Fortsetzung von Update3
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Site I is a relatively flat area on the smaller lobe that may contain some fresh material, but higher-resolution imaging is needed to assess the extent of the rough terrain. The illumination conditions should also allow for longer-term science planning.
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Site J
Site J is similar to site I, and also on the smaller lobe, offering interesting surface features and good illumination. It offers advantages for the CONSERT experiment compared with Site I, but higher-resolution imaging is needed to determine the details of the terrain, which shows some boulders and terracing.
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The next step is a comprehensive analysis of each of the candidate sites, to determine possible orbital and operational strategies that could be used for Rosetta to deliver the lander to any of them. At the same time, Rosetta will move to within 50 km of the comet, allowing a more detailed study of the proposed landing sites.
By 14 September, the five candidate sites will have been assessed and ranked, leading to the selection of a primary landing site, for which a fully detailed strategy for the landing operations will be developed, along with a backup.
During this phase, Rosetta will move to within 20–30 km of the comet, allowing even more detailed maps of the boulder distributions at the primary and backup landing sites to be made. This information could be important in deciding whether to switch from primary to backup.
The Rosetta mission team are working towards a nominal landing date of 11 November, but confirmation of the primary landing site and the date will likely only come on 12 October. This will be followed by a formal Go/No Go from ESA, in agreement with the lander team, after a comprehensive readiness review on 14 October.
“The process of selecting a landing site is extremely complex and dynamic; as we get closer to the comet, we will see more and more details, which will influence the final decision on where and when we can land,” says Fred Jansen, ESA Rosetta mission manager.
“We had to complete our preliminary analysis on candidate sites very quickly after arriving at the comet, and now we have just a few more weeks to determine the primary site. The clock is ticking and we now have to meet the challenge to pick the best possible landing site.”
Quelle: ESA
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Update: 27.08.2014
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Die Qual der Wahl: Fünf Kandidaten für Landeplätze auf Churyumov-Gerasimenko
Noch nie musste ein Missionsteam einen Landeplatz auf einem Kometen auswählen - Lander Philae wird das erste Gerät überhaupt sein, das auf einem Kometen aufsetzt und vor Ort Messungen durchführt. Und Zielkomet Churyumov-Gerasimenko, zu dem die ESA-Raumsonde Rosetta vor zehn Jahren mit Philae an Bord aufgebrochen ist, macht es dem Landerteam unter Leitung des Deutschen Zentrums für Luft- und Raumfahrt (DLR) nicht einfach: Auf seiner Oberfläche sind nicht nur flache Gebiete, sondern auch zahlreiche Risse, Hänge, Krater und Brocken zu sehen. "Wenn man die außergewöhnliche Form und die globale Topographie des Kometen sieht, ist es sicherlich keine Überraschung, dass viele Gebiete gleich aus der Auswahl herausfielen", sagt DLR-Wissenschaftler Dr. Stephan Ulamec, Projektleiter für den Lander Philae. Am 24. August fiel die Entscheidung für fünf mögliche Landestellen. "Bei diesen Kandidaten, die wir nun weiterhin untersuchen, ist eine Landung nach ersten Abschätzungen technisch durchführbar - die vorläufigen Flugbahnberechnungen zeigen dies. Außerdem bieten alle zumindest teilweise flaches Terrain und bei jeder Rotation des Kometen mindestens sechs Stunden Tageslicht, um den Lander mit Energie zu versorgen."
Abwägen der Kriterien
Bei dem Auswahlprozess musste das Team aus Wissenschaftlern des "Lander Control Centers" des DLR in Köln, des SONC (Science, Operations and Navigation Centre) der französischen Raumfahrtagentur CNES und die Wissenschaftler, deren Instrumente an Bord von Philae mitfliegen, die unterschiedlichsten Kriterien berücksichtigen: So benötigt der Lander beispielsweise genügend Sonnenstunden, um nach einer 64-stündigen ersten wissenschaftlichen Untersuchungsphase über einen möglichst langen Zeitraum seine Batterien für seinen Betrieb und die weiteren wissenschaftlichen Untersuchungen aufzuladen. Kann der Lander nicht mit ausreichend Energie versorgt werden, hat dies Konsequenzen für die geplante "Long term science phase", der Phase, in der alle Instrumente die Entwicklung des Kometen auf seinem Weg in Richtung Sonne untersuchen können. Permanente Beleuchtung hingegen könnte wiederum ein Überhitzen des Landers zur Folge haben - und somit die Lebensdauer von Philae und seiner Instrumente deutlich einschränken. Und auch die Zeitspanne von der Trennung des Landers von der Muttersonde bis zur eigentlichen Landung hat Auswirkungen auf die Wissenschaft: Je länger der Landevorgang dauert, umso weniger Energie steht für die erste wissenschaftliche Phase auf der Kometenoberfläche zur Verfügung.
Landung mit Ansprüchen
Ist das Gelände zu schroff und befinden sich beispielsweise Vertiefungen, Brocken von der Größe des Landers oder steile Hänge in dem Areal, ist eine Landung riskant. Da die Position des Orbiters, wenn er Philae in Richtung Komet absetzt, nicht exakt bestimmt werden kann, können die Wissenschaftler nämlich auch das Landegebiet nur mit einem Durchmesser von etwa einem Kilometer festlegen. Setzt der Lander nicht genau im angepeilten flachen Gebiet auf, könnte er im angrenzenden Gelände auf eine sehr landeunfreundliche Umgebung treffen. Nicht zuletzt muss die Position für das Absetzen auch mit dem Rostta-Orbiter erreichbar und eine regelmäßige Kommunikation mit Philae nach dessen Landung möglich sein, um unter anderem die gewonnenen Daten zur Erde zu funken. Und ginge es ausschließlich nach den Wünschen der beteiligten Wissenschaftler, würden diese sich vor allem ein möglichst aktives, ausgasendes, aber auch ursprüngliches Gebiet auswählen, in dem das Kometenmaterial seit der Entstehung unseres Sonnensystems vor 4,5 Milliarden Jahren kaum Veränderungen erfahren hat.
