GO FOR 10 KM!
With the last week spent at about 20 km from 67P/C-G, the decision has now been made to go to just 10 km.
A series of manoeuvres will reduce Rosetta’s distance from its current 18.6 km orbit (taking 7 days) to an intermediate orbit approximately 18.6 x 9.8 km (with a period of about 5 days). From there the orbit will be circularised at about 9.8 km radius, with a period of approximately 66 hours on 15 October, and the mission will enter the “Close Observation Phase” (COP). This will provide even higher resolution images of the landing site in order to best prepare for Philae's challenging touch-down.
The new orbit will also allow a number of Rosetta's science instruments to collect dust and measure the composition of gases closer to the nucleus.
Rosetta will stay in the 10 km COP until 28 October, when it will begin the transfer to a “pre-delivery” orbit, a slightly elliptical orbit at approximately 30 km distance from the comet centre. After another orbital change taking place about two hours before separation, the spacecraft will finally deploy Philae from a distance of 22.5 km from the comet centre, at 08:35 UT on 12 November.
A detailed timeline of spacecraft events around the landing will be provided soon.
COMETWATCH – FOCUS ON THE ‘NECK’
This four-image NAVCAM montage, comprising images taken on 2 October from a distance of 19 km from the centre of 67P/C-G, provides a stunning view onto the ‘neck’ region of the comet. The image scale is 1.45 metres/pixel, so each 1024 x 1024 pixel frame is about 1.5 km square.
Four image montage of NAVCAM images taken on 2 October. Credits: ESA/Rosetta/NAVCAM
As usual, the four individual images making up the montage are provided at the end of this post. It’s interesting to see how the perspective has shifted and some of the features have changed in appearance as a result of the comet rotation and spacecraft movement in the time between the first (bottom left) and last (bottom right) image being taken. For example, look at the change in the length of shadows of features close to the ‘join’ in the bottom two images. So, a word of caution: while it is possible to make a seamless mosaic using software like Microsoft ICE, a careful check against the originals will very likely show areas that are incorrect. Note also that the diffuse ‘blob’ mentioned in Monday’s post and probably due to internal scattering in the NAVCAM optics, is visible again in the top two images in today’s post. Since the blob appears to blend into the jets seen in the top right frame, take care when cranking up the contrast!
You might also like to compare these images with those of the same region seen from a distance of about 28.5 km on 24 September.
Focusing on the lower right frame of this montage, you may spot some interesting features, in addition to the numerous boulders. For example, in the lower centre portion of the image, there appear to be exposed surfaces devoid of the smoother material that dominates the neck region (the exposed surfaces appear as brighter features with these image contrast settings). Higher-resolution images will hopefully tell us how thick this dusty covering is in this location, at least, and thus perhaps provide information about the rates of erosion.
Snaking through the centre of the frame, close to the shadowed portion on the right, is another curious feature. In some places it looks like a series of small pits; in others, they appear to merge into longer trough-like features – you can just about make out some small pits in the 24 September image. It will be interesting to get a closer view of these features, to see how they fit into the overall picture of the evolution of the comet.
As the comprehensive survey of 67P/C-G continues, efforts will also be made to learn more about the origin of the boulders and, of course, their composition. Are they relics from the comet’s interior, exposed by an erosive process that has removed material from around them; are they products of erosion from nearby cliffs; or were they exhumed by jet activity?
The scientific imaging system OSIRIS on board ESA’s spacecraft Rosetta has caught a spectacular glimpse of one of the many boulders that cover the surface of comet 67P/Churyumov-Gerasimenko.
This image of the surface of Comet 67P/Churyumov-Gerasimenko was taken by Rosetta’s OSIRIS narrow-angle camera on 19 September 2014, from a distance of 28.5 km.
The image features a large boulder casting a long shadow on the surface of the comet. The boulder has a maximum dimension of about 45 metres and is the largest structure within a group of boulders located on the lower side of the comet’s larger lobe. This cluster of boulders reminded scientists of the famous pyramids at Giza near Cairo in Egypt, and thus it has been named Cheops for the largest of those pyramids, the Great Pyramid, which was built as a tomb for the pharaoh Cheops (also known as Kheops or Khufu) around 2550 BC.
Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
With a maximum dimension of approximately 45 metres, it is one of the larger boulders seen on the comet. It stands out among a group of boulders in the smooth region located on the lower side of 67P/C-G’s larger lobe.
This cluster of boulders reminded scientists of the famous pyramids at Giza near Cairo in Egypt, and thus it has been named Cheops for the largest of those pyramids, the Great Pyramid, which was built as a tomb for the pharaoh Cheops (also known as Kheops or Khufu) around 2550 BC.
This choice also introduces a more general Egyptian naming scheme agreed by Rosetta scientists that will be used for many of the features on 67P/C-G, in keeping with the spirit of the overall mission and spacecraft names.
Cheops was seen for the first time in images obtained in early August upon Rosetta’s arrival at the comet (see below – can you spot it?). In the past few weeks, as Rosetta has navigated closer and closer to the comet, OSIRIS imaged the unique structure again – but this time with a much higher resolution of 50 centimetres per pixel.
