NASA will host a media teleconference at 1 p.m. EDT Wednesday, Oct 5, to discuss select science investigations launching on the next Orbital ATK commercial resupply flight to the International Space Station.
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Friday, 30 September 2016
NASA, France to Collaborate on Aircraft Noise Research
During bilateral meetings in Daejeon, South Korea, NASA and France’s Office National d'Etudes et de Recherches Aerospatiales (ONERA) signed an agreement Tuesday to collaborate on research that focuses on mitigating the effects of civil air transportation noise.
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NASA TV to Broadcast Hispanic Heritage Event, Aspira con NASA / Aspire with NASA
NASA will celebrate Hispanic Heritage Month at the agency’s headquarters in Washington Tuesday, Oct. 4, with stories of aspiration, inspiration and exploration. The event will air live on NASA Television and the agency’s website at 10 a.m. EDT.
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Loss of signal confirmation
Spacecraft Operations Manager Sylvain Lodiot confirms loss of signal (LOS) and end of Rosetta operations at 13:19 CEST, 30 September 2016, via the voice loop in the Main Control Room at ESA's space operations centre, Darmstadt, Germany.
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Comet landing descent image – 51 m
This is Rosetta's last image of Comet 67P/Churyumov-Gerasimenko, taken shortly before impact, only 51 m above the surface.
Close-up view of the surface of Comet 67P/C-G imaged with the OSIRIS wide-angle camera, shortly before Rosetta's final impact on the comet. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
The image was taken with the OSIRIS wide-angle camera on 30 September 2016. The image scale is about 5 mm/pixel and the image measures about 2.4 m across.
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Last call from Rosetta
Going... going... gone! A sequence of screenshots showing the signal from Rosetta seen at ESA's ESOC mission control centre via NASA's 70m tracking station at Madrid during comet landing on 30 September 2016. The peak of the spectrum analyser is strong at 12:19 CEST, and a few moments later, it's gone.
Rosetta's radio signal starts to fade... Credit: ESA
Rosetta's radio signal starts to fade... Credit: ESA
Rosetta's radio signal is almost gone Credit: ESA
Rosetta's radio signal is gone Credit: ESA
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Comet landing descent image – 1.2 km
Rosetta’s descent continues. Here's an OSIRIS narrow-angle camera Comet 67P/Churyumov-Gerasimenko captured at 10:14 GMT from an altitude of about 1.2 km on 30 September.
The image scale is about 2.3 cm/pixel and the image measures about 33 m across.
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Rosetta’s landing site
Here's a sequence of images captured by Rosetta during its descent to the surface of Comet 67P/Churyumov-Gerasimenko on 30 September.
Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
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Comet landing descent image – 5.7 km
Another striking image from Rosetta's descent onto the surface of Comet 67P/Churyumov-Gerasimenko, taken with the OSIRIS narrow-angle camera at 08:21 GMT from an altitude of about 5.7 km.
Comet 67P/C-G viewed with Rosetta's OSIRIS NAC on 30 September 2016, 5.7 km from the surface. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
The image scale is about 11 cm/pixel and the image measures about 225 m across.
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ROSINA confirms pressure increase
As Rosetta approaches the surface of Comet 67P/Churyumov-Gerasimenko, the Comet Pressure Sensor (COPS) on the ROSINA instrument is measuring the gas pressure around the nucleus increasing!
The ROSINA-COPS readings on 30 September 2016. Image courtesy K. Altwegg.
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Flight dynamics team at ESOC
Throughout the entire Rosetta mission, the Flight Dynamics team at ESOC have been some of the hardest-working, behind-the-scenes wizards ensuring navigation – and today is no exception. They also have some of the coolest visualization tools in the Solar System!
Visualisation screens in the Flight Dynamics control room at ESOC. Credit: ESA
Visualisation screens in the Flight Dynamics control room at ESOC. Credit: ESA
ESOC Flight Dynamics control room during Rosetta's final descent, 30 Sep 2016. Credit: ESA
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Comet landing descent image – 5.8 km
As Rosetta continues its descent onto the Ma'at region on the small lobe of Comet 67P/Churyumov-Gerasimenko, the OSIRIS narrow-angle camera captured this image at 08:18 GMT from an altitude of about 5.8 km.
