SCIENCE ON THE SURFACE OF A COMET
Complex molecules that could be key building blocks of life, the daily rise and fall of temperature, and an assessment of the surface properties and internal structure of the comet are just some of the highlights of the first scientific analysis of the data returned by Rosetta’s lander Philae last November. This article is mirrored from the main ESA Web Portal.
Images taken by Philae’s ROsetta Lander Imaging System, ROLIS, trace the lander’s descent to the first landing site, Agilkia, on Comet 67P/Churyumov–Gerasimenko on 12 November 2015.
Early results from Philae’s first suite of scientific observations of Comet 67P/Churyumov-Gerasimenko were published today in a special edition of the journal Science.
Data were obtained during the lander’s seven-hour descent to its first touchdown at the Agilkia landing site, which then triggered the start of a sequence of predefined experiments. But shortly after touchdown, it became apparent that Philae had rebounded and so a number of measurements were carried out as the lander took flight for an additional two hours some 100 m above the comet, before finally landing at Abydos.Some 80% of the first science sequence was completed in the 64 hours following separation before Philae fell into hibernation, with the unexpected bonus that data were ultimately collected at more than one location, allowing comparisons between the touchdown sites.
A timeline of the science operations that Rosetta’s lander Philae performed between 12 and 15 November 2015, following touchdown on the surface of Comet 67P/Churyumov–Gerasimenko. Following Philae’s unexpected flight across the surface of the comet, the planned first science sequence had to be adapted according to the new situation. The graphic shows the approximate times (to the nearest 15 minutes) that each of Philae’s 10 instruments was activated; however, it does not indicate the success of data acquired.
After the first touchdown at Agilkia, the gas-sniffing instruments Ptolemy and COSAC analysed samples entering the lander and determined the chemical composition of the comet’s gas and dust, important tracers of the raw materials present in the early Solar System.
COSAC analysed samples entering tubes at the bottom of the lander kicked up during the first touchdown, dominated by the volatile ingredients of ice-poor dust grains. This revealed a suite of 16 organic compounds comprising numerous carbon and nitrogen-rich compounds, including four compounds – methyl isocyanate, acetone, propionaldehyde and acetamide – that have never before been detected in comets.
Meanwhile, Ptolemy sampled ambient gas entering tubes at the top of the lander and detected the main components of coma gases – water vapour, carbon monoxide and carbon dioxide, along with smaller amounts of carbon-bearing organic compounds, including formaldehyde.
Importantly, some of these compounds detected by Ptolemy and COSAC play a key role in the prebiotic synthesis of amino acids, sugars and nucleobases: the ingredients for life. For example, formaldehyde is implicated in the formation of ribose, which ultimately features in molecules like DNA.
The existence of such complex molecules in a comet, a relic of the early Solar System, imply that chemical processes at work during that time could have played a key role in fostering the formation of prebiotic material.
Zooming in to a portion of the fractured cliff face imaged by CIVA camera 4 reveals brightness variations in the comet’s surface properties down to centimetre and millimetre scales. The dominant constituents are very dark conglomerates, likely made of organics. The brighter spots could represent mineral grains, perhaps even pointing to ice-rich materials. The left hand image shows one of the CONSERT antennas in the foreground, which seems to be in contact with the nucleus. The dimensions of the antenna, 5 mm in diameter and 693 mm long, help to provide a scale to the image. Credits: ESA/Rosetta/Philae/CIVA
This image, created from Philae’s ROLIS descent camera, focuses on the largest boulder seen in the image captured at 67.4 m above Comet 67P/Churyumov–Gerasimenko. It is best viewed with red/blue–green glasses. The 3D view highlights the fractures in the 5 m-high boulder, along with the tapered ‘tail’ of debris and excavated ‘moat’ around it. Credits: ESA/Rosetta/Philae/ROLIS/DLR
Comparing touchdown sites
Thanks to the images taken by ROLIS on the descent to Agilkia, and the CIVA images taken at Abydos, a visual comparison of the topography at these two locations could be made.
ROLIS images taken shortly before the first touchdown revealed a surface comprising metre-size blocks of diverse shapes, coarse regolith with grain sizes of 10–50 cm, and granules less than 10 cm across.
The regolith at Agilkia is thought to extend to a depth of 2 m in places, but seems to be free from fine-grained dust deposits at the resolution of the images.
The largest boulder in the ROLIS field-of-view measures about 5 m high, with a peculiar bumpy structure and fracture lines running through it that suggest erosional forces are working to fragment the comet’s boulders into smaller pieces.
The boulder also has a tapered ‘tail’ of debris behind it, similar to others seen in images taken by Rosetta from orbit, yielding clues as to how particles lifted up from one part of the eroding comet are deposited elsewhere.