The lower side of 67P/C-G’s larger lobe. The image was presented on the occasion of arrival on 6 Aug; it was taken from a distance of 130 km and the image resolution is 2.4 metres per pixel. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
The boulder-like structures that Rosetta has revealed in many places on the surface of 67P/C-G are one of the comet’s most striking and mysterious features (see yesterday’s CometWatch for a focus on the boulders on the comet’s ‘neck’ region).
Just like the many other boulders seen by both OSIRIS and the spacecraft’s NAVCAM, Cheops stands out not just physically, but also as a slightly brighter feature compared to the darker surface around it.
OSIRIS Principal Investigator Holger Sierks, from the Max Planck Institute for Solar System Research (MPS) in Germany, describes the surface of Cheops as “very craggy and irregular.”
Interspersed between the lighter lumps on the boulder’s surface are intriguing small patches of darker material, similar in brightness and texture to the ground upon which the boulder lies.
“It almost looks as if loose dust covering the surface of the comet has settled in the boulder’s cracks. But, of course, it is much too early to be sure,” says Sierks.
Apart from their size distribution, which is being measured through careful analysis of the images, almost all other properties of 67P/C-G’s boulders are still a mystery to researchers. What are they made of? What are their physical properties, including density and stability? How were they created? As Rosetta continues to survey and monitor the comet’s surface in the next months, the scientists will be looking for clues.
“For example, if the boulders are exposed by cometary activity or are displaced following the comet’s gravity field, we should be able to track this down in our images,” adds Sierks.
Tomorrow, Rosetta will begin its Close Observation Phase reaching a distance of only 10 kilometres from the comet’s surface, giving the cameras an even closer view of these features.
Spectacular Jets Erupting From Rosetta’s Active Comet
Jets are blasting from the active neck of comet 67P/Churyumov-Gerasimenko in this four-image photo mosaic comprising images taken on 26 September 2014 by the European Space Agency’s Rosetta spacecraft at a distance of 26.3 kilometers (16 miles) from the center of the comet. See the montage ot four individual navcam images below. Credit: ESA/Rosetta/NAVCAM/Marco Di Lorenzo/Ken Kremer
A spectacle of erupting jets are blasting away from the clearly active neck of comet 67P/Churyumov-Gerasimenko in stunning new imagery captured by Europe’s Rosetta spacecraft as it swoops in ever closer to this bizarre remnant from the formation of our Solar System.
COMETWATCH CHECKS UP ON CHEOPS
This four-image NAVCAM montage comprises images taken on 8 October from a distance of 16.9 km from the centre of comet 67P/C-G, so roughly 15 km from the surface.
Comet 67P/C-G on 8 October, from a distance of 16.9 km from the centre of the comet. Credits: ESA/Rosetta/NAVCAM
The view covers the bottom and side of the larger lobe of the comet, and highlights the large, smooth region that is home to a number of large boulders, including ‘Cheops’, as seen in yesterday’s close-up view from OSIRIS.
The NAVCAM image scale at this distance is about 1.25 metres/pixel, so each 1024 x 1024 pixel frame is nearly 1.3 km square (the four full-resolution individual images making up the montage are provided at the end of this post). The montage has been rotated 90 degrees clockwise to place the region containing Cheops at the top for emphasis; the four individual frames are in the original orientation.
When seen in this side-on view, Cheops appears much more like its Egyptian pyramid namesake and has a height of approximately 25 metres, compared with a width of 45 metres as seen from above in yesterday’s OSIRIS image. For reference, the real Pyramid of Cheops at Giza is 139 metres high and 230 metres across at the base.
Be sure to let your eye trace around the profile of the comet, too, and take in some great new views of dramatic cliffs and ‘spires’.
MISSION SELFIE FROM 16 KM
If you thought last month’s mission ‘selfie’ from a distance of 50 km from Comet 67P/C-G was impressive, then prepare to be wowed some more: this one was taken from less than half that distance, at just 18 km from the centre of the comet, or about 16 km from the surface.
Rosetta mission selfie a distance of about 16 km from the surface of 67P/C-G. Credits: ESA/Rosetta/Philae/CIVA
This latest image was taken by the CIVA imaging system on board Rosetta’s lander Philae, on 7 October. It captures the side of the Rosetta spacecraft and one of its 14 metre-long solar arrays, with 67P/C-G in the background. Not only does the comet appear much larger than in last month's image, the active ‘neck’ region of 67P/C-G is now clearly visible, with streams of dust and gas extending away from the comet. The primary landing site, currently known as Site J, can also be seen on the smaller lobe of the comet.
Two images, one with a short exposure time, one with a longer one, were combined to capture the whole dynamic range of the scene, from the bright parts of the solar arrays to the dark comet and the dark insulation cladding the Rosetta spacecraft.
CIVA, the Comet Infrared and Visible Analyser, is one of ten instruments on-board Philae. The CIVA-P part of the instrument comprises seven micro-cameras arranged around the top of the lander to take panorama images, while CIVA-M is a visible/infrared microscope imager/spectrometer that will the study the composition, texture, and albedo of surface samples.
