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Astronomie - ESO-Beobachtungen zeigen, dass der erste interstellare Asteroid mit nichts vergleichbar ist, was wir bisher kennen

21.11.2017

VLT enthüllt dunkles, rötliches und langgezogenes Objekt

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Zum ersten Mal haben Astronomen in unserem Sonnensystem einen Asteroiden untersucht, der aus dem interstellaren Raum stammt. Beobachtungen mit dem Very Large Telescope der ESO in Chile und anderen Observatorien auf der ganzen Welt haben gezeigt, dass dieses einzigartige Objekt für Millionen von Jahren durch den Weltraum gereist ist, bevor es zufällig auf unser Sonnensystem traf. Es scheint sich um ein dunkelrotes, langgezogenes metallisches oder felsiges Objekt zu handeln. Die neuen Ergebnisse erscheinen am 20. November 2017 in der Fachzeitschrift Nature.

Am 19. Oktober 2017 entdeckte das Pan-STARRS-1-Teleskop auf Hawai`i einen schwachen Lichtpunkt, der sich über den Himmel bewegte. Zunächst sah er aus wie ein typischer kleiner Asteroid, der sich mit hoher Geschwindigkeit bewegte, jedoch konnte seine Bahn mithilfe zusätzlicher Beobachtungen in den darauffolgenden Tagen recht genau berechnet werden. Diese Bahnberechnungen enthüllten ohne jeden Zweifel, dass dieser Körper nicht wie alle anderen jemals zuvor beobachteten Asteroiden oder Kometen aus unserem Sonnensystems stammte, sondern aus dem interstellaren Raum kam. Obwohl das Objekt ursprünglich als Komet klassifiziert wurde, lieferten Beobachtungen der ESO und anderen Stellen keine Hinweise auf kometare Aktivität, nachdem es im September 2017 der Sonne am nächsten kam. Daraufhin wurde das Objekt als interstellarer Asteroid neu klassifiziert und in 1I/2017 U1 (`Oumuamua) [1] umbenannt.

Wir mussten schnell handeln”, erklärt Teammitglied Olivier Hainaut von der ESO in Garching. „`Oumuamua war schon am sonnennächsten Punkt vorbei und bereits wieder auf dem Weg zurück in den interstellaren Raum.

Sofort wurde das Very Large Telescope der ESO miteinbezogen, um die Bahn des Objekts, seine Helligkeit und seine Farbe genauer zu bestimmen, als es mit kleineren Teleskopen möglich gewesen wäre. Die Helligkeit von `Oumuamua nahm rapide ab, als sich das Objekt von der Sonne weg und aus dem Umfeld der Erdbahn heraus bewegte. Es hatte allerdings noch weitere Überraschungen auf Lager.

Durch Kombination der Bilder, die mit dem FORS-Instrument am VLT mit vier verschiedenen Filtern aufgenommen wurden, mit den Aufnahmen anderer großen Teleskope, fand das Astronomenteam unter der Leitung von Karen Meech vom Institute for Astronomy auf Hawai`i in den USA heraus, dass sich die Helligkeit von `Oumuamua dramatisch um einen Faktor zehn ändert, da es sich alle 7,3 Stunden um seine eigene Achse dreht.

Karen Meech erklärt, warum das so bedeutend ist: „Diese ungewöhnlich starke Helligkeitsänderung deutet darauf hin, dass das Objekt sehr langgezogen sein muss: etwa zehnmal so lang wie breit, mit einer komplexen, gewundenen Form. Wir fanden auch heraus, dass es eine dunkelrote Farbe besitzt, ähnlich wie Objekte im äußersten Bereich des Sonnensystems. Außerdem konnten wir bestätigen, dass es vollständig inaktiv ist, weil wir in seiner direkten Umgebung nicht den geringsten Hinweis auf Staub finden konnten.“

Diese Eigenschaften deuten darauf hin, dass `Oumuamua ein kompaktes Objekt ist, möglicherweise aus Gestein oder mit einem hohen Anteil an Metall, keine signifikanten Mengen an Wasser oder Eis enthält und dass seine Oberfläche in Folge der Millionen Jahre anhaltenden kosmischen Bestrahlung jetzt dunkel und rötlich erscheint. Seine Länge wird auf mindestens 400 Meter geschätzt.