Over a kilometre away at Abydos, not only did the images taken by CIVA’s seven microcameras reveal details in the surrounding terrain down to the millimetre scale, but also helped decipher Philae’s orientation.
The lander is angled up against a cliff face that is roughly 1 m from the open ‘balcony’ side of Philae, with stereo imagery showing further topography up to 7 m away, and one camera with open sky above.
The images reveal fractures in the comet’s cliff walls that are ubiquitous at all scales. Importantly, the material surrounding Philae is dominated by dark agglomerates, perhaps comprising organic-rich grains. Brighter spots likely represent differences in mineral composition, and may even point to ice-rich materials.
Anaglyph view created from the stereo pair of images acquired by CIVA cameras 6 and 7 at the final landing site Abydos on 13 November 2014. The topography occupying the foreground and left hand portion of the image is estimated to be 0.8–1.2 m from the lander’s body. To the top right, the topography is likely 1.2–2 m away. In the background, towards the top left, one unit may be at least 4.5 m away, and perhaps up to 7 m away.
From the surface to the interior
The MUPUS suite of instruments provided insight into the physical properties of Abydos. Its penetrating ‘hammer’ showed the surface and subsurface material sampled to be substantially harder than that at Agilkia, as inferred from the mechanical analysis of the first landing.
The results point to a thin layer of dust less than 3 cm thick overlying a much harder compacted mixture of dust and ice at Abydos. At Agilkia, this harder layer may well exist at a greater depth than that encountered by Philae.
The MUPUS thermal sensor, on Philae’s balcony, revealed a variation in the local temperature between about –180ºC and –145ºC in sync with the comet’s 12.4 hour day. The thermal inertia implied by the measured rapid rise and fall in the temperature also indicates a thin layer of dust atop a compacted dust-ice crust.
Moving below the surface, unique information concerning the global interior structure of the comet was provided by CONSERT, which passed radio waves through the nucleus between the lander and the orbiter. The results show that the small lobe of the comet is consistent with a very loosely compacted (porosity 75–85%) mixture of dust and ice (dust-to-ice ratio 0.4–2.6 by volume) that is fairly homogeneous on the scale of tens of metres.
This diagram shows the propagation of signals between Rosetta and Philae through the comet’s nucleus, between 12 and 13 November 2014. Green represents the best signal quality, decreasing in quality to red for no signal. The signals are sent and received by the CONSERT instrument, which is on both the orbiter and the lander. The time taken for the signal to travel between the instruments, and the amplitude of the received signal offers insights into the structure of the comet’s nucleus. In particular, the travel time depends on a parameter called permittivity, which is itself linked to the nucleus porosity, composition, temperature and internal structure of the comet. The permittivity value is approximately 1.27. Credits: ESA/Rosetta/Philae/CONSERT
In addition, CONSERT was used to help triangulate Philae’s location on the surface, with the best fit solution currently pointing to a 21 x 34 m area.
“Taken together, these first pioneering measurements performed on the surface of a comet are profoundly changing our view of these worlds and continuing to shape our impression of the history of the Solar System,” says Jean-Pierre Bibring, a lead lander scientist and principal investigator of the CIVA instrument at the IAS in Orsay, France.
“The reactivation would allow us to complete the characterisation of the elemental, isotopic and molecular composition of the cometary material, in particular of its refractory phases, by APXS, CIVA-M, Ptolemy and COSAC.”
“With Philae making contact again in mid-June, we still hope that it can be reactivated to continue this exciting adventure, with the chance for more scientific measurements and new images which could show us surface changes or shifts in Philae’s position since landing over eight months ago,” says DLR’s Lander Manager Stephan Ulamec.
“These ground-truth observations at a couple of locations anchor the extensive remote measurements performed by Rosetta covering the whole comet from above over the last year,” says Nicolas Altobelli, ESA’s acting Rosetta project scientist.
“With perihelion fast approaching, we are busy monitoring the comet’s activity from a safe distance and looking for any changes in the surface features, and we hope that Philae will be able to send us complementary reports from its location on the surface.”
The 31 July 2015 Science special issue includes the following papers:
“The nonmagnetic nucleus of comet 67P/Churyumov–Gerasimenko,” by H.-U. Auster et al.
“67P/Churyumov-Gerasimenko surface properties as derived from CIVA panoramic images,” by J-P. Bibring et al.
“The landing(s) of Philae and inferences about comet surface mechanical properties,” by J. Biele et al.
“Organic compounds on comet 67P/Churyumov-Gerasimenko revealed by COSAC mass spectrometry,” by F. Goesmann et al.
