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18.02.2015
AND AWAY AGAIN – COMETWATCH 15 FEBRUARY
After Rosetta's close flyby just 6 km from the surface of Comet 67P/Churyumov-Gerasimenko on 14 February, the spacecraft has once more moved away from the comet as part of its new phase of operations, reaching a distance of 255 km from the centre of 67P/C-G on 17 February.
Today’s CometWatch entry presents a single frame image obtained halfway along this latest leg, taken with Rosetta's NAVCAM on 15 February from a distance of 125 km to the centre of 67P/C-G. The 1024 x 1024 pixel image frame has a resolution of 10.6 m/pixel and measures 10.9 km across.
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NAVCAM image of Comet 67P/C-G taken on 15 February from a distance of 125 km to the comet centre. The image has been processed to bring out the details of the comet’s activity. The exposure time of the image is 4 seconds. Credits: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
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The view focuses on the large comet lobe, but a hint of the small lobe can be seen beyond it, in the distance.
The smooth Imhotep region is seen on the underside of the large lobe and partly cast in shadows. The image provides an interesting contrast to the close-up image of this region taken during the 14 February flyby . Even at this much larger distance, it is still possible to pick out the large boulder Cheops and a few of its companions casting very long shadows.
By making a careful adjustment of the intensity scaling in Lightroom, it has been possible to preserve the details seen in the bright comet, while also revealing a delicate fan of jets along the upper edge, likely emanating from the Hapi region on 67P/C-G’s neck.
After having reached 255 km from the centre of 67P/C-G on 17 February, Rosetta is now approaching the comet again. Today, 18 February, the spacecraft is about 200 km from the comet, moving towards another flyby scheduled for Wednesday 25 February. At closest approach, this will be at the much larger distance of 76 km from the surface, as part of Rosetta’s campaign to sample the material flowing away from the comet at a range of distances.
The original 1024 x 1024 pixel image frame for today's entry is provided below:
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Quelle: ESA
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Update: 22.02.2015
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IMHOTEP FROM AFAR – COMETWATCH 16 FEBRUARY
Today’s CometWatch entry presents a four-image mosaic comprising images taken from Rosetta's NAVCAM on 16 February from a distance of 226.5 km to the centre of the comet. The image resolution is 19.3 m/pixel and the individual 1024 x 1024 frames measure 19.8 km across. The mosaic measures 37.4 × 37.2 km.
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Four-image mosaic comprising images taken on 16 February 2015 by Rosetta's Navigation Camera (NAVCAM). Rotation and translation of the comet during the imaging sequence make it difficult to create an accurate mosaic, and there may be some spurious spatial and intensity features as a result of the mosaic-making process, so always refer to the individual frames before performing any detailed comparison or drawing conclusions about any strange structures or low intensity extended emission. Credits: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
The view shows the underside of the large comet lobe, focusing on the smooth Imhotep region and its transition to Ash on the right. The mosaic has been processed in Lightroom to bring out some indication of the comet's activity, in particular the jet pointing upwards.
Today's CometWatch comes from close to the end of the trajectory arc that took Rosetta 255 km from the centre of 67P/C-G on 17 February; this is the furthest the spacecraft has been from the comet since early August 2014, just before rendezvous.
A comparison of this image with CometWatch 15 February, also featuring Imhotep, and with the close-up view of the same region from the 14 February flyby, provides a taste of this leg of Rosetta's journey receding from the comet.
Rosetta is now approaching the comet again. Today, 20 February, it is around 100 km from 67P/C-G.
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On Saturday, Rosetta passed within just 6 km of the surface of Comet 67P/Churyumov-Gerasimenko in the first dedicated close flyby of the mission. The closest approach took place at 12:41 UT over the Imhotep region on the comet’s large lobe.
Four sets of NAVCAM images taken on 14 February were downlinked from the spacecraft yesterday and we are delighted to be able to share all four with you here today.
As usual, we present either a mosaic of montage of each image, noting that at the closest distances the combined effect of the rotation of the comet and the movement of the spacecraft between the first and last image being taken is particularly large, resulting in difficulties in creating an accurate mosaic. The montages have been tweaked in Lightroom for global intensity/contrast, but we have not made any local adjustment of e.g. ghosts/vignetting or other local intensity mismatches; the mosaic has been adjusted to fix boundary intensity mismatches. The individual frames are also provided here [as a zip file] to check features and to offer you the chance to create your own mosaics.
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Four image mosaic of Comet 67P/Churyumov-Gerasimenko comprising images taken on 14 February at 14:15 GMT from a distance of 8.9 km from the surface. The image scale is 0.76 m/pixel and the mosaic measures 1.35×1.37 km across. The image focuses on the stunning features of the Imhotep region, on the comet’s large lobe.