The 7 October selfie is the last image from Philae before the lander separates from Rosetta on 12 November. The next image will be taken by CIVA shortly after separation, when the lander will look back at the orbiter to bid it a final farewell. While the lander’s ROLIS instrument will take images during the descent phase, CIVA will be tasked with making a 360 degree panoramic image of the landing site, including a section in stereo, once safely on the surface of 67P/C-G. The images and other data collected by Philae will provide important in situ information about this particular region on the comet, providing ‘ground truth’ data that can be used to complement the data collected for the whole comet from the Rosetta orbiter now and into 2015, as the comet becomes more active.
Final confirmation of the landing site and its landing scenario is under discussion today at ESA’s Lander Operations Readiness Review, being held at ESOC in Darmstadt. A formal announcement of the outcome will be made tomorrow, 15 October.
Landeplatz für Lander Philae bestätigt
Nun steht es fest: Nach weiteren detaillierten Untersuchungen ist der im September ausgewählte Landeplatz J auf dem Kometen Churyumov-Gerasimenko bestätigt worden. Das Team im Lander-Kontrollraum des Deutschen Zentrums für Luft- und Raumfahrt (DLR) kann sich somit definitiv auf die Landung auf dem Kopf des entenförmigen Kometen vorbereiten. „Der Landeplatz hat ausreichend Sonne und relativ flaches Gelände“, sagt Lander-Projektleiter Dr. Stephan Ulamec vom DLR. Einfach wird die Aufgabe dennoch nicht – bisher kennen die Wissenschaftler beispielsweise nicht die Bodenbeschaffenheit. „Die Landung ist eine große Herausforderung.“ Am 12. November 2014 um 9.35 Uhr soll der Lander von der Rosetta-Sonde getrennt werden und rund sieben Stunden später auf dem Kometen aufsetzen. Gesteuert und überwacht wird die erste Landung überhaupt auf einem Kometen aus dem Lander Control Center des DLR in Köln.
Mit der Mission, die am 2. März 2004 startete, wollen die Kometenforscher einen Blick in die Entstehungszeit unseres Sonnensystems werfen. Zum ersten Mal haben sie dabei die Möglichkeit, das ursprüngliche Material eines Kometen nicht nur bei einem weit entfernten Vorbeiflug zu untersuchen, sondern aus dem Orbit und sogar direkt vor Ort auf der Kometenoberfläche. Die Rosetta-Sonde selbst ist zurzeit nur noch zehn Kilometer von Churyumov-Gerasimenko entfernt. Sobald Philae auf dem Kometen gelandet ist, werden zehn wissenschaftliche Instrumente die ersten Daten von einer Kometenoberfläche liefern. Drei Instrumente werden dabei federführend vom DLR-Institut für Planetenforschung durchgeführt: Die Sonde MUPUS hämmert sich bis zu 40 Zentimeter tief in den Boden, um beispielsweise die Wärmeleitfähigkeit zu messen, SESAME sendet und horcht auf akustische und elektrische Signale, und die Kamera ROLIS nimmt bereits während des Abstiegs in Richtung Komet die ersten Bilder auf.
Doch zunächst muss am 12. November 2014 die anspruchsvolle Landung gelingen. Noch am Tag zuvor sowie in der Nacht zum 12. November 2014 werden mehrfach Entscheidungen getroffen, ob die Landung ausgelöst werden soll. Rund zwei Stunden vor der Separation des Landers wird die Rosetta-Sonde auf die entsprechende Flugbahn gesteuert. Rund 22,5 Kilometer über der Kometenoberfläche beginnt dann nach der Separation der Flug von Philae, der über eine vorab im DLR programmierte Computer-Sequenz autonom verlaufen wird. 509 500 000 Kilometer werden Rosetta und Philae dann von der Erde entfernt sein, und ein Signal wird über 28 Minuten aus dem All bis zum Boden benötigen. Gegen 17 Uhr erwartet das Team im Lander-Kontrollraum dann die Information, dass Lander Philae sicher auf dem Kometen aufgesetzt hat. Etwa eine Stunde nach der Landung wird das DLR-Lander-Kontrollzentrum dann mit der ersten wissenschaftlichen Phase beginnen, bei der alle Instrumente auf dem Lander in Betrieb genommen werden.
Rosetta ist eine Mission der ESA mit Beiträgen von ihren Mitgliedsstaaten und der NASA. Rosettas Lander Philae wird von einem Konsortium unter der Leitung von DLR, MPS, CNES und ASI beigesteuert.
NAVCAM’S SHADES OF GREY
Ever since early August, when ROSETTA rendezvoused with Comet 67P/Churyumov-Gerasimenko at a distance of roughly 100 km, the on-board navigation camera (NAVCAM) has been returning images that depict the many different facets of its nucleus. A complex surface emerges from these images, revealing valleys, cliffs, boulders, and craters all over the comet.
NAVCAM takes black-and-white images and the surface of 67P/C-G shows a wide dynamic range of light and dark regions, depending on the illumination conditions and surface characteristics at any given area. But what do “light” and “dark” mean for an object like 67P/C-G? Followers of this blog have asked this and similar questions, so here are some details on how NAVCAM images are taken and DISPLAYED to make a wide range of surface features possible.