“Properties of the 67P/Churyumov–Gerasimenko interior revealed by CONSERT radar,” by W. Kofman et al.
“The structure of the regolith on 67P/ Churyumov–Gerasimenko from ROLIS descent imaging,” by S. Mottola et al.
“Thermal and mechanical properties of the near-surface layers of comet 67P/Churyumov–Gerasimenko,” by T. Spohn et al.
“CHO-bearing organic compounds at the surface of 67P/Churyumov–Gerasimenko revealed by Ptolemy,” by I.P. Wright et al.
Individual ROLIS and CIVA images are available via our "Landing on a comet" gallery.
Based on the most recent calculations using CONSERT data and detailed comet shape models, Philae’s location has been revised to an area covering 34 x 21 m. The best fit area is marked in red, a good fit is marked in yellow, with areas on the white strip corresponding to previous estimates now discounted. One lander candidate proposed previously in the vicinity lies 62 m from the red marked area of the new CONSERT region, suggesting this is no longer a viable candidate. Credits: ESA/Rosetta/Philae/CONSERT
Philae results shed light on the nature of comets
Landing points (SONC) on a NAVCAM image are shown. Note that a second TD2, here taken at 17:24 (observed time: 17:25:26) is not modeled by RMOC and here only an example is given, since it is not well constrained. We expect that the last hop was only a few meters.
During the first ever landing of a probe on a comet, the world held its breath as Philae survived a bouncy landing on comet 67P/Churyumov-Gerasimenko on November 12, 2014. This special issue of Science highlights seven new studies that delve further into the data that has been transmitted back by Philae.
In a detailed account, Jens Biele et al. describe the critical moments where Philae descends on 67P, only to bounce off the soft, intended landing area and finally settle on a harder surface farther away.
Analysis of the different compressive strengths of the two surfaces, based on the bouncing trajectory, sheds more light on the evolution of comets and could improve the design of future comet missions. Previously, scientists trying to understand comet surface material strength had to rely on indirect observations, which have ranged widely, and include some very low values that have raised questions about whether a comet could successfully dock on such weak material.
After analyzing the depth profile of the lander footprint features with imaging tools, Biele and colleagues believe that the Philae's feet first came in contact with a soft granular surface, which was approximately 0.25 meters (0.82 feet) thick, with a harder layer below.
This layering creates a compressive strength of about 1 kilopascal, whereas the compression strength of Philae's final, much harder landing site exceeded 2 megapascals (2,000 kilopascals), possibly contributing to why only one leg was able to anchor to this latter surface, and partially at that.
In a research article by Wlodek Kofman et al., the team found the composition of the head of the comet to be fairly homogenous. To get a better idea of 67P's interior, the team directed electromagnetic signals through the nucleus of the comet to Rosetta on the opposite side. The signals Rosetta received lacked a scattering pattern, indicating that the interior of the comet is uniform throughout.
The team used these electromagnetic measurements, which analyze the permittivity (resistance of the electrical field), to further determine that 67P has dust/ice ratio of 0.4 to 2.6 and a very high porosity of 75 to 85%. Research by Fred Goesmann and colleagues further analyzes the composition of 67P using the COmetary SAmpling and Composition (COSAC) instrument, designed to identify organic compounds in the comet and thus contribute to a deeper understanding of the history of life on Earth (some scientists think comets delivered to Earth materials that were important for its chemical and biological evolution).
The device collected molecules from 10 kilometers (6.2 miles) above the comet surface, after the initial touchdown, and at the final resting site. This process detected 16 organic compounds, four of which - methyl isocyanate, acetone, propionaldehyde, and acetamide - were previously unknown to exist on comets.
In a related study, Ian Wright et al. also analyzed organic compounds on 67P but used Ptolemy, an instrument that measures stable isotope ratios. Their measurements indicate the presence of a radiation-induced polymer on the surface of the comet. Ptolemy measurements also indicate an absence of aromatic compounds, such as benzene, on the comet.
In a study by Jean-Pierre Bibring and colleagues, the surface of 67P is analyzed in panoramic images taken by a set of seven cameras as part of the Comet Infrared and Visible Analyser (CIVA). The collection of images, taken just after Philae's initial bounce and final touchdown, reveal a fractured surface with a variety of grain scales and reflective rock structures, offering unprecedented insights into this type of primitive space matter.
As Philae approached 67P, far-field sequence and the near-field sequence images revealed a clearer picture of the comet's geography; an analysis of Rosetta Lander Imaging System (ROLIS) descent images by Stefano Mottola et al. suggests that 67P's landscape is shaped by erosion.