Credits: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
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The closest images, taken shortly before (right) and after (above) the point of closest approach respectively, provide stunning details of the contrasting terrains that we have seen so far on the comet. Particularly notable in the image set taken at 14:15 UT (above) is the long, layered and fractured exposed surface at the lower left.
Towards the centre of the frame the faint outline of raised near-circular objects with smooth floors can be seen. In the same image, several angular blocks appear to jut out from beneath the surface. Boulders, ranging in size from a few metres to a few tens of metres, lie scattered across the whole surface of the comet. Here, in Imhotep, we see boulder Cheops towards the top centre of the mosaic.During the flyby, the spacecraft passed through “zero phase” angle – i.e. with the Sun exactly behind the spacecraft – offering extremely good
illumination conditions. As well as providing the opportunity to take close-up high-resolution images of the comet surface, flybys like this also allow Rosetta’s instruments to sample the innermost parts of the comet’s atmosphere, or coma, to better understand the connection between the source of the observed activity and the wider coma.
Rosetta is now moving out for a far view of the comet – it will reach a distance of about 255 km from the comet centre tomorrow.
Quelle: ESA
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Update: 23.02.2015
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CLOSING IN AGAIN – COMETWATCH 18 FEBRUARY
Today’s CometWatch entry is a 1024 x 1024 pixel single frame NAVCAM image obtained on 18 February from a distance of 198 km from the centre of Comet 67P/Churyumov-Gerasimenko. The image has a resolution of 16.9 m/pixel and measures 17.3 km across.
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NAVCAM image of Comet 67P/C-G taken on 18 February from a distance of 198 km to the comet centre. The image has been processed to bring out the details of the comet’s activity. The exposure time of the image is 4 seconds. Credits: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
While the original image (provided at the end of this post) is dominated by the nucleus, increasing the exposure emphasises the nebulosity of the coma and reveals the impressive nature of the comet’s jets streaming out into space (see image above). This also highlights the “noisy” background – typically a combination of detector noise, background stars, and grains of material ejected from the comet, which are sometimes captured as streaks.
Meanwhile on the comet’s large lobe, the dark shadowed region of Aten and the seemingly flat face of Khepry to its left stand out. On the small lobe, the regions known as Bastet and Ma’at are most clearly seen, while the cliffs of Hathor are largely in shadow.
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A number of the regional boundaries defined for Comet 67P/Churyumov–Gerasimenko overlaid onto views of the comet
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Quelle: ESA
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Update: 26.02.2015
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COMETWATCH 20 FEBRUARY
Today’s CometWatch entry is a single frame NAVCAM image obtained on 20 February from a distance of 118.5 km from the centre of Comet 67P/Churyumov-Gerasimenko. The image has a resolution of 10.1 m/pixel and measures 10.3 km across.
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Single frame (1024 x 1024 pixel) NAVCAM image of Comet 67P/C-G taken on 20 February from a distance of 118.5 km to the comet centre. The image has been processed to bring out the details of the comet’s activity. The exposure time of the image is 3 seconds. Credits: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
The image nicely complements that published on Monday, this time showing the comet’s small lobe to the right and the large lobe to the left. Some 80 km closer than Monday’s image, details on the comet nucleus are now a little clearer again. For example, the smooth Imhotep region on the large lobe at the far left can be distinguished from the adjacent terrain of Ash, while on the small lobe the cliffs of Hathor stand out (refer to the regional maps here and here if you are unfamiliar with the comet's regions).
Rather captivating is the shadow of the larger lobe that is projected onto Hathor. The silhouette of the ‘underside’ of the comet can also be seen against the diffuse nebulosity to the lower-left of the nucleus. Around the sunlit portion of the nucleus, the nebulosity of the comet’s innermost coma can be seen together with denser streaks that likely correspond to regions of greater surface activity.
As usual, background features are typically a mix of detector noise, stars, and material ejected from the comet.
The original image is provided below:
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Quelle: ESA
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Update: 28.02.2015
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COMETWATCH: THE CHALLENGES OF A CLOSE FLYBY
In today’s CometWatch post, we not only present a previously unseen NAVCAM image from the close flyby on 14 February, but we also reflect on some of the challenges encountered in operating in this new environment.
The image below is a single frame (1024 x 1024 pixel) NAVCAM image taken at 16:12 UT on 14 February, from a distance of about 15.3 km from the surface. The image scale is 1.3 m/pixel and the frame measures 1.3 km across. The image was taken over the Imhotep region on Comet 67P/C-G's large lobe (see the supporting images below for context).