Vorläufige Bahnberechnungen legen nahe, dass das Objekt ungefähr aus der Richtung des hellen Sterns Vega im nördlichen Sternbild Leier (lat. Lyra) stammt. Doch selbst mit einer halsbrecherischen Geschwindigkeit von etwa 95.000 Kilometern/Stunde dauerte die Reise des interstellaren Objekts zu unserem Sonnensystem so lange, dass Vega nicht in der Nähe dieser Position war, als der Asteroid vor etwa 300.000 Jahren dort war. `Oumuamua könnte seit Hunderten Millionen von Jahren durch die Milchstraße gereist sein, ohne an irgendein Sternsystem gebunden zu sein, bevor das Objekt zufällig auf das Sonnensystem traf.

Astronomen schätzen, dass etwa einmal pro Jahr ein interstellarer Asteroid wie `Oumuamua durch das innere Sonnensystem hindurchfliegt. Da solche Objekte aber sehr lichtschwach und damit schwer zu finden sind, wurde bisher nie eins beobachtet. Erst seit kurzem sind Durchmusterungsteleskope wie Pan-STARRS leistungsfähig genug, um überhaupt eine Chance zu haben, sie zu entdecken.

Wir beobachten dieses einzigartige Objekt weiterhin“, schießt Olivier Hainaut, „und wir hoffen, genauer bestimmen zu können, woher es kam und wohin es auf seiner Reise durch die Galaxis als nächstes fliegt. Und jetzt, da wir den ersten interstellaren Gesteinsbrocken gefunden haben, bereiten wir uns auf die nächsten vor!

Endnoten

[1] Dem Antrag des Pan-STARRS-Teams, dem interstellaren Objekt einen Namen zu geben, wurde von der Internationalen Astronomischen Union (IAU), die für die offizielle Namensvergabe für Körper im Sonnensystem und darüber hinaus zuständig ist, stattgegeben. Der Name ist hawaiisch, weitere Details finden sich hier. Die IAU erstellte für interstellare Asteroiden eine neue Klasse, dieses Objekt ist also das erste, dass diese Bezeichnung erhält. Die korrekte Weise, auf dieses Objekt zu verweisen ist nun: 1I, 1I/2017 U1, 1I/`Oumuamua oder 1I/2017 U1 (`Oumuamua). Das Symbol vor dem O ist ein Okina. Den Namen spricht man also H O u  mu a mu a aus. Vor der Einführung des neuen Namenschemas wurde das Objekt als A/2017 U1 bezeichnet.

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Kombinierte tiefe Aufnahme von `Oumuamua vom VLT und anderen Teleskopen (beschriftet))

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Diese sehr tiefe kombinierte Aufnahme zeigt den interstellaren Asteroiden ‘Oumuamua in der Mitte des Bildes. Er ist umgeben von den Spuren lichtschwacher Sterne, die zu Strichpunktlinien verschmiert wurden, als das Teleskop den sich bewegenden Asteroiden verfolgt hat. Das Bild wurde aus mehreren Aufnahmen des Very Large Telescope der ESO sowie des Gemini-South-Teleskops zusammengestellt. Das Objekt ist mit einem blauen Kreis gekennzeichnet und erscheint als Punktquelle ohne umgebenden Staub.

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Die Flugbahn von ‘Oumuamua

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Das Diagramm zeigt die Flugbahn des interstellaren Asteroiden ‘Oumuamua auf seinem Weg durch das Sonnensystem. Im Gegensatz zu allen anderen Asteroiden und Kometen, die bisher beobachtet wurden, ist dieser Körper gravitativ nicht an die Sonne gebunden. Seine hyperbolische Flugbahn ist stark geneigt und es scheint nicht, als ob es auf seinem Weg einem anderen Sonnensystemobjekt nahegekommen wäre.