Boulders jutting out of granular areas are surrounded by depressions that are reminiscent of the wind tails observed on Earth, the result of wind erosion and deposition. The authors speculate that some of the erosion occurs from "splashing", the ejection of one or more soil particles by the impact of an incoming projectile, which they confirmed using models.
Finally, to determine 67P's thermal and mechanical properties, Tilman Spohn and colleagues analyzed data from Multi Purpose Sensors for Surface and Subsurface Science (MUPUS) thermal and penetrating sensors on board Philae. Due to the lander's unintentional final resting spot, the sensors were unable to penetrate the hard surface to attain subsurface temperature readings; however, the data reveals that 67P's daytime surface temperature varies between 90 and 130 Kelvin.
By analyzing thermal inertia and soil composition, the team found that the surface at the final landing spot is covered with a highly compact, microporous, dust-ice layer with a porosity of 30 to 65%. Collectively, the unique data that Philae has sent back brings us closer than ever before to understanding the nature of comets.
Churyumov-Gerasimenko: Fast wie Firn und geformt im Hagel
Blick aus 67,4 Metern Höhe auf Kometen
Bereits während des langsamen Abstiegs von Lander Philae am 12. November 2014 auf Komet Churymov-Gerasimenko starteten die ersten Instrumente an Bord mit Messungen. Drei Mal setzte Philae im Laufe der spektakulären ersten Landung auf einem Kometen auf, streifte noch einen Kraterrand und kam schließlich um 18.31 Uhr mitteleuropäischer Zeit am ungeplanten Landeplatz Abydos zur Ruhe. "Wir hätten es wohl nie gewagt, eine Landung in einem so rauen Gelände wie Abydos zu versuchen“, erläutert Philae-Projektleiter Dr. Stephan Ulamec vom Deutschen Zentrum für Luft- und Raumfahrt (DLR). Doch so konnten die Instrumente an zwei unterschiedlichen Stellen des Kometen messen. Und was Philae seinen Wissenschaftlern am Boden mit den ersten jemals auf einer Kometenoberfläche erfassten Daten mitteilte, bestätigte zum Teil das bisherige Bild von Kometen - zum Teil ließ er die Wissenschaftler aber auch staunen: Eine Oberfläche, die mit grobem Material bedeckt ist, ein überraschend harter Untergrund, der es der Hammersonde MUPUS schwer machte, und Moleküle, wie man sie bisher noch nicht in der Umgebung von Kometen festgestellt hatte. "Die Experimente direkt vor Ort haben zu neuen, zum Teil unerwarteten Einsichten in die Natur des Kometen geführt“, fasst DLR-Planetenforscher Dr. Ekkehard Kührt, der den wissenschaftlichen DLR-Anteil an der Mission betreut, zusammen. "Manches lässt sich eben nur messen, wenn man ganz nah dran ist.“ In einer Sonderausgabe von "Science“ werden diese Ergebnisse am 31. Juli 2015 veröffentlicht.
Grober Schutt und Ablagerungen
Den ersten - und besten - Ausblick auf Komet Churyumov-Gerasimenko hatte die Kamera ROLIS (Rosetta Lander Imaging System), die an der Unterseite des Landers Philae montiert ist, und bereits im Landeanflug alle zehn Sekunden ein Bild der Kometenoberfläche aufnahm. "Niemals zuvor wurde eine kometare Oberfläche in einer so hohen Auflösung von bis zu einem Zentimeter pro Bildpunkt aufgenommen“, erläutert Dr. Stefano Mottola vom DLR-Institut für Planetenforschung. Die Kamera blickte am ersten Landeplatz Agilkia dabei nicht auf die erwarteten Staub-Ablagerungen, sondern vielmehr auf eine Oberfläche mit grobem Schutt, Kies und Felsen mit Abmessungen von einigen Zentimetern bis zu fünf Metern. Die Regolith-Partikel der Kometenoberfläche sind dunkel und absorbieren in einem hohen Maß das Licht. Zudem ist die Verteilung der Partikel-Größen auf dem Kometen identisch mit der Verteilung der vom Kometen ins All geschleuderten Partikel: "Es gibt also einen wechselseitigen Austausch von Partikeln zwischen Oberfläche und Koma, das heißt der Gas- und Staubhülle des Kometen“, erläutert der wissenschaftliche Leiter der ROLIS-Kamera, Stefano Mottola.