Die Suche nach dem besten Kompromiss
Allerdings: Den idealen Landeplatz, an dem ein flaches Terrain, genügend Sonnenstunden, eine gute Erreichbarkeit und optimale wissenschaftliche Bedingungen gewährleistet sind, entdeckte das Lander-Team nicht auf Komet Churyumov-Gerasimenko - und musste bei der Auswahl jeweils Vor- und Nachteile abwägen und einige "Kröten schlucken". "Es ist klar, dass wir Kompromisse eingehen müssen", betont DLR-Projektleiter Stephan Ulamec. Informationen über Temperaturen, das Ausgasen oder Geländeformen wurden von den Instrumente auf dem Orbiter geliefert. Aus ursprünglich zehn möglichen Landeplätze A bis H entschied sich das Lander-Team schließlich für fünf Kandidaten auf dem Kometen, der aus einem kleineren Kopf, einem größeren Körper und einem schmalen, sehr aktiven Verbindungsstück besteht. Drei der möglichen Landestellen (B, I und J) befinden sich auf dem kleineren der beiden Kometenteile, die beiden anderen (A und C) sitzen auf dem größeren Teil, dem Körper.
Landestelle A liegt in einer interessanten Region auf dem größeren Kometenteil, die den Blick zum Kometenkopf ermöglicht. Das schmale Gebiet zwischen diesen beiden Teilen ist sehr wahrscheinlich aktiv - dort gast der Komet auf seiner Reise in Richtung Sonne bereits jetzt schon aus. Zunehmend höher aufgelöste Aufnahmen sollen nun genauere Untersuchungen ermöglichen, um die Risiken durch kleineren Vertiefungen und Hängen bei der Landung besser einschätzen zu können. Auch die Beleuchtungsbedingungen werden noch detaillierter analysiert.
Landestelle B befindet sich einer kraterähnlichen Struktur am Kopf des Kometen und bietet für die Landung sehr wahrscheinlich ein relativ großes flaches Gelände im Inneren des Kraters. Allerdings ist an dieser Stelle das Tageslicht, das Philae erreicht, geringer als es ideal wäre - dies könnte zu einem Problem bei den längerfristigen wissenschaftlichen Untersuchungen führen. Mit weiteren Aufnahmen der Kometenoberfläche sollen nun die Gefahren durch die Brocken im Kraterinneren genauer abgeschätzt werden. Die Brocken im Gebiet deuten zudem daraufhin, dass es sich um verändertes und somit nicht so ursprüngliches Material handelt, wie es an anderen Orten auf dem Kometen untersucht werden könnte.
Landestelle C liegt auf dem größeren Kometenteil. Die Wissenschaftler finden hier viele unterschiedliche Strukturen wie Vertiefungen, Klippen, Hügel und ebene Gebiete - und auch Material, dass auf den Kameraaufnahmen heller als gewöhnlich erscheint und somit besonders interessant ist. Doch eben diese Oberflächenstrukturen müssen nun genauer betrachtet werden, um ihre Risiken für eine sichere Landung einzuschätzen. Landestelle C verfügt über genügend Tageslicht, von dem die späteren wissenschaftlichen Untersuchungsphasen profitieren würden.
Landestelle I ist in einem relativ flachen Gebiet und könnte neueres Material enthalten. Mit Kameraaufnahmen soll in den nächsten Wochen die Oberfläche im Detail betrachtet werden, um das Ausmaß der vorhandenen rauen Strukturen exakter bestimmen zu können. Die Beleuchtung der Landestelle hingegen ist günstig und gestattet eine länger andauernde wissenschaftliche Phase auf der Kometenoberfläche.
Der fünfte Kandidat J hat große Ähnlichkeit mit I - die Landestelle sitzt ebenfalls auf dem kleineren Kometenteil, dem Kopf, hat interessante Oberflächenstrukturen und eine gute Beleuchtung bei der Rotation des Kometen. Für das Experiment CONSERT, bei dem Radiowellen durch den Kometen zum Orbiter gesendet und empfangen werden, ist diese Landestelle günstiger als Landestelle I. Da allerdings auch hier einige Brocken und Terrassen zu sehen sind, sind höher aufgelöste Kamerabilder notwendig, um die Details des Geländes genauer zu bestimmen.
Landestellen mit viel Potenzial
"Jede Landestelle unter diesen Kandidaten hat das Potenzial für einmalige wissenschaftliche Entdeckungen", betont Lander-Projektleiter Stephan Ulamec. Das DLR-Institut für Planetenforschung ist bei vier Instrumenten der Mission in einer führenden Rolle und bei drei weiteren Experimenten wissenschaftlich beteiligt. Kamera ROLIS (Rosetta Lander Imaging System) wird beispielsweise bereits während des Abstiegs von Lander Philae von dessen Unterseite aus erste Bilder der Kometenoberfläche aufnehmen und vor Ort dann die Oberflächenstruktur des Kometen untersuchen. Thermalsonde MUPUS (Multi-Purpose Sensors for Monitoring Experiment) hämmert sich unter anderem in den Kometen, um dort bis in 40 Zentimetern Tiefe die Temperatur sowie die Wärmeleitfähigkeit zu messen. Das Experiment SESAME misst den Staubfluss, sitzt aber auch unter anderem in den Füßen des Landers und sendet und empfängt akustische und elektrische Signale.
