Part 77- CAESAR Mission Landing Location Target Anomaly

CAESAR (Comet Astrobiology Exploration Sample Return) is a proposed sample-return mission to comet 67P/ Churyumov-Gerasimenko. In December 2017 CAESAR was chosen as one of the two finalists for NASA’s New Frontiers program, mission 4. This meant further development of the mission concept would be funded. If accepted in the final round, it would launch in 2024 and return in 2038.

In January 2018 a presentation was given to the Small Bodies Assessment Group (SBAG) giving an overview of the CAESAR mission. SBAG reports to NASA.

Here’s a link to the presentation, entitled, CAESAR Project Overview

On page 47 of the overview there’s a close-up photo of the comet with the proposed landing site marked with a yellow circle. The area chosen is part of the region named as Ash. The yellow circle is marked on a clear OSIRIS camera photo. However, that photo is mirror-flipped. Many OSIRIS camera photos are mirror-flipped because the OSIRIS Narrow Angle Camera (NAC) and Wide Angle Camera (WAC) share a lens. A mirror is used to direct the image to one or other of the two cameras. Software is then used to flip the photo back to its true non-flipped orientation. Nevertheless, some of these photos remain in the OSIRIS archive as unflipped and several have been used inadvertently in peer-reviewed OSIRIS papers on 67P. I have pointed this out several times, both on the Rosetta blog and on Twitter.

This uncorrected mirror-flipping has the potential to cause confusion further down the line when interpreting any such image and its attendant data.

Here is the mirror-flipped photo on page 47 of the Project Overview (yellow circle on the right) along with its correct counterpart (yellow circle on the left, annotated on a similar view). The coloured dots are fiduciary points, used to show up the mirrored image. The originals are further below.The mirror-flipped CAESAR photo shows the landing site biased towards the right of a smooth area of the comet. This area has a characteristic shape defined by an obvious perimeter line of cliffs. As with all the smooth areas on 67P, it is (or should be) highly recognisable to scientists working on the comet. One should be able to see at a glance that the photo is flipped and correct it.

The mirror-flipping error seems to have resulted in another photo on the following page (page 48) of the Project Overview showing a completely different location for the proposed landing site. This photo shows a target which is supposed to be the same location as the yellow circle on page 47. However, the centre of the target is at a location that’s over a kilometre from the yellow circle, which means it’s at least a quarter of the way across the 4km-long body lobe. A possible reason for the misplacing of the target might be that the yellow circle in the page 47 photo is already biased in that direction (roughly east) by about 300 metres due to the mirror flip. However, the page 48 location is biased a further 1000 metres or so in the same direction. It’s no longer in that smooth area with a well defined perimeter and is now in an identifiably different area with different surface morphology (next to the big crater). The total distance between the (correctly flipped) yellow circle and the target centre is about 1.35km.

Here’s a close-up of the relevant area on the page 48 photo, followed by an annotated version of the same photo taken from the Rosetta Image Archive. The annotated version explains why the page 48 version shows the wrong location. Page 48 in the Project Overview linked above has the full-size photo although it’s not much bigger and not needed for context. A close-up view of the two locations together, and from a more favourable angle, will follow further down.


Second photo and its original: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA/A.COOPER

Here are some closer views. The first pair (annotated and original) is the closest and highest resolution of all photos in this post. However, the landing site circle from page 47 is slightly obscured and the smooth area is still rather foreshortened. Overhead shots are hard to come by for this area when well illuminated because Rosetta orbited along the terminator for much of the time although some overhead shots do exist. The second pair has a scale showing that the two locations are at least 1.35km apart and probably a little more due to foreshortening. The left hand photo of the second pair is left unannotated and gives a slightly different view:

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



It goes without question that two of the people who would be most enthusiastic about returning to 67P and continuing the data gathering would be myself and Marco Parigi. After all, we lobbied hard to put the Rosetta orbiter into hibernation through to 2020 and in a quasi orbit (inclined heliocentric orbit) at a few thousand km distant from 67P to keep it safe. It would then have returned to 67P naturally at the opposite heliocentric orbit node and at the ideal wake-up time with an almost zero delta V budget. See Part 59 of this blog:

So Marco and I have high hopes for CAESAR. But this landing site location anomaly and mirror-flipping a photo without realising it makes for a disappointing start. The CAESAR team needs to get really familiar with the regions and the morphology- and preferably do so using the actual photos. I would recommend avoiding use of the shape model as a tool of first resort. In our experience, the photos, when scrutinised in their hundreds and even thousands, give a knowledge of the morphology that is second to none. This resulted in our bringing attention to regional border errors in a regions paper published by the OSIRIS team. An erratum was published as a result.

Here’s Marco’s latest blog post (published the same day as this blog post) explaining the movement of the Anuket boulder:


The stretch blog has been on a hiatus but is far from finished. The northern hemisphere layer delaminations and slides are now fully understood but only half are documented in the 77 parts published thus far. The rest are in note form and will be published slowly in due course. Regarding the southern hemisphere most of that is understood too but with only a low-resolution understanding of Geb and part of the neck. This is mainly owing to the lack of high res photos for these areas. However, since all layer slides, right down to the 50-metre scale were found via the predictive process of establishing the tensile force vectors of stretch, one can piece together the Geb narrative via the slides in the adjacent regions of Bes and Sobek.