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Rosetta - Early Orbital Operations at Comet 67P C-G, August 6, 2014 - November 13, 2014
foxfire
post Oct 25 2014, 02:28 AM
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QUOTE (Phil Stooke @ Oct 24 2014, 11:56 AM) *
"On one of the images, we can see scallop-shaped features (see arrows). Sublimation zones?"

I had been thinking of these as avalanche scars, places where a patch of the dusty surface slipped and fell off the nearby cliff.

Phil


Although I'm inclined to sublimation (cf. 10/20/14), the lobate piles(?) of stuff on the floor below(?) support your theory.
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nprev
post Oct 25 2014, 04:31 AM
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Notice that the rocks near these features have 'dust' tails, as if there was a prevailing wind blowing from the left. I would interpret these as gas-scoured areas, and the venting is happening from inside the adjoining 'canyon'.


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fredk
post Oct 25 2014, 05:27 PM
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QUOTE (Gerald @ Oct 24 2014, 03:18 PM) *
Hipass-filtering, brightness stretching, rgb-channel combine, closer inspection, and annotation connects the two OSIRIS WAC images

I was skeptical at first (as always), but linking several streaks in the two frames appears to be fairly convincing, and they may indeed be real snowflakes/particles.

In terms of determining sizes/distances, I would say some estimate of the expected relative velocities would be our best bet, as I described in this post. (In that post I was considering dots, not streaks, so only talked about putting limits on distances, but the same idea applies with streaks, where, up to geometrical factors, you could actually estimate the distance.)
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jgoldader
post Oct 25 2014, 09:26 PM
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I have to keep reminding myself that the nucleus has very low albedo, and these images are very stretched. Has there been mention of any actual high albedo areas being identified?
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Gerald
post Oct 26 2014, 12:05 AM
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QUOTE (fredk @ Oct 25 2014, 07:27 PM) *
In terms of determining sizes/distances...

According to the ESA blog the exposure time for the high-exposure image has been 18.45 seconds.
The estimated 1.3 degrees streak should have been caused by a dust grain, which would have needed about 18.45s / 2 tan 0.65° = 18.45s * 44.07 = 813s = 13.6 min to overcome a distance the same as from the grain to the camera, simplified assuming it has been moving perpendicular to the pointing of the camera. With a more realistic assumption of 45° trajectory it would have been 9.58 min for the same distance.

For a grain at 10 meters distance its relative velocity would have been rougly 10m/600s = 1.6 cm/s. Distance assumption and relative velocity assumption are proportional.

The brightness of the streak in the short-exposure image is about 1/100 that of the brightest features on the comet, assuming a 2.2-gamma corrected image, grey values of 26/255 for the grain pixels and a grey value 213/255 for the brightest parts of the comet ((26/213)^2.2 = 1/102). The length of the streak is about 10 pixels. The exposure per pixel is hence about 1/10 of the comet. Assuming the same albedo results in an apparent diameter of about 1/3 pixel or about 7 arc seconds.

For an assumed distance of 10 m this would mean an estimated diameter of the grain of about 10m * 2 * tan 9.7e-4° = 0.34 mm.
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fredk
post Oct 26 2014, 04:45 PM
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QUOTE (Gerald @ Oct 26 2014, 12:05 AM) *
For an assumed distance of 10 m

This is still the big assumption, and could conceivably be orders of magnitude off.

The velocity will have two components - the orbital velocity of Rosetta and the ejected velocity of the particles. The former is easy to estimate given the orbital radius and period. The latter I know nothing about. I don't even know which (if either) might dominate. Indeed I'd expect some distribution of ejected velocities. But even an order of magnitude would tell us something about the size and distance of the particles we're seeing.
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Gerald
post Oct 26 2014, 07:43 PM
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QUOTE (fredk @ Oct 26 2014, 06:45 PM) *
This is still the big assumption, and could conceivably be orders of magnitude off...

Correct! There are still a few degrees of freedom in the system. Albedo/scattering and gamma-correction (for grain size estimates) are the other two, besides some minor uncertainties in image analysis.
For Rosetta's velocity relative to the center of mass of the nucleus, the escape velocity of an object at a distance of 10 km from a mass of 1e13 kg is
sqrt(2 GM/r) = sqrt(2 * 6.672e-11 Nm²/kg² * 1e13 kg / 10,000 m) = 36.5 cm/s.
The velocity for a circular orbit is slower by a factor of sqrt(2), hence 25.8 cm/s.
At 9 km distance from the center of mass it's 27.2 cm/s for the circular orbit.

If we take the 27.2 cm/s as an estimate for the relative velocity to the dust, the distance would be (using the proportionality of velocity and distance) about 10 m * 27.2 cm/s / 1.6 cm/s = 170 m.
No idea, whether this assumption is realistic. From the geometry of the jets, it may be possible to infere the velocity of the grains causing the scattering there. But it's not clear, whether a velocity obtained that way can be used for the individual observed grains. There may be a different observational bias.

There are some faint, short streaks indicating, that there are grains further away; hence the distance for the brighter streaks should be far less than the distance to the comet. This still leaves much uncertainty.
Blur (by the optics) of the dust streaks might be another approach to find some constraints for the distance.
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Phil Stooke
post Oct 27 2014, 05:05 PM
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http://blogs.esa.int/rosetta/2014/10/27/co...tch-24-october/

New mosaic alert! Yowsers, that's jagged.

Phil


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mcgyver
post Oct 28 2014, 02:06 PM
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I can't find anywhere the total amount of kilometers traveled by Rosetta till now, any help?
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Marvin
post Oct 28 2014, 10:52 PM
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QUOTE (mcgyver @ Oct 28 2014, 10:06 AM) *
I can't find anywhere the total amount of kilometers traveled by Rosetta till now, any help?


"rendezvousing with the comet required travelling a cumulative distance of over 6 billion kilometers"

http://www.esa.int/Our_Activities/Space_Sc...asked_questions

Factsheet:

Launch date: 2 March 2004

Journey milestones:
1st Earth gravity assist: 4 March 2005
Mars gravity assist: 25 February 2007
2nd Earth gravity assist: 13 November 2007
Asteroid Steins flyby: 5 September 2008
3rd Earth gravity assist: 13 November 2009
Asteroid Lutetia flyby: 10 July 2010
Enter deep space hibernation: 8 June 2011
Exit deep space hibernation: 20 January 2014
Comet rendezvous manoeuvres: May - August 2014
Arrival at comet: 6 August 2014
Philae lander delivery: November 2014
Closest approach to Sun: 13 August 2015

Mission end: 31 December 2015

http://www.esa.int/Our_Activities/Space_Sc...setta_factsheet
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charborob
post Oct 31 2014, 05:57 PM
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New images today, taken on October 28. On one of them, there is a feature that looks like a landslide (white arrows):
Attached Image

Is the configuration of this gouge compatible with the gravity gradient at that spot?
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Gerald
post Oct 31 2014, 06:15 PM
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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.
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nprev
post Oct 31 2014, 06:25 PM
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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.


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belleraphon1
post Oct 31 2014, 06:30 PM
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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?
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Gladstoner
post Nov 1 2014, 12:08 AM
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QUOTE (belleraphon1 @ Oct 31 2014, 12:30 PM) *
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.


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.

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


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?
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