The image shows dust-covered terrains, exposed walls and a few boulders on Ma'at, not far from the target impact region (not visible in this view - located below the lower edge).
Comet 67P/C-G viewed with Rosetta's OSIRIS NAC on 30 September 2016, 5.8 km from the surface. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
The image scale is about 11 cm/pixel and the image measures about 225 m across.
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Comet landing descent image – 8.9 km
As Rosetta gets closer and closer to Comet 67P/Churyumov-Gerasimenko, the OSIRIS narrow-angle camera captured this beautifully detailed image of the comet surface at 06:53 GMT from an altitude of about 8.9 km.
Comet 67P/C-G viewed with Rosetta's OSIRIS NAC on 30 September 2016, 8.9 km from the surface. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
The image scale is about 17 cm/pixel and the image measures about 350 m across.
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Comet landing descent image – 11.7 km
During Rosetta's final descent, which is currently undergoing, the OSIRIS narrow-angle camera captured this image of Comet 67P/Churyumov-Gerasimenko at 05:25 GMT from an altitude of about 11.7 km.
With dramatic shadows, the image shows the comet's 'neck' region, with the smooth terrains of Hapi on the right and the rougher Hathor on the left.
Comet 67P/C-G viewed with Rosetta's OSIRIS NAC on 30 September 2016, 8.9 km from the surface. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
The image scale is about 22 cm/pixel and the image measures about 450 m across.
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Live from Rosetta mission control
Live Twitter Periscope visit to the Rosetta mission control team on console in the Main Control Room at ESOC, Darmstadt, Germany, for today's #CometLanding. Hosted by Daniel Scuka, from the ESA web team.
LIVE on #Periscope: Live from Rosetta mission control https://t.co/BXLOwJAAb9
— ESA (@esa) September 30, 2016
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CometWatch finale: Rosetta’s last NavCam images
Rosetta's Navigation Camera captured five images shortly after the collision manoeuvre last night, which were used by flight dynamics teams to confirm the spacecraft is on track to impact its target in the Ma'at region of Comet 67P/C-G.
The NAVCAM images were acquired at 22:53, 23:25, and 23:56 UT on 29 September and 0027 and 0059 UT on 30 September (on board spacecraft time) when the spacecraft was between about 20 and 17 km from the comet centre.
The first two images show the scan of the spacecraft over the large comet lobe, featuring the Seth, Hapi and Ash regions, before the Hatmehit and Ma'at regions on the small lobe came into view.
The full set of lightly enhanced images are presented below. Click for distance and scale info.
22:53UT 29 September 2016
ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
23:25 UT 29 September 2016. ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
23:56 UT 29 September ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
00:27 UT 30 September 2016 ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
Lightly enhanced NAVCAM image taken on 30 September 2016 at 00:59UT. ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
The original images are also provided below:
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Impact site is coming in to view!
We just received this image from the OSIRIS wide-angle camera, taken at 02:17 UT at the comet. It shows the target impact region just coming in to view in the lower left –look for the distinctive shape of the Ma'at pits.
It was taken from a distance of about 15.5 km; the image scale is about 1.56 m/pixel and the image measures 3.2 km across.
ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
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Rosetta’s last NAVCAM image
Rosetta's Navigation Camera captured five images shortly after the collision manoeuvre last night, which are being analysed by flight dynamics to confirm the spacecraft is on track to impact its target in the Ma'at region of Comet 67P/C-G later today.
The last image returned from the spacecraft was taken at 00:59 UT onboard the spacecraft, and downlinked to Earth a couple of hours later. It was taken at a distance of 17.4 km from the centre of the comet. The image scale is 1.5m/pixel and the image measures about 1.5 km across.