Zum Einsatz kommen können diese Instrumente allerdings nur, wenn Philae - gesteuert und betrieben aus dem "Lander Control Center" des DLR in Köln - sicher landet. Bis zum 14. September 2014 wird das Lander-Team deshalb die fünf möglichen Kandidaten für eine Landung genauer unter die Lupe nehmen und aus ihnen den Landeplatz sowie eine Ersatz-Landestelle auswählen. Im Oktober wird dann nach noch genaueren Analysen der Landeplatz bestätigt oder auf den Ersatz-Landesplatz zurückgegriffen. Voraussichtlich am 11. November 2014 ist es dann soweit: Die erste Landung auf einem Komet findet statt und erstmals können direkt vor Ort auf einer Kometenoberfläche wissenschaftliche Untersuchungen durchgeführt werden.
Quelle: DLR
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Update: 28.08.2014
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COMETWATCH UPDATE
Regular followers of this blog will have noticed that we had a short pause in our routine “CometWatch” updates while we focused our outreach efforts on the landing site selection. NAVCAM images are of course still being taken – the navigation camera is crucial for navigating around the comet – but at the same time, the way in which the images are being taken has changed.
Comet 67P/C-G from a distance of 61 km on 23 August. The image is a 512 x 512 pixel crop of the full frame NAVCAM image below. Credits: ESA/Rosetta/NAVCAM
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Until now, each NAVCAM image has covered the whole comet in one shot, but now that Rosetta is about 50 km from the comet, the nucleus is close to overfilling the NAVCAM field, and will do as we get even closer. As a result, on Saturday we started taking NAVCAM image sequences as small 2 x 2 rasters, such that roughly one quarter of the comet is seen in the corner of each of the four images, rather than all in just one shot. An example of one of these 512 x 512 pixel ‘corner’ images is shown above, which was taken on Saturday from a distance of 61 km (the full frame 1024 x 1024 image is also shown below). At this closer distance, the details of surface features are becoming much clearer.
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Full frame NAVCAM view of comet 67P/C-G taken on 23 August. Credits: ESA/Rosetta/NAVCAM
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The primary objective of the NAVCAM is navigation and its images are used by ESOC to identify and match the location of landmarks on the comet at different times. This information is then fed into the navigation process to improve orbit determination. The raster technique is used to ensure that the comet is always seen by the spacecraft and compensates for any uncertainties in the spacecraft’s trajectory close to the comet. The four images thus cover the uncertainty in the pointing, ensuring that the comet is always seen in each raster, and that in at least one of the images enough of the comet surface is seen to allow landmark recognition.
As Rosetta is slightly repointed for each image, there is a time delay of about 20 minutes between the first and the last image being taken in each four-image sequence. In that time, Rosetta moves and the comet rotates, changing the appearance of features and shadows slightly. This complicates the making of a composite image and no software has been developed to do so, as the individual images do not need to be stitched together to meet the navigation requirement.
Today we are posting a ‘corner’ image to illustrate this change in the operations of NAVCAM. In the near-term, we aim to share more images from a given NAVCAM sequence, but the number and frequency may vary depending on the navigation activities on any given day.
Quelle: ESA
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Update: 30.08.2014
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Quelle: ESA
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Update: 1.09.2014
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MAKE A NAVCAM MOSAIC
Today’s four-image NAVCAM montage comprises images taken on 31 August from a distance of 61 km from comet 67P/C-G.
NAVCAM image sequences are now being taken as small 2 x 2 rasters, such that roughly one quarter of the comet is seen in the corner of each of the four images. The four images are taken over an approximately 20 minute period, meaning that there is some motion of the spacecraft and rotation of the comet between the images. As a result, making a clean mosaic out of the four images is not simple.
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Four image montage of comet 67P/C-G, using images taken on 31 August. Credits: ESA/Rosetta/NAVCAM
Thus, for the purpose of this montage, the four images are shown separated by black borders. There is some overlap between adjacent frames, so that some features appear in more than one image (see CometWatch update post for more about the reasons why NAVCAM images are being taken this way).
The images have been cleaned to remove the more obvious bad ‘pixel pairs’ and cosmic ray artefacts, intensities have been scaled to show a wide range of features, and the montage has been cropped to remove most of the black space around the comet. The dark feature in the centre is an imaging artefact.
In addition, we are also providing the individual JPEGs of the four full-frame ‘corners’ (click each image below for the full 1024 x 1024 frames), and warmly invite you to create your own mosaics and share them with us. You are welcome to post them on your personal profiles with the ESA/Rosetta/NAVCAM credit line, and share a link to your work in the comments box below, so that we have the possibility to follow up with you and eventually post your efforts on this blog.
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Four-image photo mosaic comprising images taken by Rosetta’s navigation camera on 31 August 2014 from a distance of 61 km from comet 67P/Churyumov-Gerasimenko. The mosaic has been rotated and contrast enhanced to bring out details. The comet nucleus is about 4 km across. Credits: ESA/Rosetta/NAVCAM/Ken Kremer/Marco Di Lorenzo
Quelle: ESA
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Update: 3.09.2014
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ESA Makes History with Rosetta
The European Space Agency, and in particular the ESA member states that chose to participate in the Rosetta comet-chaser mission, received a large and well-deserved dose of satisfaction Aug. 6 when the spacecraft finally arrived within striking distance of its destination.
After a journey of more than 10 years following a combined investment of $1.75 billion, Rosetta is now within 100 kilometers of Comet 67P/Churyumov-Gerasimenko and has sent back stunning images that are already changing the way scientists think about these mysterious objects. It has been widely assumed that comets are giant, dirty slush balls, but based on initial assessments of Rosetta’s data Comet 67P appears to more rocky than icy.
Over the coming weeks and months Rosetta will continue to track the comet as it approaches the sun, in the process kicking out increased amounts of water vapor and dust. Rosetta is the first probe to rendezvous and fall in step with a comet — previous missions to comets have been flybys.
Gradually, the spacecraft will close to within a distance of 10 kilometers before deploying a lander that will anchor itself to the surface of the comet — another first.