Four-image NAVCAM mosaic of Comet 67P/Churyumov-Gerasimenko, using images taken on 24 September 2014 when Rosetta was 28.5 km from the comet. On the left, the contrast was enhanced by setting the darkest pixels as black and the brightest ones as white; on the right, the intensities were scaled so that the mean brightness of fully illuminated regions of the comet is around 4%. CREDIT: ESA/Rosetta/NAVCAM
Let’s start with the light available to take pictures by. At present, 67P/C-G and ROSETTA are out beyond the orbit of Mars and the Sun is roughly only 10% as bright as they would see if they were in orbit around the Earth. In addition, the surface of comets can be very dark, reflecting less than 10% of the light that falls on them – something that has been known since ESA’s Giotto flyby of Comet 1P/Halley in 1986. The technical term used is that comet nuclei have a very low ‘albedo’. For 67P/C-G in particular, astronomers have combined visible light data from the Hubble Space Telescope and ground-based observatories, with infrared data from Spitzer and WISE, to determine that it has an albedo of just 4–6%, as dark as charcoal.
So combining these two facts, there’s not that much light coming from 67P/C-G with which to take a picture. But just as you would do in dimly lit situations on Earth, that can be overcome by using a longer exposure time. In particular, the exposure time needs to be long enough to get above the background noise of the detector, but not so long that any parts of the scene saturate the detector. With NAVCAM, the aim is to get the brightest parts of the comet up to roughly 75–85% of the detector saturation limit, which at present means an exposure time of 6 seconds.
Once a NAVCAM image has been captured and sent back to Earth, it is processed to remove artefacts due to electronic noise. The data are then scaled for DISPLAY according to their brightness: if left untouched, the darkest parts in the image, where there is essentially no light, will be black, while the brightest parts will be at about 75–85% grey (where 100% grey is white). In practice, some slight additional tweaking of the brightness and contrast is done to bring out the full range of features, with the result that the brightest parts of the nucleus are just about white.
While this is a perfectly standard approach, it admittedly doesn’t give a COMPLETELY accurate impression of the physical nature of the comet, where even the white parts of the picture are in fact very dark.
But actually, the human eye and brain do this all the time, constantly adjusting their sensitivity and perception of intensities to the scene at hand or even locally within a given scene. This so-called “anchoring” effect is the reason why the Moon appears white or even to shine against the dark night sky, while we know – not least from photos shot by moonwalking astronauts, as WELL as direct measurements of samples of the lunar soil – that the Moon’s surface is in fact a dark shade of grey, with an average albedo of around 12%.
A classic example is provided in the “checkershadow” optical illusion, which shows a column casting a shadow over parts of a checkerboard pattern. Because of the anchoring effect, the human eye perceives THE SQUARES in the shadow to be lighter than they really are, while those outside the shadow are seen to be lighter. In fact, squares A and B are exactly the same shade of grey, which can easily be shown by masking out the rest of the picture.
So, a human in a spaceship next to 67P/C-G may in fact perceive the comet pretty much as it is seen in the intensity-stretched NAVCAM images, if not even brighter. But one way of giving at least a suggestion of just how dark comets are is to show 67P/C-G against a number of other SOLAR SYSTEM objects exhibiting a wide range of albedos.
The montage below compares 67P/C-G with the Moon, the Earth, and Enceladus, a moon of Saturn. The brightness of each object in the montage has been scaled according to its mean albedo: for 67P/C-G, we have taken an albedo of 5%; for the Moon, 12%; for Earth, 31% (with deserts having an albedo of roughly 40%, thin clouds 30–50%, thick clouds 60–90%, and oceans 7–10%). Finally, for simplicity, an albedo of 100% has been taken for the brightest parts of the ice-covered surface of Enceladus, the most reflective body in THE SOLAR SYSTEM.
It’s hard to do this scientifically accurately, partly because the actual albedo in a given image of an object depends on a whole host of factors and because the human eye and brain don’t respond linearly to different light levels. But hopefully this comparison gives at least an impression of quite how dark 67P/C-G is and how diverse the SOLAR System’s bodies can be.
Another interesting aspect of the NAVCAM images of 67P/C-G is the very crisp and deep shadows. This is a result of a SINGLE, distant illumination source, namely the Sun, and the lack of any atmosphere surrounding the comet which would serve to diffuse the light, as on Earth: the developing coma is far too thin to scatter much light. However, on occasion, the deep shadows may be relieved by light reflecting off other parts of the comet, as seen in a number of the NAVCAM images.
Beyond the overall darkness of 67P/C-G, why are some regions nevertheless lighter and darker than the average? Some of this will likely be due to compositional differences across the surface of the comet, with some regions fresher due to activity and others more deeply covered in dust. The ROSETTA scientists are studying the composition of the surface through a combination of imaging at different ultraviolet, visible, infrared, and millimetre wavelengths, along with a range of other remote-sensing diagnostics.
But one key difference is down to the angle between the incident sunlight hitting the comet and the reflected light being measured by ROSETTA, which scientists call the ‘phase angle’. When the phase angle is large, the spacecraft sees shadows cast over the surface by the light coming in at an angle, but when the phase angle is small, few shadows are seen.
As ROSETTA has been manoeuvring around 67P/C-G, we have been seeing different regions at a wide variety of phase angles and because 67P/C-G has such a complex shape, different regions are seen at different phase angles simultaneously, further enhancing the apparent diversity of its landscape.