An Hindernissen wie dem Fünf-Meter-Brocken nahe Agilkia bilden sich zudem kleine "Wind tails“, das sind Anhäufungen, die der Wind nicht abgetragen hat - vergleichbar mit Formationen auf dem Mars. "Auf Churyumov-Gerasimenko geschieht dies allerdings überraschenderweise ohne Wind und Atmosphäre - es muss ein anderer Mechanismus dafür verantwortlich sein.“ Eine Ursache für diese "Wind tails“ auf dem Kometen sind höchstwahrscheinlich Staub und gröbere Teilchen, die bei den Gas-Ausstößen des Kometen mitgerissen werden, in größerer Entfernung wieder zurückfallen und wie ein "Partikel-Hagel“ die Oberfläche abschleifen - es sei denn, es besteht ein Schutz beispielsweise hinter größeren Brocken. Dies berechneten die DLR-Planetenforscher mit Computermodellen. "Die Beobachtungen mit der ROLIS-Kamera deuten darauf hin, dass es auf Kometen großflächige Bewegungen des Materials gibt, die durch die Kometen-Aktivitäten ausgelöst werden.“
Bodenhärte wie irdischer Firn
Auch die vergeblichen Versuche der Hammersonde des Experiments MUPUS (Multi-Purpose Sensors for Surface and Sub-Surface Science), sich in den Kometenboden zu hämmern, ergeben für die Wissenschaftler wertvolle Daten: "Wir sind auf eine deutlich härtere Oberfläche gestoßen, als wir uns vorgestellt haben“, sagt Prof. Tilman Spohn, Planetenforscher am DLR und wissenschaftlicher Leiter des MUPUS-Teams. Unter einer vermutlich wenige Zentimeter dünnen Staubschicht stieß die Sonde auf poröses, aber dennoch festes Eis. "Ähnlich wie Firn auf der Erde, also wie alter, fester Schnee, der verdampft und wieder gefroren wurde.“ Bei diesen Sinterprozessen "verbacken“ zuvor lockere Bestandteile miteinander. Vergleichbar ist das Material auch mit Glasschäumen, die in der Bau-Industrie zum Dämmen verwendet werden. "Vielleicht kann man es als die größte Überraschung des Kometen bezeichnen, dass Abydos einen so harten Boden hat“, schätzt DLR-Planetenforscher Tilman Spohn. "Die gemessenen vier Megapascal sind der höchste Wert, der je für Kometen gemessen wurde.“
Die Temperaturmessungen, die MUPUS vorgenommen hat, liegen zwischen minus 180 und minus 140 Grad Celsius - und bestätigen die Erwartungen. "Wir sind ja auch in einer dunklen, kalten Ecke gelandet.“ Die Wärmeleitfähigkeit des Bodens, das heißt das Vermögen, Wärmeenergie durch das Material zu leiten, scheint etwas höher zu sein als vermutet - liegt aber im Rahmen dessen, was die Wissenschaftler von Kometen und Asteroiden erwarten. Abydos könnte eventuell eine etwas staubigere oder porösere Oberfläche haben als der übrige Komet.
Abprallen statt Versinken
Die erstaunliche Festigkeit des Kometen bestätigt auch das Team des Lander-Kontrollzentrums des DLR in Köln. "Man kann diese Festigkeit der Oberfläche nicht aus der Ferne messen, sondern muss tatsächlich vor Ort sein“, sagt Dr. Jens Biele vom DLR-Nutzerzentrum für Weltraumexperimente (MUSC). Vor der Landung auf Churyumov-Gerasimenko war unter den Kometenforschern die fast einhellige Meinung, der Lander würde auf recht weichen Kometenboden treffen. "Stattdessen sind wir mehrfach abgeprallt, nachdem die Harpunen des Landers nicht auslösten, um den Lander im Boden zu verankern.“ Mit den Daten des Landers und den Daten der bereits im Flug aktiven Instrumente rekonstruierte ein Team um DLR-Wissenschaftler Jens Biele den Weg, den Philae nach dem ersten Abprallen genommen hat sowie die Festigkeit des ersten Landeplatzes.
Fehlende Magnetisierung und unerwartete Moleküle
Die Bildung von Kometen verstehen die Wissenschaftler beispielsweise durch ROMAP (Rosetta Lander Magnetometer and Plasma Monitor) nun besser: Das Instrument registrierte bereits während des Abstiegs auf den Kometen, dass dieser kein eigenes, messbares Magnetfeld besitzt. "Das bestätigt uns, dass bei der Bildung von Kometen aus dem solaren Nebel die vorhandenen Magnetfelder nicht stark genug waren, um die einzelnen Staubteilchen magnetisch auszurichten und im Kometenmaterial eine dauerhafte Magnetisierung zu erzeugen“, erläutert DLR-Kometenforscher Dr. Ekkehard Kührt. "Keine überraschende, aber eine wichtige Erkenntnis für die Entstehungsmodelle.“
Das Instrument COSAC (Cometary Sampling and Composition) hat in die Vergangenheit des Sonnensystems “hineingeschnüffelt". Eventuell könnten sogar kleine Staubteilchen in die an der Bodenplatte des Landers installierten Rohre gelangt sein und dort ausgegast haben. Insgesamt 16 organische Molekülarten spürte COSAC auf - darunter vier, die noch nie zuvor in Kometen festgestellt wurden. "Einige davon sind präbiotische Moleküle, also Bestandteile, die eine Rolle bei der Entstehung von Leben spielen“, sagt Dr. Stephan Ulamec, DLR-Wissenschaftler und Mitglied im COSAC-Team.
