-G is periodic (~6.45 yrs) with a perihelion that has been reduced from 2.7 to 1.3 AU, it has had multiple episodes of "emissivity." At 1 AU, the moon's sunlit surface can reach temperatures exceeding 250 degrees Fahrenheit (123 Celsius) and its dark side has temperatures exceeding these values on the minus side of the scales. Assuming there are sufficiently complex molecules to cross-link--or "dusts" that can lithify--as components of the out-gased materials, these could be either directly sprayed onto the surface by the vents or fall back on it later. Near perihelion these materials could melted and annealed. Indeed, this could even happen during one rotation cycle for certain parts of the comet. It would seem possible that as the comet receded from the sun, the consolidated materials would be relatively immune from further "boiling" away, and yet, some of the residual gases and dusts could be captured by the comet. I agree that UV and cosmic radiation could continue to then operate to bind the "final coating." And, there is always the ongoing, fortuitously captured "micrometeor rain" adding to the regolith.
I would still kind of like to see snow or something analogously snow-like. There would still probably be layers like glacial firn (neve).

Color images would be much better to understand surface features.
OSIRIS camera has even sixteen color filter! http://www.planetary.org/explore/resource-...tta-osiris.html
Why do we only see B/W pictures published?
Is there a full archive of raw images, like it happens for NASA Mars rovers? We could at least try building color images by ourselves by combining different filters; we all know it will be never be "true color", but at least it will not be just gray.

All the images from the mission will end up on the PDS when the proprietary period has ended. (And that's the end of that discussion)

The surface is spectrally very uniform. So the error between images taken at slightly different times and slightly different light is almost at the same level as the difference in spectral reflectivity between filters. It is NOT a trivial thing to do right. You need a very good shape-model and very good registration of images. You also have to compensate for the lighting differances. Otherwise you risk creating color that just isn't there. I believe that they will release images when they feel that they have something that is good.

I see no other problems in creating a true color image. The surface is dark but it is still very much visible. The sunlight out there is about 10% of what we have here on earth. The comet reflects about 4 percent of that so yes it is really dark now. but still very visible. (The exposure times they use for OSIRIS is less than a second.)

Remember that the moon reflects about 12% and it is very bright when you look at it at night. So if you normalize the light to sunlight at 1Au the comet would be just short of half as bright as the moon.

There is obviously the usual problem of creating sRGB from the spectra. But given the filter ranges and the spectral uniformity it should work well to create a spectra, convert to cie and from there to sRGB.

'Color images would be much better to understand surface features.'

It may be very difficult to make a color image with the viewing angle and shadows changing between different filtered exposures, depending on the interval between them. They may have to do a more complicated version of what people do with Jupiter spacecraft photos and project the frames onto a 3D model rotated into place to register the three images. These would show color fringing in the shadows unless they decided to let the longest shadows 'fill in' the others. They may wait a rotation between exposures until the lighting is similar in all three to provide color information unaffected by lighting angle differences. But vantage point differences between such images could introduce further complications as color coverage gaps etc.

While anticipating a color product some time, is there Earth based spectral data one can use for overall average color information? I would guess the comet is a black-brown color.

How long does it take to OSIRIS to change color filter?



Thats' actually what would be interesting to know/see. There should be CO2 ice, water ice and carbon and many other stuff in different percentages down there; I guess color images could even highlight underground multylayering in zones where fracturing shows underground terrain.

http://www.esa.int/spaceinimages/Images/20..._October_2014_a

Has anyone seen the swear jar?

A google search for "osiris instrument rosetta pdf"

Reveals this paper http://pdssbn.astro.umd.edu/holdings/ro-a-.../osiris_ssr.pdf

Which states....



It later states

http://www.esa.int/spaceinimages/Images/20...de_of_the_comet

Our first glimpse of the night side! I thought it would involve a lot more waiting until after perihelion.

Talking of colors, this is one of a few images where the comet really looks like carbon:
Click to view attachment
http://www.esa.int/spaceinimages/Images/20..._October_2014_a


Talking of filters change time:
in 1 second the comet should rotate 2*pi/45720 = 0.0014 rad (12.7 hours period); assuming 2 km radius this would mean 0.0014*2000 = 2.8 meters surface shifting in 1 second; assuming 0.5m/pixel resolution this would mean 6 pixel "smear" for each color channel.
This should result in acceptable color images (if my math is correct)

Is there any picture showing the rotation axis of the comet? I can't find anyone.

ADMIN: From this same thread. Don't forget to use the Forum's search tool or read back through the topic.

