The comet boulder that changed places

Khonsu · before and after — ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; cropped from ESA’s Comet changes showcase by VectorPoint
Processed image. Actual Rosetta OSIRIS photographs: 2 May 2015 at left (about 2.3 metres per pixel), 7 February 2016 at right (about 0.8 metres per pixel). Arrows mark the roughly 30-metre boulder; the dotted circle marks its former position, about 140 metres away. Dates, arrows and unequal scale bars are retained in this crop of ESA’s licensed Comet changes montage. These are two observations, not continuous footage of movement. The early ESA caption suggested movement before perihelion; Tang and colleagues’ 2024 archive reanalysis instead brackets 3 October 2015, after perihelion.
CC BY-SA 3.0 IGO — crop of the licensed Comet changes montage
· Original image source · Image licence

A roughly 30-metre boulder moved about 140 metres on comet 67P. Explore Rosetta’s actual before-and-after photographs, then follow the researchers who narrowed the missing journey to a single day.

World: Comets
Case date or period: 3 October 2015 · movement window inferred from the archive

A displaced boulder, not a moving camera trick

ESA’s paired OSIRIS views locate a roughly 30 metre boulder in two different places in Khonsu on comet 67P. The earlier image was acquired on 2 May 2015 and the later on 7 February 2016. Arrows identify the boulder; a dotted circle marks its previous location. The published displacement is about 140 metres. The photographs establish a changed position between acquisitions. They do not provide a continuous movie of the journey, its speed or the exact path taken. Their unequal ground resolutions, about 2.3 and 0.8 metres per pixel, also matter when comparing fine details.

Putting the change in its geological setting

Ramy El-Maarry’s 2017 surface-change study placed the displacement within a larger picture of weathering, erosion and cometary activity. ESA’s account distinguishes several mechanisms: heating and cooling can weaken consolidated material, sublimating ice can release gas, and changes in spin can impose mechanical stresses. The Khonsu movement was interpreted as possibly caused by erosion beneath the boulder or a sufficiently powerful outburst. Those were candidate mechanisms, not a recorded trigger. Nearby outbursts motivated the comparison, but proximity alone does not decide whether gas lifted the boulder or undermined its support.

A useful comparison is still a separate event

The same mission account describes a much smaller boulder near fractures in Anuket, moving roughly 15 metres between observations. It explicitly leaves uncertain whether that movement and the fracture extension were linked. This comparison shows why cometary changes need individual timelines and local settings. It does not turn every shifted rock into a repetition of Khonsu’s mechanism. The wider surface-change survey also found localized alterations without a major reshaping of the entire nucleus; the dramatic small scene should not be presented as the comet breaking apart.

A narrower window emerged from the archive

Tang, Shi and El-Maarry revisited the images in 2024, matching observations to a detailed shape model. They constrained movement to 3 October 2015, within about 14 hours, roughly two months after perihelion. The relevant spacecraft views were unusually distant and low resolution, so shadows and modelled illumination helped identify the boulder. This improves the earlier broad interval, but does not replace missing camera detail with a direct observation. Synthetic images in the paper simulate geometry and lighting; they must stay labelled separately from the OSIRIS exposures used for comparison.

The night-time activity was a clue, not a verdict

Shi and colleagues’ 2023 conference abstract described an archive search for the closest before-and-after pair and related dust emissions. A night-time outburst occurred during the movement interval near the new location. Their preliminary thermophysical analysis found different heating histories on the boulder’s northern and southern faces, with heat penetrating below the surface. This supplied a reason to investigate delayed thermal effects instead of assuming the immediate sunlight controlled everything. It remained a proposed relationship requiring further modelling, not a measurement of the force that moved the rock.

A mechanism remains open

The 2024 study considered direct ejection by an outburst, destabilisation by nearby activity and propulsion from the boulder’s own outgassing. It did not confirm or eliminate the previous explanations and allowed combined triggers. These alternatives connect actual displacement to testable physical processes while keeping the cause uncertain. The scientific result is a better constrained sequence and a revised thermal interpretation. A before-and-after photograph can prove relocation without proving precisely how it happened; the distinction is part of the discovery rather than a reason to discard it.