Compare it to the frame taken at 14:19 UT to see much of the same region at a different viewing angle, as the comet rotated and spacecraft moved during the two hours between the images.
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Single frame NAVCAM image taken at 16:12 UT on 14 February. The image has been lightly processed to remove vignetting and to increase the local contrast. Credits: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
The data acquired during Rosetta’s close flyby will provide incredible new details of the comet’s surface and near coma environment, complementing that collected earlier in the mission while on bound orbits.
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Imhotep in context. This wider field view (captured on 15 August from 91 km) shows the Imhotep region on the comet's large lobe.
Credits: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
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For the close flyby, Rosetta had to move away from the terminator (the division between day and night) and towards the sunlit side of the comet. (Check this video to get an idea of the trajectory.) This provided a good opportunity to perform unique science: the illuminated side is the most active, and the geometry was such that the spacecraft passed through the point where the Sun was directly behind Rosetta, allowing the reflection properties of the surface to be determined.
But flying closer to the surface means flying through denser regions of outflowing gas and dust, which results in the spacecraft and its large solar arrays being exposed to significantly more ‘drag’.
In general, Rosetta’s solar panels are oriented perpendicular to the Sun, to capture the most sunlight. Usually, terminator orbits are preferred, with the solar panels edge-on to the outflowing material to minimise the drag. But in this flyby geometry, the flat face of the solar panels was oriented more or less parallel to the comet’s surface and thus perpendicular to the outflowing material, maximising the drag. Imagine holding your hand under a stream of water – it’s like the difference between the relative ease at moving your hand through the stream edge on, thumb or little finger first, compared to doing it palm first.
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Annotated map of the 'underside' of the comet's large lobe, showing the Imhotep region. Rosetta passed over this region at a distance of just 6 km during the 14 February flyby. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
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However, for the 14 February event, it seems that flying this close to the comet also had another effect: the spacecraft effectively couldn’t see the usual number of “signposts” for navigation, namely stars.
The operations team report that already two hours before closest approach, the spacecraft began to lose the ability to identify enough stars to track correctly, and occasionally locked onto “false” stars – a result of the comet’s coma environment. This resulted in the spacecraft switching to some backup units (including its backup star tracker) in order to try and address the issue. However, the backup also subsequently experienced tracking problems due to the lack of stars in the field-of-view during the flyby.
“With a lot of luck, the spacecraft did not end up in ‘safe mode’,” says Sylvain Lodiot, Rosetta spacecraft operations manager. “Although in this case we could have recovered the spacecraft and resumed operations as planned, the science instruments would have automatically switched off in the meantime. By the time they had been switched back on, we would have been relatively far away from the comet again.”
“Doing science during a close flyby like this is not easy, and we are looking at ways to better prepare for future flybys so that we can maximise science while not jeopardising the safety of the spacecraft,” adds Matt Taylor, ESA’s Rosetta project scientist.
Indeed, the team are discussing plans to fly through one of the comet’s jets this summer, taking into account the lessons learned from this first close flyby.
The original 1024 x 1024 pixel frame from today’s entry is provided below:
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Quelle: ESA
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Update: 2.03.2015
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SIDEWAYS – COMETWATCH 26 FEBRUARY
This four-image mosaic comprises images taken from a distance of 94.3 km from the centre of Comet 67P/Churyumov-Gerasimenko on 26 February. The image resolution is 8.0 m/pixel and the individual 1024 x 1024 frames measure 8.2 km across. The mosaic measures 14.6 × 14.7 km.
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Four image mosaic comprising images taken on 26 February 2015 by Rosetta's Navigation Camera (NAVCAM). There may be some spurious spatial and intensity features as a result of the mosaic-making sequence, so always refer to the individual frames before performing any detailed comparison or drawing conclusions about any strange structures or low intensity extended emission. Credits: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
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Cropped and annotated version of CometWatch 26 February. Credits: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
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In this slant view, the comet's large lobe is on the right, and the small lobe on the left.
On the left, the image is dominated by the cliffs of Hathor with hints of the smooth Hapi region, both on the comet's neck. Moving towards the right, we see most of the Ash region on the large comet lobe and, on the underside, the Imhotep region with its constellation of boulders. Indication of activity is also visible in the upper part of the image.
We prepared an annotated view of today's CometWatch to help you find your way around the image. You can also find regional maps of Comet 67P/C-G here and here.
