posted 09-28-2017 09:57 AM               

Unexpected surprise: a final image from Rosetta

Scientists analysing the final telemetry sent by Rosetta immediately before it shut down on the surface of the comet last year have reconstructed one last image of its touchdown site.

Above: A 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. (ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)

After more than 12 years in space, and two years following Comet 67P/Churyumov–Gerasimenko as they orbited the Sun, Rosetta's historic mission concluded on 30 September with the spacecraft descending onto the comet in a region hosting several ancient pits.

It returned a wealth of detailed images and scientific data on the comet's gas, dust and plasma as it drew closer to the surface.

But there was one last surprise in store for the camera team, who managed to reconstruct the final telemetry packets into a sharp image.

"The last complete image transmitted from Rosetta was the final one that we saw arriving back on Earth in one piece moments before the touchdown at Sais," says Holger Sierks, principal investigator for the OSIRIS camera at the Max Planck Institute for Solar System Research in Göttingen, Germany.

"Later, we found a few telemetry packets on our server and thought, wow, that could be another image."

Above: In order to give a feeling of scale, this artist impression of the Rosetta spacecraft is superimposed on an OSIRIS wide-angle camera image of the region in which it landed on 30 September 2016. Also marked on the image are the approximate locations of the final two images taken by the spacecraft from around 20 m altitude. The cross indicates the estimated centre of touchdown of Rosetta. (Image: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; spacecraft: ESA/ATG medialab)

During operations, images were split into telemetry packets aboard Rosetta before they were transmitted to Earth. In the case of the last images taken before touchdown, the image data, corresponding to 23 048 bytes per image, were split into six packets.

For the very last image the transmission was interrupted after three full packets were received, with 12 228 bytes received in total, or just over half of a complete image. This was not recognised as an image by the automatic processing software, but the engineers in Göttingen could make sense of these data fragments to reconstruct the image.

Owing to the onboard compression software, the data were not sent pixel-by-pixel but rather layer-by-layer, which gives an increasing level of detail with each layer. The 53% of transmitted data therefore represents an image with an effective compression ratio of 1:38 compared to the anticipated compression ratio of 1:20, meaning some of the finer detail was lost.

That is, it gets a lot blurrier as you zoom in compared with a full-quality image. This can be likened to compressing an image to send via email, versus an uncompressed version that you would print out and hang on your wall.

Above: Annotated image indicating the approximate locations of some of Rosetta's final images. Note that due to differences in timing and viewing geometry between consecutive images in this graphic, the illumination and shadows vary. (ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)

The camera was not designed to be used below a few hundred metres from the surface but a sharper image could be achieved using the camera in a special configuration: while the camera was designed to be operated with a colour filter in the optical beam, this was removed for the last images. This would have resulted in the images being blurred for the normal imaging scenario above 300 m, but they came into focus at a 'sweet spot' of 15 m distance.

Approaching 15 m therefore improved the focus and thus level of detail, as can be seen in the reconstructed image taken from an altitude of 17.9–21.0 m and corresponding to a 1 x 1 m square region on the surface.

In the meantime, the altitude of the previously published last image has been revised to 23.3–26.2 m. The uncertainty arises from the exact method of altitude calculation and the comet shape model used.

The sequence of images progressively reveals more and more detail of the boulder-strewn surface, providing a lasting impression of Rosetta's touchdown site.

posted 10-28-2020 11:20 AM               

Philae's second touchdown site discovered at 'skull-top' ridge

After years of detective work, the second touchdown site of Rosetta's Philae lander has been located on Comet 67P/Churyumov-Gerasimenko in a site that resembles the shape of a skull. Philae left its imprint in billions-of-years-old ice, revealing that the comet's icy interior is softer than cappuccino froth.

Above: Rosetta's Philae lander touched down on Comet 67P/Churyumov-Gerasimenko on 12 November 2014 and made multiple contacts with the surface before arriving at its final resting place. The comet topography at Philae's second touchdown site resembles the shape of a skull with a pointed 'hat' when viewed from above. This gif shows the feature that resembles a skull face, with Philae superimposed for scale (Philae's 'body' measures about 1 m across, and each leg is 1.5 m long). Philae's body compressed into the ice-dust scenery to create the skull's right eye. The dark region just above the skull's right eye is the entrance to a gap between the two boulders nicknamed 'skull-top crevice', where Philae acted like a windmill to pass between them.

Detective story

Philae descended to the surface of the comet on 12 November 2014. It rebounded from its initial touchdown site at Agilkia and embarked on a two-hour flight, during which it collided with a cliff edge and tumbled towards a second touchdown location. Philae eventually came to a halt at Abydos, in a sheltered spot that was only identified in Rosetta imagery 22 months later, a few weeks before the conclusion of the Rosetta mission.

ESA's Laurence O'Rourke, who played the leading role in finding Philae in the first instance, was also determined to locate the previously undiscovered second touchdown site.

"Philae had left us with one final mystery waiting to be solved," says Laurence. "It was important to find the touchdown site because sensors on Philae indicated that it had dug into the surface, most likely exposing the primitive ice hidden underneath, which would give us invaluable access to billions-of-years-old ice."