Lightly enhanced NAVCAM image taken on 30 September 2016 at 00:59UT. ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
The five images were used by the flight team to update the estimate of the landing time and final pointing of the spacecraft. The revised impact time is now predicted as 10:38:32 UT+/- 2 minutes at the comet.Because of the 40 minute signal travel time between Rosetta and the Earth today, confirmation of the mission's end will arrive at ESA's mission control at 11:18 UT/ 13:18 CEST +/- 2 minutes.
The full set of five images will be published later this morning.
Follow rosetta.esa.int for live coverage.
The original unprocessed image is provided below:
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Impact time update: 10:38 UT
Based on the Navigation Camera images taken shortly after last night's collision manoeuvre, flight dynamics analysis has refined the predicted time of Rosetta's impact into the Ma'at region on the small lobe of Comet 67P/C-G to 10:38:32 UT+/- 2 minutes at the comet.
Because of the 40 minute signal travel time between Rosetta and the Earth today, confirmation of the mission's end will arrive at ESA's mission control at 11:18 UT/ 13:18 CEST +/- 2 minutes.
Follow rosetta.esa.int for live coverage later today.
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Descent images begin!
We've started to get images from Rosetta's descent. This one was taken by the OSIRIS narrow-angle camera at 01:20 UT, from a distance of around 16 km.
The image scale is about 30 cm/pixel and the image measures about 614 m across.
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Collision manoeuvre complete
Rosetta has completed its final manoeuvre and is now on a collision course with Comet 67P/C-G.
A small thruster burn starting 20:48:11 UTC and lasting 208 seconds has set the craft on course towards its final destination.
The spacecraft's navigation cameras will soon take a set of five images to confirm that the spacecraft is on target, and to refine the predicted impact time.
These are expected to be downlinked by 0300 UT / 0500 CEST and we therefore expect that the next report, with the updated time and at least one of those NAVCAM images, will be around 0400 UT / 0600 CEST.
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NASA Awards Contract for Information Technology Support Services
NASA has awarded its Goddard Information Technology Integration Support Services (GITISS) contract to Business Integra, Inc. of Bethesda, Maryland.
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Thursday, 29 September 2016
Earlier today…!
Rosetta's OSIRIS wide-angle camera captured this image at 11:49 GMT on 29 September 2016, when the spacecraft was 22.9 km from Comet 67P/Churyumov–Gerasimenko.
Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
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Rosetta Legacy Highlights
Over the past two years, the Rosetta mission has captured the imagination of many people worldwide, stimulating them to produce art and music, and to undertake other creative activities with friends and families – some even made further education or career choices inspired by the mission.
This video features a selection of contributions that were shared on the Rosetta Legacy tumblr before 21 September 2016:
We also asked some of the contributors to tells us more about how the mission influenced their study, career and life in general:
The Rosetta Legacy campaign will run until 7 October 2016. Share your stories, images and videos on http://ift.tt/2c3orHm and be in with a chance to win spot prizes (Rosetta and Philae plush toys) and a visit at ESA's ESTEC as top prize.
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The cometary zoo
The ROSINA instrument on Rosetta has been “sniffing” the environment of Comet 67P/Churyumov-Gerasimenko for the past couple of years, obtaining unprecedented measurements of the gases found in a comet's atmosphere. Besides the main component – water vapour – ROSINA detected a wide variety of chemical species, from simple atoms to increasingly complex molecules, including some ingredients that were crucial for the origin of life on Earth.
In a humorous take on this “cometary zoo”, Kathrin Altwegg, ROSINA principal investigator from University of Bern and an enthusiast of animals, tells us more about the variety of bizarre “creatures” they've found at the comet.
Let's start from the volatile species, our beautiful butterflies, including CO, CO2, nitrogen, and the unexpected oxygen.
Then, we found many carbon chains, from methane and ethane to long chains – propane, butane, pentane... up to heptane. These are our giraffes. In fact, we might have found even longer chains, but we are still working to find out the corresponding molecule.
We also found many elephants, with a round shape and large mass: the aromatic ring compounds, from benzene to naphthalene – which also suggests there are no moths at Comet 67P/C-G!
There are many alcoholic compounds in the comet's atmosphere – not all of them have the potential to make you look funny, but we still like to group them in the “monkey” zone.
