Today's ISSI seminar with Ralf Jaumann (HRSC PI) is an absolute must-see for members of this forum. Superb citations throughout, and older hands will smile at the methane discussion after the talk

(There is a global atmospheric image at approx 50 mins. on the recording, from orbit 16472, where Ralf discusses a possible sighting of a defunct orbiter. Is this new?)

Mars Express with Ralf Jaumann

Yes, and about 53.5 minutes they violate forum rule 1.3!

They can do that; it's not their rule. But discussions here have to follow 1.3.

Very interesting.

Would the range estimate be derived from parallax, i.e. was it viewed in multiple images? Does the orbiter candidate align with the image pixel grid? Given the blocky/rectangular appearance and unfamiliar geometry I have a hard time understanding how this could be a physical object.

The blocky appearance is just the original pixels of the image, or to be more precise, it shows which pixels contain part of the object. We don't know how the histogram has been displayed. HRSC takes several images in one sequence to provide stereo and spectral data, so possibly the object is in multiple images. However, only one image in the sequence will be at the maximum resolution (presumably that must be the one we see here), so all the rest will probably just place one or two pixels on the object. From that there might be enough parallax to get the range, and from that the size of the object.

It would be interesting if they can get the location and size pinned down well enough to identify the orbiter, but the older an orbiter is the less we will know about its orbit after decades of evolution under the influence of the planet, Phobos, Jupiter and the Sun.

Phil

The image sequence in question was taken on 1 January 2017. The only sequence taken during this day. Apparently, the speck of pixels appears only in the near-infrared channel (955 nm).