Together with a team of mission scientists and engineers, he set about pulling together data from both Rosetta and Philae instruments to find and confirm the 'missing' touchdown site.

Above: This graphic presents the data collected by Philae's ROMAP instrument – a magnetometer boom – during the time of the second touchdown on Comet 67P/Churyumov-Gerasimenko on 12 November 2014, matched with imagery showing evidence of the key moments of Philae's interaction with the surface.

The star of the show

Although a bright patch of 'sliced ice' observed in high-resolution images from Rosetta's OSIRIS camera proved crucial in confirming the location, it was Philae's magnetometer boom, ROMAP, that turned out to be the star of the show. The instrument was designed to make magnetic field measurements in the comet's local environment, but for the new analysis the team looked at changes recorded in the data that arose when the boom – which sticks out 48 cm from the lander – physically moved as it struck a surface. This created a characteristic set of spikes in the magnetic data as the boom moved relative to the lander body, which provided an estimate of the duration of Philae stamping into the ice. The data could also be used to constrain the acceleration of Philae during these contacts.

ROMAP's data were cross-correlated with those collected by Rosetta's RPC magnetometer at the same time to determine Philae's attitude and exclude any influence from the background magnetic field of the plasma environment around the comet.

"We weren't able to make all the measurements we planned in 2014 with Philae, so it is really amazing to use the magnetometer like this, and to combine data from both Rosetta and Philae in a way that was never intended, to give us these wonderful results," says Philip Heinisch, who led the analysis of the ROMAP data.

A reanalysis of the touchdown data found that Philae had spent nearly two full minutes at the second touchdown site, making at least four distinct surface contacts as it ploughed across it. One particularly notable imprint revealed in the images was created as Philae's top surface sank 25 cm into the ice on the side of a crevice, leaving identifiable marks of its drill tower and sides. The spikes in the magnetic field data arising from the boom movement showed that it took Philae three seconds to make this particular depression.

Skull face

"The shape of the boulders impacted by Philae reminded me of a skull when viewed from above, so I decided to nickname the region 'skull-top ridge' and to continue that theme for other features observed," says Laurence.

Above: This animated gif focuses on the second touchdown site, which is characterised by a bright patch of exposed water-ice covering an area of about 3.5 square metres. Although the ice was mostly in shadow at the time of the landing, the Sun was directly illuminating the area when these images were taken 22 months later, lighting it up like a beacon to stand out against everything around it. In this animated gif, every second image has its brightness/contrast fully reduced to show how the ice in the crevice is brighter than all of the surrounding regions.

"The right 'eye' of the 'skull face' was made by Philae's top surface compressing the dust while the gap between the boulders is 'skull-top crevice', where Philae acted like a windmill to pass between them."

Analysis of images and data from OSIRIS and Rosetta's spectrometer VIRTIS confirmed that the bright exposure was water-ice covering an area of about 3.5 square metres. Although the ice was mostly in shadow at the time of the landing, the Sun was directly illuminating the area when the images were taken months later, lighting it up like a beacon to stand out against everything around it. The ice was brighter than the surrounds because it had not been previously exposed to the space environment and undergone space weathering.

"It was a light shining in the darkness," says Laurence, noting that it was located just 30 m away from where Philae was finally imaged on the comet surface.

Cappuccino froth

While an exciting conclusion in the search for the second touchdown site, the study also provides the first in situ measurement of the softness of the icy-dust interior of a boulder on a comet.

"The simple action of Philae stamping into the side of the crevice allowed us to work out that this ancient, billions-of-years-old, icy-dust mixture is extraordinarily soft – fluffier than froth on a cappuccino, or the foam found in a bubble bath or on top of waves at the seashore," adds Laurence.

The study also allowed an estimate of the boulder's porosity – how much empty space exists between the ice-dust grains inside the boulder – of about 75%, which is in line with the value measured previously for the whole comet in a separate study. The same study showed that the comet is homogeneous anywhere in its interior on all size scales down to about one metre. This implies that the boulders represent the overall state of the comet's interior when it formed some 4.5 billion years ago.

"This is a fantastic multi-instrument result that not only fills in the gaps in the story of Philae's bouncy journey, but also informs us about the nature of the comet," says Matt Taylor, ESA's Rosetta project scientist. "In particular, understanding the strength of a comet is critical for future lander missions. That the comet has such a fluffy interior is really valuable information in terms of how to design the landing mechanisms, and also for the mechanical processes that might be needed to retrieve samples."

Above: This graphic summarises the main touchdown sites. At 15:35 GMT Philae made first contact with the surface at Agilkia – the image shown here was taken by Philae's own camera, ROLIS, before touchdown, approximately 40 metres from the surface. Philae then took flight across the Hatmehit depression on the 'top' of the small comet lobe, colliding with a cliff edge at 16:20 GMT. This set it on course with the second touchdown site, where it interacted with the surface multiple times over a period of two minutes starting at around 17:24 GMT. Philae arrived at its final resting place at Abydos, about 30 m away, at 17:31 GMT. The image has been enhanced to allow Philae, hiding in the shadows just 30 metres away from the second touchdown location, to be seen.