Some of the "creatures" found by ROSINA at Comet 67P/C-G. Credit: ESA
Glycine, the amino acid found in proteins and crucial for life as we know it on Earth, is of course the king of the zoo – the lion – who feeds on zebras... In fact, the “smelly manure” zebra-molecules, ammonia, methylamine and ethylamine, are precursors to glycine.
We also found other unpleasantly smelly molecules at the comet: the sulphur-bearing skunks (hydrogensulphide, carbonylsulphide, sulphur monoxide and dioxide, and carbon disulphide) and the dart frogs (including some coloured species of sulphur: the blue S2, the yellow S3, and the red S4).
Some species we found at the comet are not only unpleasant to smell, but could be poisonous to some extent: the snakes – including acetylene, hydrogen cyanide and formaldehyde.
We found some noble gases at the comet: argon, xenon and krypton, which are beautiful and solitary – they don't react easily with other molecules – like the aloof peacock.
There are also hard, non-volatile species sputtered by the solar wind, like sodium, potassium, silicon, and magnesium. These are the oysters, as the findings contain some rare “pearls”, for example, isotopic ratios for silicon that were never measured before.
We also found hydrogen chloride – which can produce sea salt when in contact with sodium – and grouped it with other species as seawater, tropical fish. In this group we included also phosphorus, a key component of DNA and cell membranes.
There are also some complex, hard to classify oxygen- and nitrogen-bearing molecules, which are more complex than others found at the comet: they have no long chains, no rings, but they branch, so they became our exotic birds.
Finally, we also found cyanogen, which was always there in our measurements but somehow hiding, and it took us two years to detect: a true chameleon!
And the search is not over... we see hints of more “beasts” on the horizon, but we have not fully identified them yet.
Even if they are only inanimate chemical species – comets do not contain life, that's for sure! – looking at this colourful and diverse zoo we cannot but wonder what would happen if a piece of the comet were to fall in an ocean of water – after all, the ingredients for life are there.
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The results from ROSINA were discussed by Kathrin Altwegg during today's science briefing at the European Space Operations Centre (ESOC) in Darmstadt, Germany.
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A comet’s life – a new sonification of RPC data
Now, after two years of monitoring the plasma around the comet, the RPC team present a new song based on data collected during the entire mission, describing the comet's evolution from the point of view of Rosetta's magnetometer.
While the nucleus of Comet 67P/C-G is itself not magnetised, as measured by plasma instruments on both Rosetta and the lander Philae, it is embedded in the interplanetary magnetic field carried throughout the Solar System by the solar wind – a continuous flow of electrically charged particles streaming from the Sun. As the comet pours water vapour and other molecules into space, disturbing the solar wind, interesting phenomena take place in the surrounding plasma.
Analogy between the pile-up of the magnetic field near a comet nucleus and the pile-up of water in front of an obstacle. Image courtesy K-H Glassmeier
The interaction between comets and the solar wind had been studied in the past during the flybys of previous cometary missions, but Rosetta was the first probe to observe this phenomenon for an extended period of time, following its evolution as the comet swung around the Sun.
“When Rosetta arrived at the comet, in 2014, the activity was very low,” explains RPC-Mag principal investigator Karl-Heinz Glassmeier from the Institute for Geophysics and extraterrestrial Physics at Technische Universität Braunschweig, Germany.
At the interface between the solar wind and the comet's fuzzy atmosphere, or coma, gas molecules released by the comet may lose one or more electrons. As observed at other comets, these ionised molecules are normally picked up by the solar wind, building up boundaries in the plasma environment of the comet.
However, no spacecraft had ever visited a comet when its activity was as low as that of Comet 67P/C-G at the time of Rosetta's arrival and, when the RPC magnetometer measured some unexpected waves, it took the scientists by surprise.
Shortly after the first detection in September 2014, the RPC-Mag team collaborated with musician and composer Manuel Senfft to create a sonification of these puzzling measurements, representing the data in an audible form and creating 'A singing comet' (read more about it in the original blog post and in the 'Behind the scenes' follow up). The audio track, released on the day before Philae landed on the comet, became a worldwide sensation, providing an extra element to the experience of “being there” at the comet with Rosetta and Philae.