Very nice shot. But I don't buy the description in the caption:
The coma is far fainter than the directly illuminated surface, and I can't see the coma detectably illuminating the dark side. If you look at the full frame, I think it's pretty clear what's going on. The visible portion of the dark side, circled in my pic, is being illuminated (arrow) by sunlight scattered from the far bits of directly-illuminated surface that I've drawn an ellipse around:
Click to view attachment

http://www.esa.int/spaceinimages/Images/20..._October_2014_a

To my eye--at least for portions of the photograph--there seems to be a system of strata generally running in the 11:00-5:00 direction composed of alternating dark and light bands. These are well seen on the dimly lit "inner wall" of the central "cave" as well as at the very margin of the comet surface where it is bounded by the blackness of space on the upper right of the photograph (especially in the expanded view). The different bands appear to have a differential resistance to whatever "erosive" process can occur, if the "up-and-down 'laddering'" seen on the well-lit left "exterior surface of the cave" is representative.
Should these alternating bands reflect different mechanisms of deposition/consolidation during periods of perihelion "emissivity" from those operating during the subsequent periods quiescence of C-G, then they may be used to estimate the age of the periodic comet--somewhat like tree rings or varves.
I personally don't know enough to be calculate the thickness of the layers given the data accompanying the photograph, but I am sure this can be done. Once known, one can start to give some ballpark measure of the volume of material required to form the layers; which may support or undermine the "theory of periodicity."

My guess is the striations are a joint pattern (if that is what it would be called in this case) that resulted from 'tectonic' stresses and large impacts.

Speaking of tectonics, I wonder if the barbell shape of the comet could have formed due to a progressive deterioration of the neck region caused by the combined effects of rotational centrifugal force and cometary erosion (de-icing of matrix and outgassing erosion). If so, such nuclei that start out merely elongated (egg-shaped) may tend to end up with barbell shapes before they split outright. That may explain similar shapes with other comet nuclei as well as with commonly observed cometary splits.

Boy, it's good to see some new OSIRIS images being released. Extremely impressive! (I just filled a swear jar to overflowing with this most recent release.)

It's going to be a very exciting time when the OSIRIS images hit the international version of the PDS and we can see more than a handful of them... maybe even more exciting than the preceding six months.

-the other Doug (With my shield, not yet upon it)

i like that model, but am also adding here armchair speculation in suspecting the configuration forms when two mutually rotating clumps lose centrifugal energy to the tidal stress/rearrangement of rubble, eventually settling together and building the junction into a sort of barbell isthmus by infalling and settling flotsam/jetsam over the eons as the two lobes contract and their surface remnants recede from the junction, so it should devolve towards a barbell cylindric rather than thinning the barbell isthmus towards eventual collapse. The pole being perpendicular to the long axis seems to support such a scenario, ..not sure if that is also the case for other barbell comets though.

I can see that happening as well. Perhaps such comets could follow two possible paths:

1. Erosion proceeds more quickly than deposition/settling, which leads to thinning of the neck until the comet splits. 67P may be moving in this direction.

2. Deposition/settling proceeds more quickly than erosion, which leads to slowing and possibly termination of sublimation and erosion of the neck area. Comet 103P/Hartley may be an example of this.

Since cometary processes can be a messy affair, it would be hard to predict an ultimate outcome. Of course there are additional wild cards -- e.g. variation of distribution of materials within the nucleus, gas pocket outbursts -- that could 'shake things up' dramatically. Also, the 'barbelling' process may be more likely with smaller comets than with larger ones.

Since we haven't yet seen the southern side of the comet, we don't really know that it is a 'barbell'. If later images show that the neck extends to the south, it's a sort of barbell, but if not, the 'neck' is just a depression on one side of the nucleus. So speculation might be a bit premature.

Phil

If it is a barbell, and if it formed by the joining of two bodies, I might expect the two to be more loosely connected than if it formed by the erosion of one body. In that case, around the time of greatest activity near perihelion I might expect differing jet reaction forces across the nucleus to lead to shifts in one body perpendicular to the line joining the two. That should lead to cracks (like we appear to have seen already) opening or closing near the neck. Depending on the details of the gravitational field and connection of the two joined pieces, we might even see vibrational modes - periodic opening and closing of cracks.

I can't wait for perihelion...

I have updated my Rosetta gallery, here are the 4 most recent NavCam mosaics:

2di7 and Titanio44 had published an awesome color of the comet, chosen as APOD the September 15th.

Two more :


Comet Churuymov Gerasimenko 18 October 2014 vc par 2di7 & titanio44, sur Flickr


67P RosettA navcam 26 October 2014 vc par 2di7 & titanio44, sur Flickr

The originals are bigger.

I will link the picture of the site J on Philae's topic.

Just one more, though I can't remember if Emily didn't linked it from her blog. But if not :


Comet 26 09 14 NavCam Mosaic vc par 2di7 & titanio44, sur Flickr

Your caption of the NAVCAM image from 4 November 2014 is wrong. The LARGER lobe of the comet is seen in the foreground.