The four-image montage and the individual 1024 x 1024 frames are provided below:
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Quelle: ESA
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Update: 3.03.2015
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COMET FLYBY: OSIRIS CATCHES GLIMPSE OF ROSETTA’S SHADOW
Images from the OSIRIS scientific imaging camera taken during the close flyby on 14 February have now been downlinked to Earth, revealing the surface of Comet 67P/C-G in unprecedented detail, and including the shadow of the spacecraft encircled in a wreath of light.
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Close-up view of a 228 x 228 m region on the Imhotep region on Comet 67P/Churyumov-Gerasimenko, as seen by the OSIRIS Narrow Angle Camera during Rosetta’s flyby at 12:39 UT on 14 February 2015. The image was taken six kilometres above the comet’s surface, and the image resolution is just 11 cm/pixel. Rosetta’s fuzzy shadow, measuring approximately 20 x 50 metres, is seen at the bottom of the image.
Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
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This OSIRIS shape model is marked with the position of the narrow-angle camera field of view taken during the 14 February flyby. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
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The image released today shows an area near the edge of the comet’s “belly” close to the Imhotep-Ash regional boundary, where a mesh of steep slopes separates smooth-looking terrains from a craggier area. The image was taken from a distance of 6 km from the comet’s surface and has a resolution of 11 cm/pixel. It covers an area of 228 x 228 m.
To better identify the exact region on the comet, in the graphic below we compare the new OSIRIS narrow-angle camera image with a wider view of the comet, along with the NAVCAM image taken at 14:15 UT, noting that there are uncertainties in the distance to the surface and change in perspective between the images. Indeed, while the match on the smooth-looking region at the bottom of the NAC image in the displayed orientation is good, it is harder to match the upper half because of the lack of shadows in the NAC image, and because the geometry/viewing perspective has changed between the images. This means that the NAC image would have to be distorted and "draped" over the surface to fit the NAVCAM properly.
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The OSIRIS narrow-angle camera image from the close flyby shown here in context with a NAVCAM image. Note the region outlined in the upper left is approximate due to the change in perspective.
Credits: NAVCAM: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0;
OSIRIS: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
During the flyby, Rosetta not only passed closer by the comet than ever before, but also passed through a unique observational geometry: for a short time the Sun, spacecraft, and comet were exactly aligned. In this geometry, surface structures cast almost no shadows, and therefore the reflection properties of the surface material can be discerned.
“Images taken from this viewpoint are of high scientific value,” says OSIRIS Principal Investigator Holger Sierks from the Max Planck Institute for Solar System Research (MPS) in Germany. “This kind of view is key for the study of grain sizes.”
As a side effect of this exceptional observational geometry, Rosetta’s shadow can be seen cast on the surface of Comet 67P/C-G as a fuzzy rectangular-shaped dark spot surrounded by a bright halo-like region.
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Graphic to illustrate the difference between how a sharp shadow is generated by a point source (left) and a fuzzy shadow by a diffuse source (right). Credits: Spacecraft: ESA/ATG medialab. Comet background: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
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The shadow is fuzzy and somewhat larger than Rosetta itself, measuring approximately 20 x 50 metres. If the Sun were a point source, the shadow would be sharp and almost exactly the same size as Rosetta (approximately 2 x 32 m). However, even at 347 million km from 67P/C-G on 14 February, the Sun appeared as a disc about 0.2 degrees across (about 2.3 times smaller than on Earth), resulting in a fuzzy “penumbra” around the spacecraft’s shadow on the surface. In this scenario and with Rosetta 6 km above the surface, the penumbra effect adds roughly 20 metres to the spacecraft’s dimensions, and which is cast onto the tilted surface of the comet.
If you were standing on the surface with Rosetta high above you, there would be no place in the shadow where the entire Sun would be blocked from view, which explains why there is no fully dark core to the shadow.
Rosetta is not the first spacecraft to capture its own shadow in this way. In 2005, JAXA’s Hayabusa spacecraft captured its shadow on asteroid Itokawa. However, because Hayabusa was only a few tens of metres above the surface, the penumbral effect was much less, resulting in a sharper and darker shadow of the spacecraft.
Also, the comet surface surrounding Rosetta’s shadow on Comet 67P/C-G appears significantly brighter than the rest of the surface seen in the image. Scientists refer to this effect as the ‘opposition surge’ and it is commonly observed when highly-structured regolith surfaces on planets and moons are illuminated directly behind the observer. For example, astronauts on the lunar surface saw the effect surrounding their own shadows. The primary cause of opposition surge is ‘shadow hiding’. When the Sun is directly behind the observer, the shadows cast by small grains disappear from the perspective of the observer, hidden behind the grains themselves, leading to a pronounced increase in brightness. There may also be a contribution from coherent backscatter due to the retro-reflective properties of small dust grains.