After several months analysing the data, the scientists figured out the physical processes that led to the comet's song.
“Because of the comet's low activity, the ions are not fully picked up by the solar wind and move perpendicularly to the magnetic field, forming what is called a cross-field electric current,” adds Karl-Heinz.
“But this electric current is unstable, giving rise to the oscillations we measured back in 2014 – that's what made the comet sing.”
Measurements of the magnetic field obtained with RPC-Mag on 10 October 2014, showing the characteristic oscillations of the 'singing comet'. Image courtesy C. Goetz.
The details of the 'singing comet' are described in a paper by Ingo Richter et al, which was published in Annales Geophysicae in August 2015. By then, however, the music at Comet 67P/C-G had already changed.
“As the comet moved closer to the Sun in 2015, it was pouring increasingly larger amounts of gas into its surroundings and, with part of this gas becoming ionised, interaction with solar wind particles intensified,” says Charlotte Goetz of the Institute for Geophysics and extraterrestrial Physics at Technische Universität Braunschweig Braunschweig, Germany.
Between April and June 2015, plasma regions with different properties – velocity, temperature, density – started to form around the nucleus along with boundaries separating the different regions. Rosetta observed at least one such boundary, which was likely moving, as described in a recent paper by Kathy Mandt and collaborators.
As the comet's activity was not strong enough yet, these boundaries could not last long, leading to a phase of chaotic variations in the magnetic field, which was jumping up and down by 30–40 nT over time scales of seconds to hours according to Rosetta's measurements.
“We like to think that, in this period, the comet was 'confused' much like a teenager: the activity was not as weak as when Rosetta first arrived and detected the 'singing comet' waves, but it was neither strong enough to create stable boundaries,” adds Charlotte.
The situation changed when 67P/C-G was approaching perihelion – the closest point to the Sun along its orbit – and finally achieving full 'cometary maturity'. Ever since June 2015, only a few weeks before the comet's perihelion on 13 August, Charlotte and her colleagues started observing a diamagnetic cavity: a region near the comet nucleus where the solar wind cannot penetrate and the magnetic field is practically zero.
Measurements of the magnetic field obtained with RPC-Mag on 26 July 2015, showing the detection of a diamagnetic cavity (where the magnetic field goes drops to zero). Image courtesy C. Goetz.
“We were really happy to detect the diamagnetic cavity because we had almost given up on finding it: at the time, because of the intense comet activity, Rosetta was flying at large distances from the nucleus – farther away than where we thought the cavity would be. Fortunately, the cavity turned out to be much bigger and dynamic than we had expected, so we were able to measure it,” says Charlotte.
The RPC-Mag team found several hundreds of such magnetic-field free regions until February 2016. The discovery of a diamagnetic cavity at Comet 67P/C-G was first described in a paper by Charlotte Goetz et al, published in March 2016 in Astronomy & Astrophysics.
After that, as the comet moved away from the Sun and its activity declined, plasma boundaries became unstable again and the magnetic field turned chaotic one more time. Eventually, the activity became as low as it had been at the time of Rosetta's first measurements: the oscillations returned and, with them, the 'singing comet'.
“Having detected the waves early in the mission, we later predicted they would return as soon as the activity would reduce... and they did! This was definitely a pleasant surprise,” says Karl-Heinz.
To convey the life of the comet as monitored by the RPC magnetometer, the scientists teamed up again with Manuel, who could now transform into sound not just one day's worth of measurements, but a selection of data from two years.
“I was very fascinated by the symmetry of the magnetic field readings: first the oscillations, then chaos, then the magnetic field was gone... then chaos was back and then, in the end, also the oscillations,” says Manuel.
The composer used this symmetry to construct the dramaturgy of the new piece, called 'A comet's life' and centred on the perihelion of Comet 67P/C-G. The piece starts with the famous 'singing comet' waves, building up to a climax as the comet's activity reaches its maximum, then slowly peters out, coming to an end with the return of the waves.