The resulting scientific discoveries — indeed the imagery alone — promise to be breathtaking, assuming success, of course. Already, however, top space officials with the participating nations — notably Germany, France and Italy — are rightly touting Rosetta’s arrival as validation of their substantial investments in the mission. Germany’s space minister also praised ESA’s smart use of social media to engage young people as the mission progresses.
Notably, these same countries face tough negotiations later this year that will shape the future of ESA’s launcher program and participation in the international space station.
But for the moment, why not bask in Rosetta’s glow? Though tricky maneuvers lay ahead, Rosetta is already a showcase of ESA’s technical prowess — it’s worth noting that this mission was designed in the 1990s — and a reminder that investment in flagship-class science missions is money well spent.
Quelle: Spacenews
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Moving closer, #67P! Over the next week I’ll head towards a distance of 30 km:
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Quelle: ESA
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Update: 5.09.2014
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NASA Instrument aboard European Spacecraft Returns First Science Results
A NASA instrument aboard the European Space Agency’s (ESA's) Rosetta orbiter has successfully made its first delivery of science data from comet 67P/Churyumov-Gerasimenko.
The instrument, named Alice, began mapping the comet’s surface last month, recording the first far-ultraviolet light spectra of the comet’s surface. From the data, the Alice team discovered the comet is unusually dark -- darker than charcoal-black -- when viewed in ultraviolet wavelengths. Alice also detected both hydrogen and oxygen in the comet’s coma, or atmosphere.
Rosetta scientists also discovered the comet’s surface so far shows no large water-ice patches. The team expected to see ice patches on the comet’s surface because it is too far away for the sun’s warmth to turn its water into vapor.
"We’re a bit surprised at just how unreflective the comet’s surface is and how little evidence of exposed water-ice it shows," said Alan Stern, Alice principal investigator at the Southwest Research Institute in Boulder, Colorado.
Alice is probing the origin, composition and workings of comet 67P/Churyumov-Gerasimenko, to gather sensitive, high-resolution insights that cannot be obtained by either ground-based or Earth-orbiting observation. It has more than 1,000 times the data-gathering capability of instruments flown a generation ago, yet it weighs less than nine pounds (four kilograms) and draws just four watts of power. The instrument is one of two full instruments on board Rosetta that are funded by NASA. The agency also provided portions of two other instrument suites.
Other U.S. contributions aboard the spacecraft are the Microwave Instrument for Rosetta Orbiter (MIRO), the Ion and Electron Sensor (IES), part of the Rosetta Plasma Consortium Suite, and the Double Focusing Mass Spectrometer (DFMS) electronics package for the Rosetta Orbiter Spectrometer for Ion Neutral Analysis (ROSINA). They are part of a suite of 11 total science instruments aboard Rosetta.
MIRO is designed to provide data on how gas and dust leave the surface of the nucleus to form the coma and tail that gives comets their intrinsic beauty. IES is part of a suite of five instruments to analyze the plasma environment of the comet, particularly the coma.
To obtain the orbital velocity necessary to reach its comet target, the Rosetta spacecraft took advantage of four gravity assists (three from Earth, one from Mars) and an almost three-year period of deep space hibernation, waking up in January 2014 in time to prepare for its rendezvous with 67P/Churyumov-Gerasimenko.
Rosetta also carries a lander, Philae, which will drop to the comet’s surface in November 2014.
The comet observations will help scientists learn more about the origin and evolution of our solar system and the role comets may have played in providing Earth with water, and perhaps even life.
Rosetta is an ESA mission with contributions from its member states and NASA. Rosetta's Philae lander is provided by a consortium led by the German Aerospace Center in Cologne; Max Planck Institute for Solar System Research in Göttingen; French National Space Agency in Paris; and the Italian Space Agency in Rome.
NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California, manages the U.S. contribution to the Rosetta mission for the agency’s Science Mission Directorate in Washington. JPL also built the MIRO instrument and hosts its principal investigator, Samuel Gulkis. The Southwest Research Institute, located in San Antonio and Boulder, developed Rosetta’s IES and Alice instruments and hosts their principal investigators, James Burch (IES) and Alan Stern (Alice).
Quelle: NASA
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Rosetta-Alice spectrograph obtains first far ultraviolet spectra of a cometary surface while orbiting Churyumov-Gerasimenko
The Alice ultraviolet imaging spectrometer will be the first to study a comet up close. The shoebox-sized instrument is one-third to one-half the mass of comparable UV instruments, yet with more than 10,000 times as many imaging pixels as the spectrometer aboard Galileo.
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Boulder, Colo. — Sept. 4, 2014 — NASA’s Alice ultraviolet (UV) spectrograph aboard the European Space Agency’s Rosetta comet orbiter has delivered its first scientific discoveries. Rosetta, in orbit around comet 67P/Churyumov-Gerasimenko, is the first spacecraft to study a comet up close.
As Alice began mapping the comet’s surface last month, it made the first far ultraviolet spectra of a cometary surface. From these data, the Alice team discovered that the comet is unusually dark at ultraviolet wavelengths and that the comet’s surface — so far — shows no large water-ice patches. Alice is also already detecting both hydrogen and oxygen in the comet’s coma, or atmosphere.
“We’re a bit surprised at both just how very unreflective the comet’s surface is, and what little evidence of exposed water-ice it shows,” says Dr. Alan Stern, Alice principal investigator and an associate vice president of the Southwest Research Institute (SwRI) Space Science and Engineering Division.
Developed by SwRI, Alice is probing the origin, composition and workings of the comet, gaining sensitive, high-resolution compositional insights that cannot be obtained by either ground-based or Earth-orbital observations. The ultraviolet wavelengths Alice observes contain unique information about the composition of the comet’s atmosphere and the properties of its surface.
“As the mission progresses, we will continue to search for surface ice patches and ultraviolet color and composition variations across the surface of the comet,” says Dr. Lori Feaga, Alice co-investigator at the University of Maryland.
