Scientists find a surprise final image from comet-chaser Rosetta

A year after Europe’s Rosetta space probe crashed onto a comet, an unexpected, final, close-up image from the craft has been discovered by mission scientists.

An artist’s impression of Rosetta shortly before hitting Comet 67P/Churyumov–Gerasimenko on 30 September 2016. Image credit: ESA/ATG medialab

On 30 September 2016, almost a year ago, Rosetta touched down on Comet 67P/Churyumov-Gerasimenko and stopped transmitting. After more than 12 years in space and 2 years following the comet, it had shown us that 67P is far more complex than the dirty ice balls comets were once thought to be.

Rosetta beamed a wealth of data back to Earth, which researchers will be analysing for years to come – and now it has sent back one final photograph.

The Rosetta mission didn’t send complete images to Earth – it split them into packets of data. The final images were split into six packages each, making up 23048 bytes per image. This final image is not complete – only three telemetry packages were received.

This final image from Rosetta, shortly before it made a controlled impact onto Comet 67P/Churyumov–Gerasimenko on 30 September 2016, was reconstructed from residual telemetry. The image has a scale of 2 mm/pixel and measures 1 m across. Image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

With only 12228 bytes received in total, or just over half of a complete image, automatic processing software didn’t recognise it as a photograph. Happily, engineers at the Max Planck Institute for Solar System Research in Göttingen, Germany, realised that they could reconstruct an image from the data received.

To compress the image, the data were sent in layers of increasing detail rather than being transmitted pixel-by-pixel. Because only around 53% of a complete image made it back to Earth, some of the finer details were lost; as such, when zooming in, the image looks blurrier than a higher-quality, complete photograph.

An annotated image indicating the approximate locations of some of Rosetta’s final images. Top left is a global view of Comet 67P/Churyumov–Gerasimenko showing the Ma’at region landing zone on the smaller of the two comet lobes. Top right is an image taken by the OSIRIS narrow-angle camera from an altitude of 5.7 km, during Rosetta’s descent. The final touchdown point, named Sais, is seen in the bottom right of the image. Middle is an OSIRIS wide-angle camera image taken from an altitude of about 331 metres during Rosetta’s descent. It shows a mix of coarse and fine-grained material. Bottom right is the penultimate image, which was the last complete image taken and returned by Rosetta from an altitude of 24.7±1.5 metres. Bottom left is the final image, reconstructed after Rosetta’s landing, was taken at an altitude of 19.5±1.5 m. The image has a scale of 2 mm/pixel and measures about 1 m across. Image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

That’s impressive considering that the camera wasn’t designed to photograph 67P this close up. Ordinarily, it wouldn’t have operated much less than a few hundred metres above the comet’s surface. The camera was specially reconfigured to be able to capture sharper images – by removing its colour filter, photographs taken above 300m would be blurred. However, images would come into focus around 15m above the surface.

The previously published last image was probably taken around 23.3-26.2m above the ground (the uncertainty comes from models of the comet’s shape and from the precise details of how the altitude was calculated). The final image Rosetta sent back was probably taken 17.9-21.0m above 67P and is correspondingly slightly more in focus.

Although the mission is long over, Rosetta has left a lasting legacy. This last image is only one piece out of the many, many pieces of data sent back to Earth, but for the many people who sent heartfelt messages, made their own paper models and even crafted jewellery, it is a fitting not-quite-anniversary reminder of just what inspired them to care.

Related: We talk to Rosetta’s lead scientist on eve of crash landing

Related: Rosetta ends mission reunited with Philae on comet


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