Quelle: ESA
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Update: 5.03.2015
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ALL-ROUND ACTIVITY – COMETWATCH 25-26-27 FEBRUARY
Today’s CometWatch features not one, but four single-frame NAVCAM images taken between 25 and 27 February at distances around 80 to 100 km from the centre of Comet 67P/Churyumov-Gerasimenko. More details on the distance, size, and resolution for the individual images are provided in the captions below.
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Montage of four single-frame images of Comet 67P/C-G taken by Rosetta’s Navigation Camera (NAVCAM) at the end of February 2015. The images were taken on 25 February (top left), 26 February (top right) and on two occasions on 27 February (bottom left and right). The images have been processed to bring out the details of the comet’s activity. The exposure time for each image is 2 seconds Credits: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
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While most of Rosetta’s NAVCAM images are taken for navigation purposes, these images were obtained to provide context in support of observations performed at the same time with the Alice ultraviolet (UV) imaging spectrograph on Rosetta.
Alice makes its observations through a narrow, long slit (you can read about Alice’s first far-UV spectra and find an animation showing the size and shape of its field of view in an earlier blog post), and scientists use NAVCAM images taken at the same time as the spectra to identify where the slit was located on the comet, including which parts were pointing to an illuminated portion of nucleus, which to a region in shadow, or off the nucleus altogether. Earlier in the mission, these NAVCAM context images were taken as strips centred on the position of the Alice slit, but lately full-frame NAVCAM images have been taken in support of the Alice observations instead.
The exposure time used for the four images in today’s CometWatch post was roughly 2 seconds. Also, the phase angle was very small at the time, meaning that the Sun was illuminating the comet from behind Rosetta. This geometrical configuration combined with the relatively long exposure time has caused some parts of the nucleus to be slightly over-exposed.
The over-exposure is not a concern for Alice, as the scientists only need these images to locate the position of the slit with respect to the comet’s nucleus. However, as the images are partly saturated, we decided to process them in a way that focuses on activity in the coma rather than on the details of the nucleus.
The four images show the nucleus at different orientations, providing a good overview of the comet’s activity over the time interval between 25 and 27 February 2015.
Below we give a brief description of some of the regions visible in these four images, to help you find your way around the comet in the various orientations. You can also find regional maps of 67P/C-G here and here.
In the 25 February image, the nucleus is oriented with the small comet lobe up and the large one below. On the comet’s neck, the Anuket region is visible, while Hathor (to its left) is in shadow. Parts of the smooth Hapi region can also be seen at the bottom of the neck.
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NAVCAM image of Comet 67P/C-G taken on 25 February from a distance of 81.9 km to the comet centre. The image has a resolution of 7.0 m/pixel and measures 7.1 km across. The image has been processed to bring out the details of the comet’s activity. Credits: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
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In the 26 February image, the nucleus is oriented diagonally, with the small comet lobe to the upper left and the large lobe to the lower right. The view is dominated by the regions on the comet’s neck, namely Hathor, Anuket, and Hapi.
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NAVCAM image of Comet 67P/C-G taken on 26 February from a distance of 87.1 km to the comet centre. The image has a resolution of 7.4 m/pixel and measures 7.6 km across. The image has been processed to bring out the details of the comet’s activity. Credits: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
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The first 27 February image shows the small lobe in the foreground, offering a great view on the Hatmehit depression. A very long streak, possibly created by a particle in the foreground, is also visible in the shadow cast by the small lobe.
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NAVCAM image of Comet 67P/C-G taken on 27 February from a distance of 98.2 km to the comet centre. The image has a resolution of 8.4 m/pixel and measures 8.6 km across. The image has been processed to bring out the details of the comet’s activity. Credits: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
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The second 27 February image shows the comet with the large lobe to the right and the small one to the left. The image also provides a nice view on the cliffs of Hathor and the smooth Hapi region on the comet’s neck.
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NAVCAM image of Comet 67P/C-G taken on 27 February from a distance of 101.7 km to the comet centre. The image has a resolution of 8.7 m/pixel and measures 8.9 km across. The image has been processed to bring out the details of the comet’s activity. Credits: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
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The orientation of the last image is very similar to that of the four-image mosaic published earlier this week as CometWatch 26 February, albeit with small differences, as Imhotep (on the underside of the large lobe) is now almost completely cast in shadow. Comparing these two images reveals a very similar level of activity in the upper part of the frame, suggesting that there are no rapid changes in the outflow structure.
Quelle: ESA
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