Rosetta - Early Orbital Operations at Comet 67P C-G, August 6, 2014 - November 13, 2014 |
Rosetta - Early Orbital Operations at Comet 67P C-G, August 6, 2014 - November 13, 2014 |
Oct 25 2014, 02:28 AM
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#541
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Newbie Group: Members Posts: 7 Joined: 5-June 08 Member No.: 4184 |
"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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Oct 25 2014, 04:31 AM
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#542
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Merciless Robot Group: Admin Posts: 8783 Joined: 8-December 05 From: Los Angeles Member No.: 602 |
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'.
-------------------- A few will take this knowledge and use this power of a dream realized as a force for change, an impetus for further discovery to make less ancient dreams real.
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Oct 25 2014, 05:27 PM
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#543
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Senior Member Group: Members Posts: 4246 Joined: 17-January 05 Member No.: 152 |
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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Oct 25 2014, 09:26 PM
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#544
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Junior Member Group: Members Posts: 95 Joined: 5-September 07 Member No.: 3662 |
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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Oct 26 2014, 12:05 AM
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#545
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Senior Member Group: Members Posts: 2346 Joined: 7-December 12 Member No.: 6780 |
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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Oct 26 2014, 04:45 PM
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#546
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Senior Member Group: Members Posts: 4246 Joined: 17-January 05 Member No.: 152 |
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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Oct 26 2014, 07:43 PM
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#547
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Senior Member Group: Members Posts: 2346 Joined: 7-December 12 Member No.: 6780 |
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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Oct 27 2014, 05:05 PM
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#548
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Solar System Cartographer Group: Members Posts: 10150 Joined: 5-April 05 From: Canada Member No.: 227 |
http://blogs.esa.int/rosetta/2014/10/27/co...tch-24-october/
New mosaic alert! Yowsers, that's jagged. Phil -------------------- ... because the Solar System ain't gonna map itself.
Also to be found posting similar content on https://mastodon.social/@PhilStooke NOTE: everything created by me which I post on UMSF is considered to be in the public domain (NOT CC, public domain) |
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Oct 28 2014, 02:06 PM
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#549
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Member Group: Members Posts: 107 Joined: 1-August 14 Member No.: 7227 |
I can't find anywhere the total amount of kilometers traveled by Rosetta till now, any help?
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Oct 28 2014, 10:52 PM
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#550
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Member Group: Members Posts: 102 Joined: 12-August 12 Member No.: 6540 |
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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Oct 31 2014, 05:57 PM
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#551
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Senior Member Group: Members Posts: 1074 Joined: 21-September 07 From: Québec, Canada Member No.: 3908 |
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Oct 31 2014, 06:15 PM
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#552
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Senior Member Group: Members Posts: 2346 Joined: 7-December 12 Member No.: 6780 |
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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Oct 31 2014, 06:25 PM
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#553
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Merciless Robot Group: Admin Posts: 8783 Joined: 8-December 05 From: Los Angeles Member No.: 602 |
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. -------------------- A few will take this knowledge and use this power of a dream realized as a force for change, an impetus for further discovery to make less ancient dreams real.
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Oct 31 2014, 06:30 PM
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#554
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Member Group: Members Posts: 813 Joined: 29-December 05 From: NE Oh, USA Member No.: 627 |
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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Nov 1 2014, 12:08 AM
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#555
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Member Group: Members Posts: 714 Joined: 3-January 08 Member No.: 3995 |
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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