Kometenwand von Nahem
Neben der ROLIS-Kamera war zudem mit CIVA (Comet Infrared and Visible Analyzer) ein Panorama-Kamerasystem im Einsatz, dass die direkte Umgebung von Philae an seinem eher ungemütlichen Landeplatz abbildete. Der "Rundumblick“ von CIVA sieht unter anderem auf die Risse in der Kometenwand, an der der Lander nun steht. Dabei zeigt das Kometenmaterial eine komplexe Struktur und unterschiedliche Korngrößen. Erkennbar sind auch weiße Flecken, die auf eine unterschiedliche Zusammensetzung hinweisen.
Wissen für zukünftige Missionen
Der Verlauf der Landung und die Messungen der verschiedenen Instrumente werden nicht nur unsere Vorstellung von Kometen verändern und auf den neuesten Stand bringen, sondern auch bei der Planung zukünftiger Missionen helfen. So besteht beispielsweise bei der NASA Interesse, das Wissen über den Kometen und seine Oberfläche für geplante, zukünftige Kometenmissionen zu verwenden. "Wir haben auf jeden Fall eines mit dieser ersten Kometenlandung gelernt: Das Abprallen ist ein größeres Problem als das mögliche Versinken im Boden“, sagt Philae-Projektleiter Dr. Stephan Ulamec.
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.
Fünf-Meter-Brocken in 3D
Perspektivische Ansicht der Landestelle Agilkia
FIRST RELEASE OF ROSETTA COMET PHASE DATA FROM FOUR ORBITER INSTRUMENTS
ESA’s Rosetta downlink and archive teams are very happy to announce the release today of the first wave of Rosetta instrument data from the “comet pre-landing phase” via the Planetary Science Archive. Data from four instruments are included in this release: COSIMA, OSIRIS, ROSINA, and RPC-MAG.
The Rosetta orbiter will analyse comet 67P/Churyumov-Gerasimenko and its environment using a suite of 11 instruments:
ALICE: Ultraviolet Imaging Spectrometer – (characterising the composition of the comet nucleus and coma)
CONSERT: Comet Nucleus Sounding Experiment by Radio wave Transmission (studying the internal structure of the comet with lander Philae)
COSIMA: Cometary Secondary Ion Mass Analyser (studying the composition of the dust in the comet’s coma)
GIADA: Grain Impact Analyser and Dust Accumulator (measuring the number, mass, momentum and velocity distribution of dust grains in the near-comet environment)
MIDAS: Micro-Imaging Dust Analysis System (studying the dust environment of the comet)
MIRO: Microwave Instrument for the Rosetta Orbiter (investigating the nature of the cometary nucleus, outgassing from the nucleus and development of the coma)
OSIRIS: Optical, Spectroscopic, and Infrared Remote Imaging System Camera (a dual camera imaging system consisting of a narrow angle (NAC) and wide angle camera (WAC) and operating in the visible, near infrared and near ultraviolet wavelength range)
ROSINA: Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (determining the composition of the comet's atmosphere and ionosphere, and measuring the temperature, velocity and density of the gas flow, comprising: DFMS (Double-focusing mass spectrometer), RTOF (Reflectron Time-Of-Flight mass spectrometer) and COPS (Comet Pressure Sensor))
RPC: Rosetta Plasma Consortium (studying the plasma environment of the comet, comprising: ICA (Ion Composition Analyser), IES (Ion and Electron Sensor), LAP (Langmuir Probe), MAG (Fluxgate Magnetometer), MIP (Mutual Impedance Probe), PIU (Plasma Interface Unit))
RSI: Radio Science Investigation (tracking the motion of the spacecraft to infer details of the comet environment and nucleus)
VIRTIS: Visible and Infrared Thermal Imaging Spectrometer (studying the nature of the comet nucleus and the gases in the coma)
After Rosetta woke up from 31 months hibernation in January 2014, its scientific instruments were turned back on and checked out before being used to study Comet 67P/Churyumov-Gerasimenko during the approach, rendezvous, and escort phases.