A particular effect occurs on many airless bodies in THE SOLAR SYSTEM whose surfaces are covered in dust, including the Moon, asteroids, and comets, when the phase angle reaches zero: with the light coming from behind the observer, a significant brightening is seen, the so-called ‘opposition surge’. This is in part due to the fact that, under these conditions, there are no shadows visible anymore: they are all ‘hidden’ behind the objects (e.g. dust particles and rocks) being illuminated, as seen from the observer. Another effect may occur if the dust particles are roughly the same size as the wavelength of light being measured: this can result in them acting as little retro-reflectors, again causing an increase in brightness at zero phase angle.
Thus, despite the very dark nature of the comet, there is nevertheless much to be learned about its surface structure and composition from OBSERVING the small differences in that reflected light, even if it is very faint. Results are expected over the coming months as the scientists analyse the data being collected at this enigmatic body, 67P/C-G.
COMETWATCH AT 10 KM
This four-image NAVCAM montage comprises images taken on 15 October from a distance of 9.9 km from the centre of comet 67P/C-G – just ~7.9 km from the surface!
KLIM CHURYUMOV AND SVETLANA GERASIMENKO: MEET THE DISCOVERERS OF ROSETTA’S COMET
Svetlana Gerasimenko and Klim Churyumov, co-discoverers of comet 67P, pictured in 1975 with the 40-cm Zeiss astrograph in Dushanbe, Tajikistan. Image courtesy of K. Churyumov.
ROSETTA's comet was discovered in 1969 by two Ukranian astronomers, Klim Churyumov and Svetlana Gerasimenko, who first observed it from the Institute of Astrophysics in Alma-Ata (now named Almaty), Kazakhstan. To their delight, comet 67P/Churyumov-Gerasimenko was selected in 2003 as the destination for the Rosetta mission. Since then, they have eagerly followed the progress of the mission that is now unveiling the many facets of 'their' comet.
Forty-five years after the discovery, Klim and Svetlana are still active researchers in the field of cometary science. They recently SHARED with us the story of how they stumbled on the object that would later become – as Svetlana calls it – a 'superstar among comets'.
As with many tales from the history of science, this was a lucky discovery.
In September 1969, the two astronomers were on an expedition to Alma-Ata to OBSERVE known comets and – hopefully – to discover new ones. Using the Institute's 50-cm telescope, they took photographs of the night sky, observing each region twice at an interval of 20-30 minutes. This is a common technique used to reveal unknown comets or transient objects in the night sky: by comparing subsequent photos of the same patch of the sky, unexpected 'visitors' appear to move across the sky against the background of the fixed stars, and can be easily identified.
Those were the days of analogue photography: astronomers would COAT a photographic plate with light-sensitive emulsion and expose it to the night sky to record light from astronomical sources. At the end of the observation shift, they would develop the plates and eventually, sometimes only after days or even weeks, measure the position and brightness of the celestial bodies recorded in the images and try to make sense out of them.
Photographic plates showing comet 67P/Churyumov-Gerasimenko, taken on 21 September 1969 by Klim Churyumov. Image courtesy K. Churyumov.
It was because of an incident during the processing of one of the photographic plates that ROSETTA'S comet was discovered.
One of the objects that Klim and Svetlana were studying at the time was comet 32P/Comas Solà, a periodic comet that had been discovered in the 1920s. Over the course of SEVERAL nights they photographed the region of the sky containing this comet. As Svetlana was processing one of these plates on 11 September, she realised she didn't have much solution to develop the plates, but decided to process that one anyway, before preparing a fresh solution. As a result, that plate was underdeveloped.
Svetlana almost threw the defective plate away, but she didn't. After all, a tiny splotch of light was visible at the centre, and they thought it was comet 32P/Comas Solà. Perhaps some useful information could be extracted from the good part of the plate. Little did they know how important that 'defective' plate would turn out to be!
In October, when the two were back in Kiev and started analysing the plates, it became clear to them that the comet imaged on the underdeveloped plate was not the one they had been aiming for - comet 32P/Comas Solà was almost two DEGREES away. That blotch of light at the centre was something else entirely. They had discovered a new comet – the remarkable 67P/Churyumov-Gerasimenko!
Update: 21.30 MESZ
COMETWATCH – 18 OCTOBER
This four-image NAVCAM montage comprises images taken on 18 October from a distance of 9.8 km from the centre of comet 67P/C-G – about 7.8 km from the surface. The four 1024 x 1024 pixel images MAKING UP the montage are provided at the end of this post.
Comet 67P/C-G on 18 October, from a distance of 9.8 km from the centre of the comet.
At a distance of 7.8 km from the surface, the image scale is about 66.5 cm/pixel, so each 1024 x 1024 pixel frame is about 680 m across (although if we assume the furthest point away is an additional ~1 km further from the centre, the image scale is about 92 cm/pixel).
The combined effect of the comet rotating between the first and last images taken in the sequence and the spacecraft moving in the same time is particularly apparent if you try to match features in the lower left and lower right images, which are the first and last images in the 20-minute sequence, RESPECTIVELY.
In this orientation, the larger lobe of the comet is to the left, the smaller lobe to the right. The image highlights the features of the comet’s neck, including the active region (you might like to compare with the previous, more distant, view of 26 September). A fracture-like feature seen in the 19 September image is also visible in this montage, about 1/3 down from the top of the lower right frame. In the same frame, we also have a much closer look at a group of boulders, some of which appear to be more like protrusions, perhaps exposed by the removal of surrounding material.