Alice is one of three instruments funded by NASA flying aboard Rosetta. Alice has more than 1,000 times the data-gathering capability of instruments flown a generation ago, yet it weighs less than 4 kilograms and draws just 4 watts of power. A sister Alice instrument was developed by SwRI and was launched aboard the New Horizons spacecraft to Pluto in January 2006 to study that distant world’s atmosphere. It will reach Pluto in July 2015. SwRI also built and operates Rosetta’s Ion and Electron Spectrograph (IES), another instrument with miniaturized electronic systems. With a mass of 1.04 kilograms, IES achieves sensitivity comparable to instruments weighing five times more.
To reach its comet target, the Rosetta spacecraft executed four gravity assists (three from Earth, one from Mars) and a nearly three-year period of deep space hibernation, waking up in January 2014 in time to prepare for its rendezvous with Churyumov-Gerasimenko. Rosetta also carries a lander, Philae, that will drop to the comet’s surface in November 2014, attempting the first-ever direct observations of a comet surface.
Rosetta is an ESA mission with contributions from its member states and NASA. Rosetta’s Philae lander is provided by a consortium led by DLR, MPS, CNES and ASI. Airbus Defense and Space built the Rosetta spacecraft. NASA’s Jet Propulsion Laboratory (JPL) manages the U.S. contribution of the Rosetta mission for NASA’s Science Mission Directorate in Washington, under a contract with the California Institute of Technology (Caltech). JPL also built the Microwave Instrument for the Rosetta Orbiter and hosts its principal investigator, Dr. Samuel Gulkis. SwRI (San Antonio and Boulder, Colo.) developed the Rosetta orbiter’s Ion and Electron Sensor and Alice instrument and hosts their principal investigators, Dr. James Burch (IES) and Dr. Alan Stern (Alice).
Quelle: Southwest Research Institute
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Update: 15.00 MESZ
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Alice obtains first far ultraviolet spectra of comet 67P/C-G
The Alice ultraviolet (UV) spectrograph aboard ROSETTA has delivered its first scientific discoveries, making the first far ultraviolet spectra of a cometary surface.
Alice is probing the ORIGIN, composition and workings of comet 67P/Churyumov-Gerasimenko, gaining sensitive, high-resolution compositional insights that cannot be obtained by either ground-based or Earth-orbital observations.
From data collected over the last month, the Alice team discovered that the comet is unusually dark in the ultraviolet and that the comet’s surface – so far – shows no large water-ice patches. Alice is also already detecting both HYDROGEN and oxygen in the comet’s coma, or atmosphere.
“We’re a bit surprised at just how unreflective the comet’s surface is and how little evidence of exposed water-ice it shows,” says Dr. Alan Stern, Alice principal investigator and an associate vice president of the SwRI Space Science and ENGINEERING Division.
“As the mission progresses, we will continue to search for surface ice patches and ultraviolet color and composition variations across the surface of the comet,” says Dr. Lori Feaga, Alice co-investigator at the UNIVERSITY OF MARYLAND.
Alice was developed by Southwest Research Institute and is one of three instruments funded by NASA flying aboard ROSETTA.
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HOW STEINS’ CRATERS GOT GEMSTONE NAMES
Guest post by Sebastien Besse, Research Fellow at ESTEC
On 5 September 2008, ESA’s ROSETTA comet chaser completed its first encounter with a main belt asteroid when it flew past Steins, which revealed itself as a DIAMOND-shaped object with dimensions of 6.67 × 5.81 × 4.47 km.
At the time, I was a PhD student working with the OSIRIS imaging team in France studying the craters on Steins. With the asteroid’s diamond shape for inspiration, I began to think about naming the craters after different gemstones. I tried to select names that would be familiar or appealing to the general public, and easy to pronounce.
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As is the case with the official place names given to all worlds in THE SOLAR SYSTEM, they have to be approved by the International Astronomical Union (IAU). We had quite some discussions because we had to choose names of gems or precious minerals/stones that would not be confusing for studying the true mineralogy of the asteroid’s surface. In the end, the OSIRIS imaging team agreed that the names of 23 gemstones or precious minerals would be recommended to the IAU.
TO DATE, about 40 craters have been identified on Steins, but only the most prominent of them have been given names. In some cases we tried to name craters geographically close to each other with names from the same gemstone family. So for example, Aquamarine and Emerald are both from the Beryl family of gemstones, and Agate, Amethyst and Citrine are all types of quartz. Another example is that the crater Alexandrite sits inside the crater named Chrysoberyl – I named them like this because Alexandrite is a type of Chrysoberyl.
The most obvious crater is an impact feature near the south pole (top in the images) – now known as Diamond crater – which measures 2.1 km in diameter and is 300 m deep. The relatively LARGE SIZE and depth of this crater compared with the asteroid itself indicates that Steins was probably shattered by the force of the collision, meaning much of its interior resembles a rubble pile rather than solid rock.
A circular crater 650 m wide and 80 m deep named Topaz sits in the centre of the asteroid. Topaz is the easiest feature to see in this region and so is used to mark the asteroid’s centre of longitude.
Another noticeable feature is the chain of craters that stretches from the asteroid’s north pole (the pointed area at the bottom of the images) all the way to DIAMOND crater. This feature may be linked to the shattering impact that created Diamond crater at the south pole. At least some of the craters in the chain may have been formed by loose material sinking into a subsurface fracture. Several of these craters are named after familiar gemstones such as Opal and Jade. I included Citrine, since it sounds like the French word for lemon: citron.
As a geologist it feels good to have PROMOTED the names of these gemstones out in space! But as is the case with the naming of geological features on any world, it also provides a solid framework for discussions about certain features. Rather than having to say “you know that crater next to the funny shaped one…”, we can refer to named features, which makes discussions and writing papers a lot easier!
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Asteroid Steins seen from a distance of 800 km, taken by the OSIRIS imaging system from two different perspectives during the flyby.
