67P/Churyumov-Gerasimenko. A Single Body That’s Been Stretched- Part 16.


This is an ESA mosaic showing a top-down view of 67P (from exactly above the head). It confirms the 22 matches made in Parts 1 and 2 of this series. Yellow dots- head rim; terracotta dots- shear line. Please refer to the appendix to this post for the original component photos of the mosaic and consequent assurance regarding the correct interpretation of one small anomaly in the stitched version.


This was going to be a single post before resuming with rock D in Part 17. But this post eventually became long enough to split into three: another part on simple matches and a further part which will be a hypothesis on the nature and formation of the wavy line from Parts 1 and 2. So Part 17 will now be a continuation of the simple matches theme, Part 18 will be rock D, and somewhere past Part 20, the wavy line will get another airing.


In Parts 14 and 15 we looked at giant monoliths that had floated across the surface of 67P and we’ll look at another one, rock D, in Part 18. In the meantime, it seemed appropriate to have a rest from rotation planes and spin rates and return to the basic evidence that suggests 67P is a stretched comet. This is because a more recent NAVCAM photo (Cometwatch 21st January; ESA Rosetta blog) has confirmed the matches found in Parts 1 and 2. Those matches were between the underside of the head lobe rim and the body lobe ‘shear line’. The shear line is the line along which the head sheared from the body. The 22 separate matches followed the twists and turns of a wavy line along the head rim (sometimes a little way in from it on the Hathor cliff) that was replicated along the shear line (similarly, sometimes a little way in). That is to say, 22 turning points on the head wavy line matched 22 similar turning points on the body wavy line, one set being the mirror image of the other. Their set-back from the head rim therefore allowed us to plot the position of the actual shear line on the body. This was especially accurate for the well-documented rectangle (Parts 5 and 7) which has an elongated slurry pile running to some 400 metres along its back perimeter that corresponds to the frilly edge of the head rim above it. The frilly edge was likely pushed up by the escaping slurry and gases. 

To be quite clear, the matching lines in the header photo aren’t the result of some sudden, breathless “Eureka” moment after hours of trawling the comet photos for some match, any match. The dotted lines show the circa 1-kilometre section of matching head rim and shear line, whose 30+ matches were described in minute detail in Parts 1-7 of this series, to the point where I could quite forgive any detractors for saying, “please, just put a lid on it and just show us the match in one incontrovertible photo!” Well, this is that photo.

Another property of the dotted lines in the photo is that as well as being the same shape they obey the law of perspective. The photo was taken at a distance of 27.9 km from the comet centre. This puts the terracotta line at a distance of about 27km and the yellow line at a distance of 25.5-26km. That makes the yellow line roughly 94-96% as distant as the terracotta line and so the terracotta line ought to be 94-96% of the yellow line’s length. It appears to be 92.5-94.5% with about a 2% error margin for measurement error. So if the head was pushed back down onto the body, the two lines would match in length as well as shape. 

And finally, the yellow and terracotta lines in the header photo betray exactly the same 15 degrees of anticlockwise head rotation as the only other top-down match to date, shown in Part 3. Although this is an afterthought, tacked onto the end of a post, it’s another stunning piece of evidence against the argument that two disparate bodies drifted together at random to form a contact binary. The chance of this angle being replicated exactly in both of the two top-down matches to date is 1 in 180 at the very least (assuming a conservative, 2-degree error in the measurement of the angle).

Here are both photos, reproduced with the angle of rotation drawn in red dots (zoom required). Apologies for the sloppy estimation of 5-10 degrees in past posts- this time it was measured with a protractor to within a degree or so.


The header photo should not be regarded in isolation. Although the match is near-perfect, it is just confirmation of a wealth of evidence catalogued in Parts 1-7 and should come as no surprise at all to those who have read those parts. Its appeal is in the fact that the 1-kilometre match is captured in one photo but the evidence in Parts 1-7 is, frankly, even more compelling.

Copyright ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0


The left-hand portion of the terracotta line, after the sharp turn, appears to run through a non-descript, fuzzy area. This is because it is where the mosaic of the four NAVCAM photos was stitched together. A more faithful representation of this small portion was cut from the bottom left of the four photos. This was presumably done by ESA in favour of melding the top right and bottom right photos in the mosaic.

The path of the terracotta line through the fuzzy area faithfully traces the path of a ridge that is clearly apparent in the discarded bottom-left portion from the original set. This ridge comprises the end of the famed rectangle. It’s a very small section which has been specifically referred to on numerous occasions, most importantly as being the site of the 3D-matched ‘gull wings’ described in Part 5. Here is the original ESA NAVCAM  set of four from Cometwatch 21st January:


One thought on “67P/Churyumov-Gerasimenko. A Single Body That’s Been Stretched- Part 16.

  1. That first image is a classic case of “Look! An obvious distraction” when I first saw it on the Rosetta Blog I was mostly curious about the visible outgassing visible in the shadows of one of the cliffs. The fact that it was a fabulous angle to see both sides of the matching Shear line, was lost on me. As usual, I was disoriented, and I don’t have names in my head for the individual features. The more angles I see these matches from, the less this will be a problem.


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