Elsewhere, and particularly in the upper frames, you may notice a number of bright streaks. Some of these will likely be dust grains ejected from the comet, captured in the six-second exposure time of the images.
COMETWATCH – CHEOPS’ NEIGHBOURS
This four-image NAVCAM montage comprises images taken on 18 October from a distance of 9.9 km from the centre of Comet 67P/C-G – about 7.9 km from the surface, not long after the image published Monday was taken. The four 1024 x 1024 pixel images making up the montage are provided at the end of this post. Assuming the 7.9 km distance, the image scale is about 67 cm/pixel, so each 1024 x1024 pixel frame is about 690 m across.
Comet 67P/C-G on 18 October, from a distance of 9.9 km from the centre of the comet.
In this image, we’re looking at the underside of the larger lobe of Comet 67P/C-G, which provides another look at boulder Cheops (lower left frame; see here and here for previous views) and a much more detailed look at the feature-rich surrounding terrain.
Numerous circular depressions can be seen towards the centre of the region, some with rather well-defined rims and smooth floors. While some craters and pits on 67P/C-G may be direct tracers of active locations, it is also expected that sublimation of volatiles from beneath the surface dust could induce collapse of the overlying area into pits. Of course, based on these images alone, alternative interpretations cannot be ruled out. Some circular depressions may be evidence of past impact cratering events or perhaps fine-grained material filled in some previously existing depressions, with the surrounding material later eroded away to give the appearance of a raised rim.
The lower-right frame also hosts a wealth of other interesting features, such as the exposed cliff faces bearing cross-cutting scars that run in numerous directions. On top of this feature, in the lower centre of the frame, an area of overlying dust appears to have been shaped into a surprisingly linear form – perhaps aided by underlying topography. Moving left from the largest exposed cliff face in the bottom right corner of this frame, it is possible to trace the outline of another ‘ledge’ emerging from the dusty covering. If this is indeed an erosive process, and if it can be applied to this entire scene, then perhaps the long curved depression that takes centre-stage in the lower left frame is also an expression of the topography that lies beneath. Alternatively, different rates of erosion/sublimation could be responsible for carving out these shapes in the surface material, independent of what is below.
Meanwhile, back towards the top of the lower right frame, rougher textures similar to those seen in the cliff faces are interspersed with the smoother material, boulders occasionally dotted in a seemingly random fashion. More boulders are visible towards the top of the image, casting long shadows onto the scenery below.
THE ‘PERFUME’ OF 67P/C-G
With inputs from Kathrin Altwegg, ROSINA science team, University of Bern.
Since early August, the Rosetta Orbiter Sensor for Ion and Neutral Analysis (ROSINA) has been ‘sniffing the fumes’ of 67P/C-G with its two mass spectrometers.
As reported previously in this blog, even though the comet is still more than 400 million kilometres from the Sun, the mixture of molecules detected in the comet’s coma is surprisingly rich already. Before arriving at 67P/C-G, the ROSINA team thought that at these vast distances from the Sun, its relatively low intensity would only release the most volatile molecules via sublimation, namely carbon dioxide and carbon monoxide.
However, ROSINA has detected many more molecules. Indeed, as of our 11 September report, ROSINA’s inventory of detected gases 67P/C-G looked like this:
Carbon monoxide (CO)
Carbon dioxide (CO2)
But today we can report that the following have also been detected:
Hydrogen sulphide (H2S)
Hydrogen cyanide (HCN)
Sulphur dioxide (SO2)
Carbon disulphide (CS2)
High resolution mass spectrum from ROSINA's Double Focusing Mass Spectrometer (DFMS), taken on 10 October at a distance of 10 km from the comet centre. The plot shows the detection of hydrogen sulphide and the heavier isotope of sulphur, 34S, which is a fragment of all sulphur bearing species. The plot shows intensity vs. the mass-to-charge ratio*. Image courtesy K. Altwegg, University of Bern
If you could smell the comet, you would probably wish that you hadn’t
As the Kathrin Altwegg, principal investigator for ROSINA, put it: “The perfume of 67P/C-G is quite strong, with the odour of rotten eggs (hydrogen sulphide), horse stable (ammonia), and the pungent, suffocating odour of formaldehyde. This is mixed with the faint, bitter, almond-like aroma of hydrogen cyanide. Add some whiff of alcohol (methanol) to this mixture, paired with the vinegar-like aroma of sulphur dioxide and a hint of the sweet aromatic scent of carbon disulphide, and you arrive at the ‘perfume’ of our comet.”
While this is unlikely to be a particularly attractive perfume, remember that the density of these molecules is very low, and that the main part of the coma is made up of water and carbon dioxide, mixed with carbon monoxide.
The key point, however, is that a detailed analysis of this mixture and how it varies as 67P/C-G grows more active will allow scientists to determine the comet’s composition. Further work will show how 67P/C-G compares with other comets, for example by revealing differences between comets originating from the Kuiper Belt (like 67P/C-G) and comets that hail from the distant Oort cloud (like Comet Siding Spring, which recently flew past Mars). The goal is to gain insights into the fundamental chemical make-up of the solar nebula from which our Solar System and, ultimately, life itself emerged.