Quelle: ESA
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Update: 7.09.2014
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Rosetta Comet is Darker than Charcoal
A NASA instrument aboard the European Space Agency’s (ESA's) Rosetta orbiter has successfully made its first delivery of science data from comet 67P/Churyumov-Gerasimenko.
The instrument, named Alice, began mapping the comet’s surface last month, recording the first far-ultraviolet light spectra of the comet’s surface. From the data, the Alice team discovered the comet is unusually dark -- darker than charcoal-black -- when viewed in ultraviolet wavelengths. Alice also detected both hydrogen and oxygen in the comet’s coma, or atmosphere.
Rosetta scientists also discovered the comet’s surface so far shows no large water-ice patches. The team expected to see ice patches on the comet’s surface because it is too far away for the sun’s warmth to turn its water into vapor.
"We’re a bit surprised at just how unreflective the comet’s surface is and how little evidence of exposed water-ice it shows," said Alan Stern, Alice principal investigator at the Southwest Research Institute in Boulder, Colorado.
Alice is probing the origin, composition and workings of comet 67P/Churyumov-Gerasimenko, to gather sensitive, high-resolution insights that cannot be obtained by either ground-based or Earth-orbiting observation. It has more than 1,000 times the data-gathering capability of instruments flown a generation ago, yet it weighs less than nine pounds (four kilograms) and draws just four watts of power. The instrument is one of two full instruments on board Rosetta that are funded by NASA. The agency also provided portions of two other instrument suites.
Other U.S. contributions aboard the spacecraft are the Microwave Instrument for Rosetta Orbiter (MIRO), the Ion and Electron Sensor (IES), part of the Rosetta Plasma Consortium Suite, and the Double Focusing Mass Spectrometer (DFMS) electronics package for the Rosetta Orbiter Spectrometer for Ion Neutral Analysis (ROSINA). They are part of a suite of 11 total science instruments aboard Rosetta.
MIRO is designed to provide data on how gas and dust leave the surface of the nucleus to form the coma and tail that gives comets their intrinsic beauty. IES is part of a suite of five instruments to analyze the plasma environment of the comet, particularly the coma.
To obtain the orbital velocity necessary to reach its comet target, the Rosetta spacecraft took advantage of four gravity assists (three from Earth, one from Mars) and an almost three-year period of deep space hibernation, waking up in January 2014 in time to prepare for its rendezvous with 67P/Churyumov-Gerasimenko.
Rosetta also carries a lander, Philae, which will drop to the comet’s surface in November 2014.
The comet observations will help scientists learn more about the origin and evolution of our solar system and the role comets may have played in providing Earth with water, and perhaps even life.
Quelle: NASA
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Update: 8.09.2014
This image of comet 67P/C-G was taken with ESO's Very Large Telescope on 11 August 2014. It was made by stacking 40 short exposure images (each lasting 50 seconds) and then removing the background stars. The comet's coma extends at least 19,000 km from the nucleus. The central pixel includes the Rosetta spacecraft, presently at comet 67P/C-G, which is too small to be resolved in the image. The coma is asymmetric, as the dust is swept away from the Sun – located beyond the lower right corner of the image – to form the beginnings of a tail. Credit: Colin Snodgrass/ESO/ESA
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COSIMA CATCHES COSMIC DUST
While many of us spend time trying to eliminate dust, Rosetta’s COSIMA team have a different attitude – they are actively attempting to catch it. Today we heard that they succeeded and have the picture to prove it!
At the European Planetary Science Congress (EPSC) held in Lisbon, Portugal, the COSIMA team presented an image of the first dust grains collected by the COSIMA instrument when Rosetta was at a distance of less than 100 km from the nucleus of comet 67P/Churyumov-Gerasimenko.
COSIMA studies dust in situ by capturing grains on small (1 cm by 1 cm) target plates, first imaging these with an optical microscope and then analysing the composition of selected grains using a secondary ion mass spectrometer. The instrument is designed to investigate dust grains larger than about 10 microns.
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COSIMA catches first dust grains. Left: an image of the target plate (measuring 1 cm by 1 cm) on which the grains were collected; right: a section of the plate showing the state on 17 August (top) when no dust grains were visible and 24 August (bottom) when some large dust grains were detected. The plate is illuminated from the right by LEDs and the length of the shadows is proportional to the height of the dust grains. The resolution of the image is 14 microns per pixel.
CREDIT: ESA/Rosetta/MPS for COSIMA Team MPS/CSNSM/UNIBW/TUORLA/IWF/IAS/ESA/
BUW/MPE/LPC2E/LCM/FMI/UTU/LISA/UOFC/vH&S
Early in the morning of 11 August, the first of COSIMA’s 24 target plates was exposed to space. As the team mentioned in a previous blog post, models of 67P/C-G’s coma suggest that at the present low level of activity, it should be comparable to a high-quality cleanroom – in other words, there should not be many dust particles. They therefore decided to expose this plate for at least one month and to check back periodically to see if they had been lucky and something had been picked up at this early stage.
On 24 August, when the COSIMA team looked at the image of the plate they saw a number of large dust grains from the comet on a target that had been pristine when examined one week before. A first examination of the plate indicates that the largest two grains are about 50 microns and 70 microns in width, comparable to the width of a HUMAN HAIR.
Scientists from the COSIMA team are now examining the image of the target plate in detail to determine the locations of the dust grains. Some will be selected for further analysis: the target plate will be moved to place each chosen grain under an ion gun which will then ablate the grain layer by layer. The material is then analysed in a secondary ion mass spectrometer to determine its composition.
The results of these investigations are eagerly awaited since these are among the first dust grains to have been collected from beyond the SOLAR System’s snow line – the distance from the Sun at which ice grains can form.