The magnetic field measurements obtained by Rosetta's RPC-Mag during its entire mission at Comet 67P/Churyumov-Gerasimenko. Image courtesy C. Goetz
Manuel employed a similar technique to that developed for his 2014 sonification, using one of the three components of the magnetic field to modulate the pitch of the sound itself, and using variations in the other two components to control the location of the sound in a ‘sound stage’ with respect to the listener, moving it back and forth and left and right.
This time, given the richness of the data, he also made use of a new synthesiser to alter the 'character' of the sound.
“For example, I added distortion to the sound, making it more noisy, to better represent the period of chaotic variations, while I used silence to reflect the absence of magnetic field in the cavity,” Manuel explains.
The result is a spacey, 1:30 minute-long, binaural piece that takes the listener along and lets it fly with Rosetta, following the comet as its activity rose and declined while its Sun-bound journey unfolded.
“The first 'singing comet' was as exciting as hearing a baby's first cry, but the new track is even better: it's a lifetime experience, and it's our experience, living with this incredible comet over the past two years,” concludes Karl-Heinz.
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The results from the RPC-Mag instrument and the audio track, 'A comet's life', were presented by Charlotte Götz during today's science briefing at the European Space Operations Centre (ESOC) in Darmstadt, Germany.
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Comet Landscapes
During today's science briefing at ESA's ESOC, Ramy El-Maarry (University of Bern) presented a series of highlights about the landscape of Comet 67P/Churyumov-Gerasimenko.
Over the past two years, Rosetta mapped the entire surface of the comet at high-resolution, resolving very small features and monitoring surface changes with time.
The comet nucleus is indeed very dark (see our 2014 blog post NAVCAM's shades of grey), reflecting only about 4% of the light that hits it. But why is it so dark?
Measurements from both Rosetta and Philae report that most of the surface is dry (with the exception of icy boulders) and covered in organic material, most of which had never been detected on the surface of a comet (on this topic, see previous blog posts: Extremely dark, dry and rich in organics and Science on the surface of a comet).
The surface presents a rich diversity in texture, with the northern hemisphere mostly covered in dust, produced by the comet's activity – for example in the Ma'at and Ash regions.
Philae's multiple landings allowed scientists to observe in details two distinct regions on the comet's surface: the smooth-covered terrains at the first touchdown point, Agilkia, and the hard surface at its final resting point, Abydos.
Some of the smooth material, for example in Imhotep, also gives rise to the most dynamic features that have been observed on the comet so far. Surface changes have been observed around the time of perihelion (see the 2015 post Comet surface changes before Rosetta’s eyes).
Finally, a look at the southern hemisphere, which could not be observed until mid-2015 and gradually became illuminated shortly before the comet's perihelion. The surface morphology of the southern hemisphere is very different, showing a clear dichotomy with its northern counterpart mainly because of the absence of wide-scale smooth terrains, dust coatings and large unambiguous depressions.
This is likely caused by the uneven distribution of seasons on the comet. Due to the double-lobed shape of 67P/C-G and the inclination of its rotation axis, the northern hemisphere experiences a very long summer, lasting over 5.5 years, while the southern hemisphere undergoes a long, dark and cold winter.
However, a few months before the comet reaches perihelion – the closest point to the Sun along its orbit – the situation changes, and the southern hemisphere transitions to a brief and relatively hot summer.
As the southern summer is much shorter and more intense than in the northern hemisphere, the erosion is higher there by a factor of 3, giving rise to flattened out features. Scientists believe that, due to the more intense activity, dust is ejected at higher speed and it is either lost to space or falls back on the northern hemisphere (read more about the dust transfer across the comet in the recent blog post The surprising comet).
Below are a series of maps highlighting the various regions on the southern hemisphere of Comet 67P/C-G:
And finally a set of views of regions across the entire comet, both in the northern and southern hemisphere:
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This blog is based on the presentation by Ramy El-Maarry during today's science briefing at the European Space Operations Centre (ESOC) in Darmstadt, Germany.