COMET ACTIVITY IS ON THE INCREASE
OSIRIS image of Comet 67P/C-G on 10 September 2014, showing jets of cometary activity along almost the entire body of the comet.
Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/ INTA/UPM/DASP/IDA
Comet 67P/Churyumov-Gerasimenko is showing a gradual, but clear, increase in activity, as can be seen in the latest images provided by the OSIRIS team.
While images obtained a few months ago showed distinct jets of dust leaving the comet, these were limited to the ‘neck’ region. More recently, images obtained by Rosetta’s scientific imaging system, OSIRIS, show that dust is being emitted along almost the whole body of the comet. Jets have also been detected on the smaller lobe of the comet.
“At this point, we believe that a large fraction of the illuminated comet’s surface is displaying some level of activity,” says OSIRIS scientist Jean-Baptiste Vincent from the Max Planck Institute for Solar System Research (MPS) in Germany.
From these images, the team wants to derive a better understanding of the evolution of cometary activity and the physical processes driving it.
“Being able to monitor these emissions from up close for the first time gives us much more detailed insights,” says OSIRIS Principal Investigator Holger Sierks. “But one image alone cannot tell us the whole story; from one image we cannot discern exactly where on the surface a jet arises.”
Instead, the researchers compare images of the same region taken from different angles, in order to reconstruct the three-dimensional structure of the jets. And, since under normal circumstances the comet’s nucleus would outshine the jets, the necessary images must be drastically overexposed to reveal the details of the jets, as shown in the image pair presented here (see below).
While 67P/C-G’s overall activity is clearly increasing, the mission’s designated landing site on the smaller lobe still seems to be rather quiet. However, there is some indication that new active areas are waking up about one kilometre from the landing site. These will allow the lander’s instruments to study the comet’s activity from an even closer distance.
Today, 67P/C-G is about 470 million kilometres from the Sun. Based on a rich history of ground-based observations, scientists expect a comet’s activity to pick-up noticeably once it comes to within 300 million kilometres of the Sun; for 67P/C-G – and Rosetta – this ‘boundary’ will be crossed in late March 2015.
NAMING SITE J – UPDATE
We're one step closer to knowing the name of Philae's landing site.
By the time the competition closed, we had received more than 8000 proposals for a new name for the location, currently known as Site J. A huge big THANK YOU to all of you who participated for sharing your creativity and good will with us!
Between now and next Wednesday, the list of proposals will be whittled down to a shortlist of 30 and this will be presented to the Philae Steering Committee. They will have the onerous task of choosing the best name for Philae's landing site.
The winner will be announced on 3 November. The competition is being run and coordinated by ESA, DLR, CNES and ASI. Entries sent to any of these agencies will be considered together, with one winner selected to attend the landing event at the Mission Control Centre in Darmstadt, Germany on 12 November.
COMETWATCH – 20 OCTOBER
This four-image NAVCAM montage comprises images taken on 20 October from a distance of 9.4 km from the centre of comet 67P/C-G – about 7.4 km from the surface. In this orientation the smaller lobe of the comet is on the left, with the larger lobe on the right.
Four image NAVCAM montage comprising images taken on 20 October 2014. Credits: ESA/Rosetta/NAVCAM
Assuming the 7.4 km distance, the image scale is about 63cm/pixel, so each 1024 x 1024 pixel frame is about 645 m across (although if we assume the furthest point away is an additional ~1 km further from the centre, the image scale is about 72 cm/pixel).
Rosetta team shows that science fact is stranger than fiction
The European Space Agency has surprised many by turning to science fiction as the latest ploy to promote its remarkable Rosetta mission to Comet 67P/Churyumov-Gerasimenko. ESA teamed up with an Oscar-nominated movie director, Tomek Bagiński, to produce a short film called Ambition that highlights the importance of what Rosetta aims to achieve. Just seven minutes long, the film stars two big-name actors—Aidan Gillen (Game of Thrones) and Aisling Franciosi (The Fall). A teaser trailer was launched for it some weeks ago, and created something of a buzz among scifi fans when little could be found on the web to explain it.
The European Space Agency has surprised many by turning to science fiction as the latest ploy to promote its remarkable Rosetta mission to Comet 67P/Churyumov-Gerasimenko. ESA teamed up with an Oscar-nominated movie director, Tomek Bagiński, to produce a short film called Ambition that highlights the importance of what Rosetta aims to achieve. Just seven minutes long, the film stars two big-name actors—Aidan Gillen (Game of Thrones) and Aisling Franciosi (The Fall). A teaser trailer was launched for it some weeks ago, and created something of a buzz among scifi fans when little could be found on the web to explain it.Tomek commented: "My aim was always to create a short that mirrored the ambition of all those behind Rosetta. These characters battle with their own human limitations, yet find the strength and determination to go beyond imposed boundaries and showcase that rich and relentless spirit of exploration that makes mankind great. “This project is a tribute to that spirit, and to our collective need to discover.” Key members of the Rosetta team were present for the movie’s launch, including Rosetta Project Scientist Matt Taylor. Recalling how NASA’s Curiosity landing had been described as “seven minutes of terror”, he said that Philae’s slow attempt to land would provide ESA with “hours of anguish” instead. The BFI audience was also treated to a presentation from space as ESA astronaut Alexander Gerst, aboard the International Space Station, delivered a pre-recorded presentation that included his own zero-G demo on how to land on a comet. Ambition is impressive. But Lukasz Sobisz, of Platige Image’s effects team, put his finger on the irony of having to make it. He says, in remarks that conclude the making-of movie: “The actual campaign idea is certainly well targeted. But, shooting myself in the foot a bit, I’m very surprised you need something like this at all now. Mankind send a probe into space to catch a comet and land on it. And we need a great film, director and actors to convince people this is interesting.”