COSIMA is one of three INSTRUMENTS on ROSETTA that will study cometary dust, the other two being GIADA and MIDAS. In mid August, the GIADA team reported the detection of four dust grains, ranging in width from a few tens of microns up to a few hundreds of microns, collected when Rosetta was between 814 and 179 km from the comet.
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A PRELIMINARY MAP OF ROSETTA’S COMET
Scientists working on images of comet 67P/Churyumov-Gerasimenko have divided the comet’s surface into a number of different regions based on their morphology, revealing a unique, multifaceted world.
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SEVERAL morphologically different regions are indicated in this preliminary map, which is oriented with the comet’s ‘body’ in the foreground and the ‘head’ in the background.
CREDITS: ESA/ROSETTA/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
The map and new high-resolution images from the OSIRIS instrument were presented during the Rosetta special session at EPSC today.
With various areas dominated by cliffs, DEPRESSIONS, craters, boulders or even parallel grooves, 67P/C-G displays a multitude of different terrains. Some areas even appear to have been shaped by the comet’s activity.
This preliminary analysis provides the basis for a detailed scientific description of the comet’s surface, but a substantial amount of work involving more detailed OSIRIS images and data from other Rosetta INSTRUMENTS lies ahead to determine what each region represents in terms of their composition and evolution. One recent image from the OSIRIS narrow-angle camera is also shown here.
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Jagged cliffs and prominent boulders are visible in this image taken by OSIRIS on 5 September 2014 from a distance of 62 kilometres from comet 67P/Churyumov-Gerasimenko. The left part of the image shows a side view of the comet’s 'BODY', while the right is the back of its 'head'. One pixel corresponds to 1.1 metres.
CREDITS: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
As both 67P/C-G and Rosetta travel closer to the Sun over the next months, the OSIRIS team will MONITOR the surface looking for changes. While the scientists do not expect the borderlines of the comet’s regions to vary dramatically during this one passage around the Sun, more subtle transformations of the surface may nevertheless help to explain how cometary activity created such a breath-taking world.
Next weekend, on 13 and 14 September 2014, the maps will play a key role as Rosetta’s Lander Team and the Rosetta orbiter scientists gather at CNES, Toulouse to determine a primary and BACKUP landing site from the earlier pre-selection of five candidates.
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COMETWATCH – 7 SEPTEMBER
Four-image NAVCAM montage comprising images taken on 7 September from a distance of 51 km from comet 67P/C-G.
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Four image montage of comet 67P/C-G, using images taken on 7 September. The four images are shown separated by black borders and there is some overlap between adjacent frames, so that some features appear in more than one image; the montage has been cropped to remove most of the black space around the comet. CREDITS: ESA/Rosetta/NAVCAM
Regular followers of this blog are familiar with how NAVCAM image sequences are now being taken, but you can read more about it in previous blog posts: CometWatch update, Make a NAVCAM mosaic and CometWatch – 2 September.
As before, we INVITE you to create a mosaic using these four images (posted below as individual JPEGs of the four 1024 x 1024 pixel full-frame ‘corners’). An example, with some adjustment of the intensity and cleaning of the background noise, is shown here:
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Four image montage of comet 67P/C-G, using images taken on 7 September (with some intensity adjustment and background noise cleaning). CREDIT: ESA/Rosetta/NAVCAM
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VIRTIS MAPS COMET ‘HOT SPOTS’
The VIRTIS team have produced maps of the surface temperature of comet 67P/C-G showing how the temperature varies across regions and with local time. Some examples of these were shown today during the ROSETTA special session at the European Planetary Science Congress (EPSC).
Over the past two months, the VIRTIS imaging spectrometer has recorded about three million spectra of the surface of 67P/C-G as Rosetta closed in from about 14000 km to less than 100 km from the comet’s nucleus. One thing that can be extracted from these spectra is the surface temperature, which is DERIVED from measurements of the radiation emitted in the spectral range 4.5 to 5.1 microns.
Back in mid-July, the first temperature measurements – made when the comet occupied just a few pixels in the VIRTIS field of view – yielded an average surface temperature of 205 K, already at that time ruling out a surface covered exclusively in ice.
By early August, as the comet filled more of the VIRTIS field of view, variations in the temperature could be seen, modulated by the comet’s rotation period.
Following Rosetta’s arrival at the comet on 6 August, the VIRTIS team have been able to map variations in the surface temperature across the nucleus. These data have been an essential input to the on-going landing site selection process since one of the criteria for choosing a site is that it be neither too hot nor too cold for the Philae lander.
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Temperature measurements of the surface of comet 67P/Churyumov-Gerasimenko GENERATED from data recorded by the VIRTIS instrument. The maps, on an orthographic projection of the comet’s surface centred on the 0° meridian (left) and the 180° meridian (right), show the temperature for local time between 12h and 14h. The data were obtained in July and August 2014 when the comet was between 3.6AU and 3.45AU from the Sun, and the spacecraft was closing in from 14000km to less than 100 km from the comet nucleus. The locations of the five candidate landing sites for the Philae lander are indicated. CREDIT: ESA/ROSETTA/VIRTIS/INAF-IAPS/OBS DE PARIS-LESIA/DLR
At the Rosetta special session today at the EPSC, the VIRTIS team described how they have made temperature maps of the entire sunlit surface of the comet, noting that temperatures as high as 230 K have been recorded. They have also been able to map how the surface temperature varies with local time – as can be seen in the figure above.
Measurements of the surface temperature can provide clues to the composition and physical properties of a comet. These new VIRTIS measurements have allowed the team to rule out some models of the comet surface and to favour a comet surface composed of a porous and highly thermally insulating dusty crust that is depleted of water ice. As they reported today, this is also consistent with the VIRTIS global measurements of thermal inertia – a measure of a body’s resistance TO CHANGES in temperature – that is compatible with the value for high porosity dusty materials.