The images of the southern hemisphere and the comet maps are from the paper 'Regional surface morphology of comet 67P/Churyumov-Gerasimenkofrom Rosetta/OSIRIS images: The southern hemisphere' by El Maarry et al, which was published earlier this year in Astronomy and Astrophysics.
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Beneath the surface of Comet 67P
While scientists and the public alike have been astounded by the unexpected shape of Comet 67P/ Churyumov-Gerasimenko’s nucleus, what lies beneath the surface is just as important scientifically.
Comet interiors preserve a unique record from the formation of the Solar System 4.6 billion years ago. Reaching that information was one of the key tasks of Rosetta and Philae. There were two principal experiments designed to 'see' inside the comet's nucleus: the COmet Nucleus Sounding Experiment by Radio-wave Transmission (CONSERT) instrument and the Radio Science Investigation (RSI).
Image taken with the navigation camera (NavCam), 28 January 2016. Credit: ESA/Rosetta/NAVCAM, CC BY-SA IGO 3.0
Comets are known to be mixtures of dust and ice. Yet puzzlingly, measurements show that 67P/C-G's density is much lower than ice. This means the comet has a high porosity, and could be an indicator that there are huge empty caverns inside it. However, neither CONSERT nor RSI find any evidence to support this.
Instead, the investigations have shown that the porosity comes about because the comet is a more or less uniform mixture of ice and 'fluffy' dust grains, rather than a honeycomb structure featuring large voids.
CONSERT works by 'sounding' the comet's nucleus. This involves beaming radio waves through the comet from Philae to the orbiter.
"We know the characteristics of the signal used for the sounding. After propagation through the nucleus the signal is received by Rosetta and the way it has been modified provides information about the nucleus' interior," says Valérie Ciarletti, LATMOS, Guyancourt, France, and co-investigator on CONSERT.
This diagram shows the propagation of signals between Rosetta and Philae through the nucleus of Comet 67P/Churyumov-Gerasimenko, between 12 and 13 November 2014. Green represents the best signal quality, decreasing in quality to red for no signal. Credit: ESA/Rosetta/Philae/CONSERT
This image shows the path on the comet’s surface where waves transmitted by Philae during the First Science Sequence, between 12 and 13 November 2014, emerged from the nucleus. Green represents the best signal quality, decreasing in quality to red for no signal. Credit: ESA/Rosetta/Philae/CONSERT
As the waves pass through the comet they are slowed down by the ice and dust they encounter. When they are detected by Rosetta, the delay in their transmission and the way the radio pulse's shape has changed allows the team to compare the results with theoretical models of the comet interior to see which fits the data best.
This work shows that the comet's low density might be an intrinsic property of the material that makes up the interior. For example, instead of being a compacted solid, the dust in the interior is 'fluffy'. This fits with the kind of dust grains that Rosetta's COSIMA and GIADA instruments have detected being given off by the comet.
CONSERT's results are backed up by those of RSI. Situated on Rosetta, the instrument measures the distribution of mass inside the comet based upon the way the comet's gravity pulls the orbiting spacecraft. The movement is measured by changes in the frequency of the spacecraft's signals when they are received at Earth.
It is a manifestation of the Doppler effect, which is produced whenever there is movement between a source and an observer. This same effect causes emergency vehicle sirens to change pitch as they pass by.
The variations in Rosetta's signals were analysed to give a picture of the gravity field across the comet. Large internal caverns would have been noticeable by a tell-tale drop in the spacecraft's acceleration. Again, no such signal was recorded.
What does the comet's shape tell us?
When the strange shape of the comet was first seen by Rosetta, scientists began asking whether the two lobes of 67P/C-G started as two different comets that collided at low speed and stuck together, or if it is the result of erosion caused by the nucleus activity trigged when the nucleus gets closer to the Sun.
Images from the OSIRIS camera show the two lobes appear very different and so are almost certainly two different bodies. On-going analysis with CONSERT data is further investigating this idea.