As agreed between ESA and the funding bodies responsible for Rosetta’s instruments, their data are initially made available to the corresponding scientific teams for first analysis, with a significant number of Rosetta papers published based on these data, as described in previous blog posts.
After a nominal period of 6 months, the scientific data themselves are to be delivered to ESA to be placed in the public domain via the Planetary Science Archive (PSA) for use by all scientists and the wider public. In practice, it was agreed to deliver data to ESA for release in blocks, and the first of those blocks was defined as the ‘pre-landing phase’, i.e. covering the period from January 2014 to just after Philae’s landing on the comet in mid-November 2014.
This led to a date of 19 May for the instrument teams to deliver the data from that phase to ESA, following which, a very significant effort had to be made by the ESA team to process the datasets of the many instruments involved and prepare them for release.
What did this processing entail? Firstly, the data and the associated metadata had to be checked for completeness and for compliance with standard formats to ensure that they can be downloaded and analysed by other scientists in a transparent manner.
This involved interactions between all of the instrument teams, the ESAC-based downlink and archive teams, and the Planetary Data System (PDS – Small Bodies Node) team located in the US to ensure the data format and contents complied with the PDS standards.
These complex checks and interactions have taken quite some time, in part because this was the first major data delivery from the comet phase to ESA from the whole instrument suite. For some instruments, the datasets are vast, with up to 8 months of scientific measurements, pushing processing systems to the limit. In the majority of cases, this procedure has necessitated updates to an instrument’s data pipeline, resulting in the need for the data to be re-processed, re-delivered, and checked once more.
Today sees the first release of pre-landing phase data from four of Rosetta’s instruments, representative of the wide scientific scope of the orbiter. COSIMA collects and analyses dust grains around the comet; OSIRIS uses its Narrow and Wide Angle Cameras to take multi-wavelength visible and near-infrared images of the nucleus, activity rising from its surface, and the immediate coma; ROSINA has two mass spectrometers to sniff and analyse the gases and RPC-MAG studies the magnetic field in the environment of the comet,
The data being released are described in more detail below:
COSIMA (COmetary Secondary Ion Mass Analyser):
This dataset includes images of dust particles collected in the environment of Comet 67P/C-G from the nucleus approach phase until 19 November 2014, along with secondary ion mass spectra for some of those particles.
OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System)
This dataset covers the first part of the pre-landing phase from 20 March to 12 June 2014, comprising four sets each for the Narrow Angle Camera and the Wide Angle Camera, and consisting of 2203 images in total. It contains the early light curve observations used to make a precise determination of rotation period and orientation of the rotation axis of Comet 67P/C-G. It also includes the outburst observed in April/May 2014 and the development of the dusty coma around the cometary nucleus. The data were processed with the new, re-developed OSIRIS calibration pipeline including recent updates from the in-flight calibration campaigns.
ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis)
This dataset covers the period March to November 2014, and contains 100 Gbytes of data consisting of 685,234 tables and 1.3 billion rows. These are currently “level 2” data, meaning more or less as received from the spacecraft, followed by decompression and the addition of metadata including the distance to the comet, direction of the Sun, and the spacecraft pointing. Future releases will be at a higher, more directly usable level containing mass spectra with physical units (e.g. detected particles vs. mass); further work is necessary to ensure that this is achieved in a consistent manner.
RPC-MAG (Rosetta’s Plasma Consortium MAGnetometer)
This dataset contain time series of the magnetic field measurements made in situ on Rosetta. RPC-MAG comprises two tri-axial fluxgate magnetometers that are able to register the three components of the magnetic field vector at a maximum sampling rate of 20 Hz. Observations have been made quasi permanently since May 2014, and the dataset covers from then until 19 November. Further processing of the data is on-going in order to reduce contamination due to changing spacecraft bias fields.
OSIRIS images added to Archive Image Browser
In addition to the data being released today to the PSA, the downlink and archive team is pleased to release an update of the Archive Image Browser. As well as images from Rosetta’s NAVCAM, this “easy access tool” now includes OSIRIS images from the Earth, Mars, and asteroid fly-bys that occurred between 2005 and 2010 en-route to the comet, and a “first processed” version of the images from today’s pre-landing phase release. Further processing of the OSIRIS pre-landing images in the image browser will be performed in the mid-late August timeframe.
Screenshot from ESA's Archive Image Browser showing OSIRIS cruise phase albums
Upcoming data deliveries
While it has taken quite some time to process and release all the pre-landing datasets delivered, we anticipate that future processing will be more efficient and timely. The Philae lander instrument teams have delivered the data that formed the basis of the seven papers published in Science last week, and the next comet phase data delivery from the orbiter instrument teams is planned for mid-September (covering 19-Nov to 10-Mar 2015). With the corrected pipelines now producing data formats consistent with the archive standards, we expect to be able to archive and release those data more rapidly thereafter. The aim is to have the current Rosetta and Philae datasets released by mid-September.