The RELATIVELY high spatial resolution of the VIRTIS measurements have also allowed the team to investigate thermal shocks that happen when a region enters or exits from shadow. This is of interest because thermal shocks can give rise to stresses in the surface, which can lead to micro-cracks and eventually result in fractures in the surface.
The team have also been poring over the spectra to search for hints about the chemical makeup of the surface of comet 67P/C-G. Among the preliminary results reported today was no evidence of water ice on a global scale, confirming that the outer surface is generally dehydrated.
On the other hand, they see some strong hints of carbon-bearing compounds, with some spectral features that are compatible with the complex macromolecular carbonaceous materials found in the most primitive carbonaceous meteorites. These materials are often referred to as “organics”, even if their origin is unrelated to life.
The picture of comet 67P/C-G that is beginning to emerge from these early VIRTIS measurements is of a dark, dry, and dusty comet surface with a rich and complex chemistry.
Quelle: ESA
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Update: 10.09.2014
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DOWN, DOWN WE GO TO 29 KM – OR LOWER?
Editor’s note: The Rosetta mission control team are transitioning Europe’s intrepid craft onto the next stage of cometary operations: the Global Mapping Phase (GMP). The update below is based on inputs provided by Flight Director Andrea Accomazzo at ESOC and Project Scientist Matt Taylor at ESTEC.
Before you read today’s post, take a quick look at our trajectory video to refresh your memory (we know you’ve already seen this – but it’s a very helpful aide memoire), which illustrates what Rosetta is doing now (skip through to the 00:58 – Global Mapping)
Note also that some round-figure references to “30, 20 or 10km” refer, specifically, to 29, 18.6 or 9.8 km, respectively.
We are now in the Transition to Global Mapping (TGM) phase, that is, the two hyperbolic segments of the trajectory that move us from the 50km pyramid of the Close Approach Phase (CAT) to the 30-km gravitationally bound orbit of the Global Mapping Phase (GMP).
“The aim of GMP is to get as close as we can and gather as high-res science data as we can to best characterise the potential landing sites – helping us to make the best decision on prime and backup and really start to understand what kind of thing we're dealing with,” says Fred Jansen, ESA’s Rosetta Mission Manager.
“It's a real synthesis of operations, science and analysis that is a first in many ways.”
On Wednesday, 10 September, 09:00 UTC (11:00 CEST) the spacecraft will be at the terminator plane (the plane passing through the centre of the comet and perpendicular to the Sun direction) and will perform a 19-cm/s manoeuvre (thruster burn) to insert the spacecraft onto the 30-km circular orbit.
"The orbital plane is 60 degrees away from the Sun’s direction and is such that we will orbit over areas of the comet in their ‘morning hours’," says Andrea Accomazzo, Rosetta Flight Director.
"This results in orbits with periods of exactly 14 days. However, we will not fly all of each orbit!"
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Andrea explains that, instead, after 7 days (i.e. when we are again on the terminator plane) we will conduct a manoeuvre (thruster burn)that will change the orbital plane such that it will have the same characteristics as the previous orbit – but flying over ‘afternoon areas’ of the comet.
This will be the last time we fly bound orbits in front of the comet (see explanation below).
At the end of this second 30-km arc, we will let the spacecraft go onto the night side, just before dawn, to afford the instruments a look at the thermal characteristics of the comet.
Just before entering ‘night’ arc, we will do a small manoeuvre to lower the orbit such that when we come out again, that is crossing the terminator plane five days later, we will be at just 20km.
This manoeuvre, set to take place on 29 September, is actually subject to a GO/NOGO decision to be made on Thursday, 18 September, because we want to see how the spacecraft flies, and what conditions we encounter, at 30km for a week before we decide to go down any further.
Staying at 30 km
If conditions and performance dictate that we should not go any closer, we will continue until just before lander delivery in our 30-km orbits.
Two things to note:
By ‘night’ we are not referring to eclipse, i.e. when the spacecraft is not illuminated by the Sun, but rather to when the ground track on the comet surface is on the night side*.
The period of the orbit at 30x20km, that is the elliptic orbit we will use to go down to 20km, cannot be a multiple of 7 days and it is one of the few exceptions where the orbital plan cannot follow our ‘ideal’ weekly calendar (the other time when this happens will be, as you may guess, during landing); the mission control team have to be able to accommodate these exceptions.
"Once we reach the terminator plane at 20km, another small manoeuvre will set the orbital plane to coincide therewith," says Andrea.
This is done in order to expose the minimum spacecraft surface (body plus leading edge of the arrays) to the gas flow emanating radially from the comet (if we were flying, for example in front of the comet, then the big 16-m long solar arrays would be exposed face-on to the flow).
Similarly, on 6 October (after having been for one week at 20km), we will again have a GO/NOGO decision as to whether we go down to 10km.
If so, on 8 October, another manoeuvre will change the orbit to 20x10km and once down to 10km on 15 October, we will circularise there.
This orbit would then be the one maintained until 26 October when we will start phasing the orbit for the lander delivery.
ESA’s most intense science operations ever
While the description so far of the spacecraft’s operational orbit evolution may sound relatively straightforward (we’ve left off many details on the intensive flight control and flight dynamics team work going on behind the scenes), it is especially important to understand that there is a related and massive level of science operations activity taking place.
The multiplicity of possible orbits (the aforementioned 30- 20- and 10-km orbital levels) make the planning for scientific observations immensely challenging since, given the lead time necessary for science operations planning, the Rosetta Science Operations Centre (RSOC) at ESAC, Madrid, Spain, will have to provide a separate plan, for all instruments, for each case.
“We are in the most intense science operations activity ever for ESA,” says Matt Taylor.
“The workload and the need to coordinate science planning with potential spacecraft orbits – all while venturing into the unknown of an entirely new comet – are challenging to say the least.”
Matt adds that the on-going science and flight operations planning processes don’t simply stop once Rosetta releases the lander.
“We are working on two parallel plans for post-landing also.”
Quelle: ESA
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