COMETWATCH – 24 OCTOBER
This four-image NAVCAM montage comprises images taken on 24 October from a distance of 9.8 km from the centre of comet 67P/C-G – about 7.8 km from the surface. The corresponding image scale is about 66 cm/pixel, so each 1024 x 1024 pixel frame is about 676 m across.
Four image NAVCAM montage comprising images taken on 24 October 2014. Credits: ESA/Rosetta/NAVCAM
In this orientation the larger lobe occupies the upper frames, with the neck filling the lower frames. The smaller lobe of the comet is out of view towards the right.
This image set complements those taken on 18 and 20 October, which together show a number of the same features from different angles, such as the dune-like features seen in the smooth neck region, and the fracture-like feature that runs through part of the rougher neck terrain. In the top- and bottom-left corners in particular, groups of boulders appear to cling to cliff faces, perhaps exposed as surrounding material is eroded away. Meanwhile jagged spires interspersed with flat-topped plateaus make for an attractive view over the horizon.
ROSETTA ENROUTE TO PRE-DELIVERY ORBIT
Rosetta conducted a manoeuvre today – a thruster burn that lasted 82 seconds – and it was completed as planned, Spacecraft Operations Manager Sylvain Lodiot has confirmed.
"The burn looks OK from telemetry, and the spacecraft is in good health," wrote Sylvain in an email.
The burn got underway at 12:59 UTC (13:59 CET), ended at 13:01 UTC (14:01 CET) and, based on an initial analysis of post-burn telemetry, delivered a change in velocity – 'delta-v' – of 0.081 meters/sec. This pushed the spacecraft to leave the 10-km-altitude circular orbit (following the terminator line) – the so-called 'Close Observation Phase' (COP) orbit – where it's been since 15 October.
The COP orbit enabled high-resolution images of the landing site in order to best prepare for Philae's challenging touch-down.
Today's manoeuvre is important as it means Rosetta has now started the transition from the COP to the pre-lander-delivery orbit.
Specifically, today's burn has put Rosetta on a very elliptical trajectory rapidly moving the spacecraft away from the 10-km orbit. In three days, on 31 October, the mission control team will perform another manoeuvre to enter onto the pre-delivery orbit proper.
This is a slightly elliptical orbit at approximately 30 km distance from the comet centre (see the animation above).
After another orbital change taking place about two hours before separation, the spacecraft will finally deploy Philae from a distance of 22.5 km from the comet centre, at 08:35 UT on 12 November.
Update: 21.45 MEZ
COMETWATCH – 26 OCTOBER
This four-image NAVCAM mosaic comprises images taken on 26 October from a distance of 9.8 km from the centre of comet 67P/C-G – about 7.8 km from the surface. The corresponding image scale is about 66 cm/pixel, so each 1024 x 1024 pixel frame is about 676 metres across.
Changes in perspective and shadows due to rotation and translation of the comet and spacecraft with respect to each other seem to be limited across this particular set of four images, which is why we’ve decided to present a mosaic this time. The mosaic covers roughly 1200 x 1350 metres. But as usual, we urge you to be cautious in over-interpreting the mosaic: the individual images, presented at the end of this post, provide the ‘ground truth’.
Four image NAVCAM mosaic comprising images taken on 26 October. Credits: ESA/Rosetta/NAVCAM
This scene focuses on the same part of the comet as seen in the 18 October image set, but you will notice some interesting new features, as well as some previously visited areas in a new light. (See CometWatch 8 October and the OSIRIS image of 6 August for additional context.)
Visible towards the upper left is a region of brighter material that was previously cast in shadow in the 18 October image, and that appears to lie at the base of a cliff. Some additional bright material is seen at the top of the cliff. This material may be freshly revealed and yet to be covered in dust, perhaps as the result of a recent ‘landslide’. It remains to be seen what its composition is, but no doubt the science teams will be looking out for its signature in the remote sensing data.
Remember though, that this material isn’t in fact bright white on the comet; comets are blacker than coal, and the NAVCAM images are grey-scaled according to their brightness with additional contrast adjustments to bring out the full range of features (see NAVCAM’s shades of grey for a full recap of the way in which intensities are displayed in NAVCAM images).
Also not seen previously in this much detail are the two boulders just below the centre of the mosaic, one of which takes on a heart-shaped appearance from this angle. Zooming in reveals hints of a layered structure and both objects seems to be appearing from beneath the dusty layer, just like some of the smaller ‘boulders’ around them.
Finally, over on the right hand side of the mosaic, the region cast in dramatic shadow on 18 October can now be seen in much more detail. And don’t forget the boulder Cheops and friends above the centre of the mosaic too!