It is important to realise that even after today’s first release, this process remains a work-in-progress. As more is learned about the calibration of the various instruments from in-flight measurements, and as feedback is received from the instrument teams and external users, these data may be re-processed and released again into the archives. This is standard practice and ensures that the data found in the archives always reflect the current best understanding of the complex instruments that produced them. A case in point is the current OSIRIS dataset, where issues still remain with their pipeline: a second version will be released to the PSA by September to correct for such problems.
Beyond the formal validation steps, a scientific review of these Rosetta pre-landing data is scheduled to take place at the beginning of 2016. This review will assess the scientific usefulness of the data and may lead to updates being made to various instrument datasets to incorporate proposed improvements in this regard.
At a higher level, the goal of the activities described in this post is to ensure that the products entering the archive are well documented and remains compatible with changing computer hard- and software systems, so that they can be used far into the future,
For example, we can now retrieve archival data from Giotto’s mission to Comet Halley in 1986 and re-analyse it with Rosetta’s discoveries in mind. We are still finding valuable and surprising information in these old data, which in turn enhances the science return from Rosetta. But the prerequisite for this is to adhere to very strict rules: a difficult task but worthwhile in the long run to maximise the legacy of the Rosetta mission.
Seit einem Jahr bei Komet 67P/Churyumov-Gerasimenko
ESA’s Rosetta mission today celebrates one year at Comet 67P/Churyumov–Gerasimenko, with its closest approach to the Sun now just one week away.
It’s been a long but exciting journey for Rosetta since its launch in 2004, featuring Earth, Mars and two asteroid flybys before arriving at its ultimate destination on 6 August 2014. Over the following months, the mission became the first ever to orbit a comet and the first to soft land a probe – Philae – on its surface.
The mission teams have had to overcome many challenges in learning to fly in an unpredictable and sometimes inhospitable environment, and the spacecraft has returned a wealth of outstanding scientific data from this intriguing comet, spanning its interior, the dramatic surface and the surrounding cloud of dust, gas and plasma.
“This mission is about scientific discovery and every day there is something new to wonder at and try to understand,” says Nicolas Altobelli, acting Rosetta project scientist.
“A year of observations near to the comet has provided us with a wealth of information about it, and we’re looking forward to another year of exploration.”
Highlights thus far have included the discovery that the comet’s water vapour has a different ‘flavour’ to Earth’s oceans, fuelling the debate on the possible role of comets and asteroids in delivering water to our planet in its early history.
The first detection of molecular nitrogen in a comet provided important clues about the temperature environment in which the comet was ‘born’. Molecular nitrogen was common when the Solar System was forming, but required very low temperatures to become trapped in ice, so Rosetta’s measurements support the theory that comets originate from the cold and distant Kuiper Belt.
Data collected by Rosetta and Philae during the lander’s descent to the surface have allowed scientists to deduce that the comet’s nucleus is non-magnetised, at least on large scales.
Although magnetic fields are thought to have played an important function in moving small, magnetised dust grains around in the infant Solar System, the Rosetta and Philae measurements show that they did not continue to play a significant role once the particles had agglomerated to form larger building blocks metres and tens of metres across.
These are just a few of the myriad examples of the scientific discoveries being made by Rosetta, and most of them come from data taken in the early part of the comet-phase activities.
Now the comet and spacecraft are a week from perihelion, the point on its 6.5-year orbit that takes it closest to the Sun. On 13 August, they will be 186 million kilometres from the Sun, about a third of the distance at rendezvous last August.
COMETWATCH 30 JULY
Today’s spectacular CometWatch entry was taken by Rosetta’s NAVCAM on 30 July 2015 from a distance of 178 km from Comet 67P/Churyumov-Gerasimenko.
Comet 67P/C-G on 30 July (with contrasts enhanced) from a distance of 178 km. Credits: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
The image scale is 15.2 m/pixel and the image measures 15.6 km across. Although the activity is visible even in the original image (below), the contrast has been increased in the enhanced image above to better show off these details.
It is clear from the prominent jets of dust and gas streaming from the nucleus and out to the edge of the camera’s field of view that activity is becoming more intense – the comet is now just over a week from perihelion on 13 August, its closest approach to the Sun along its orbit.
But, while the increasing activity provides dramatic views, it can also lead to difficulties in navigation. Indeed, Rosetta’s star trackers have struggled to identify stars among the large amount of debris being ejected from the nucleus during the last week and is therefore currently moving to safer distances – it will be at about 300 km by Saturday.