The instrument was active during Philae's descent and the scientists measured not only the direct signal between Philae and Rosetta, but also its reflection off the surface of both lobes. Current work is aiming at an accurate analysis of the reflection signal. This will allow the surface properties of each lobe to be compared, which in turn will provide further insight into whether or not these two lobes originated as separate entities.
The lander was identified at the far right of the image, just above centre. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
Now that Philae's location is known, it confirms the scientists' suspicion that all CONSERT data received so far has been propagating only through the comet's smaller lobe. This could well be because Philae came to rest on its side and so the antenna was not in the expected orientation, namely sitting horizontal to the surface. Being on its side has meant that there is a mismatch between the antennas on Philae and Rosetta, hence the signal was probably not strong enough to pass through the bulk of the larger lobe as well.
"Now that we know precisely where Philae is and, most of all, its attitude as well as the very close environment, we are able to simulate the whole radio link from Philae to Rosetta. We might understand if the antenna's configuration is the explanation for the non-detection or if we need to also consider attenuation inside the larger lobe’s nucleus to explain why we did not detect any signal," says Ciarletti.
Probing the surface layers
The effect of the Sun's heat has had a major impact at the surface layer of the comet. Philae has already shown that the surface of the small lobe is different from the interior. The propagation of the radio waves from CONSERT is driven by the permittivity of the nucleus, which is an electrical property governed by the comet’s composition, porosity and temperature. Philae also measured the surface permittivity using its Surface Electric Sounding and Acoustic Monitoring Experiment Permittivity Probe (SESAME-PP) instrument.
This shows that at Abydos, the location where Philae has come to rest, the top metre of the surface has a significantly higher permittivity than the interior, which is consistent with a lower porosity at the surface. This indicates that the surface layers are more compacted than the interior.
There is even more science to come as Rosetta is guided to its controlled impact with the comet on 30 September. The radio link to Earth will mean that RSI continues collecting data right to the very end. The closer the spacecraft gets the comet, the deeper in its gravity field it will be and the better the precision of the readings.
Thanks to the Rosetta and Philae mission we now know 67P/C-G better than any other comet – both inside and out.
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Insights from the Rosetta mission into what lies beneath the comet's surface were presented by Valérie Ciarletti during today's science briefing at the European Space Operations Centre (ESOC) in Darmstadt, Germany.
from Rosetta – ESA's comet chaser http://ift.tt/2cYnsJb
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Science highlights briefing starting soon
The Rosetta science highlight briefing at the European Space Operations Centre (ESOC) in Darmstadt, Germany, will start shortly.
Tune in from 14:30 to the livestream viewer at rosetta.esa.int or via http://ift.tt/2d6RQlU or ESA's Facebook page to follow dedicated talks celebrating the scientific highlights of the mission.
Programme overview
- Matt Taylor (ESA’s Rosetta Project Scientist): Introduction
- Mohamed El-Maarry (OSIRIS team, University of Bern): Landscapes of Chury
- Valerie Ciarletti (CONSERT team, Universités Paris-Saclay): Getting the ground truth about the nucleus
- Thurid Mannel (MIDAS team, University of Graz): Dust under the microscope
- Jean-Baptiste Vincent (OSIRIS team, Max-Planck Institute for Solar Physics, Göttingen): Cometary activity and fireworks
- Andre Bieler (ROSINA team, University of Bern/University of Michigan): Comet activity variation and evolution
- Charlotte Goetz (RPC team, Institute for Extra-terrestrial Physics, TU Braunschweig): The singing comet
- Cecila Tubiana (OSIRIS team, Max-Planck Institute for Solar Physics, Göttingen): Rosetta’s link to Earth
- Kathrin Altwegg (ROSINA team, University of Bern): The cometary zoo
- Björn Davidsson (Asteroids, Comets and Satellites Group, JPL): Formation of our Solar System
- Matt Taylor: Final comments and close
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#LivingWithAComet blog post series – summary
Find out from Rosetta's instrument teams what it was really like "living with a comet" for two years. The blog posts include anecdotes from the teams including challenges overcome and 'scares' the instruments gave them, as well as the scientific highlights and some impressive data collection statistics along the way!
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