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Kombinierte tiefe Aufnahme von `Oumuamua vom VLT und anderen Teleskopen (nicht beschriftet)

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Diese sehr tiefe kombinierte Aufnahme zeigt den interstellaren Asteroiden ‘Oumuamua in der Mitte des Bildes. Er ist umgeben von den Spuren lichtschwacher Sterne, die zu Strichpunktlinien verschmiert wurden, als das Teleskop den sich bewegenden Asteroiden verfolgte. Das Bild wurde aus mehreren Aufnahmen des Very Large Telescope der ESO sowie des Gemini-South-Tleskops zusammengestellt. Das Objekt erscheint als punktförmige Lichtquelle und hat keinen Staub, der ihn umgibt.

Quelle: ESO

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Earth's First Known Interstellar Visitor Unmasked

In October, astronomers at the University of Hawaii's Institute for Astronomy (IfA) made a stunning discovery with the Pan-STARRS1 telescope - the first interstellar object seen passing through our Solar System. Now, an international team lead by Karen Meech (ifA) has made detailed measurements of the visitor's properties. "This thing is very strange," said Karen Meech. 

Originally denoted A/2017 U1 (with the A for asteroid), the body is now the first to receive an I (for interstellar) designation from the International Astronomical Union, which created the new category after the discovery. In addition, it has been officially given the name `Oumuamua. The name, which was chosen in consultation with Hawaiian language experts Ka`iu Kimura and Larry Kimura, reflects the way this object is like a scout or messenger sent from the distant past to reach out to us (`ou means "reach out for", and mua, with the second mua placing emphasis, means "first, in advance of"). The object's full official name is 1I/2017 U1 (`Oumuamua), and can also be correctly referred to as 1I, 1I/2017 U1, and 1I/`Oumuamua .

`Oumuamua is rapidly fading as it heads out of the Solar System and recedes from both the Sun and the Earth, so getting new observations as fast as possible was crucial. The IfA team - including those who discovered 1I - was already prepared to rapidly follow up solar system discoveries from Pan-STARRS, which is operated by the IfA and funded by NASA. "We were able to rapidly develop a follow-up strategy on a very short timescale. It is exciting to think that the brief visit by `Oumuamua gave us the opportunity to do the first characterization of a sample from another solar system," says Meech. As a result, they are the first to publish their results, appearing in the November 20th online issue of the journal Nature. 

The team gathered data from telescopes around the world, including the Canada-France-Hawaii Telescope (CFHT), the United Kingdom Infrared Telescope (UKIRT) and the Keck Telescope on Maunakea, the Gemini South telescope, and the European Southern Observatory (ESO) Very Large Telescope (VLT) in Chile. Marshalling all of these resources yielded a wealth of data that revealed `Oumuamua's unusual nature. 

"We had to act quickly," explains team member Olivier Hainaut from ESO in Garching, Germany. "`Oumuamua had already passed its closest point to the Sun and was heading back into interstellar space. This felt very much like the beginning of the famous science fiction story, Rendezvous with Rama."

"Needless to say, we dropped everything so we could quickly point the Gemini telescopes at this object immediately after its discovery," said Gemini Director Laura Ferrarese, who coordinated the Gemini South observations for Meech's group. 

"The CFHT data was absolutely critical for understanding the light curve, for our initial understanding of the orbit, and determining that this object was more like an asteroid and not a comet," noted IfA's Richard Wainscoat.

"What we found was a rapidly rotating object, at least the size of a football field, that changed in brightness quite dramatically," according to Meech. "This change in brightness hints that `Oumuamua could be more than 10 times longer than it is wide - something which has never been seen in our own Solar System," according to Meech.

"An axis ratio like that is truly extraordinary - we have never seen anything in the solar system that is this elongated", says Lance Benner, a specialist in radar imaging of near-Earth and main-belt asteroids at the Jet Propulsion Laboratory in California.

`Oumuamua does have some similarities to small objects in the outer Solar System, especially the distant worlds of the Kuiper Belt - a region of rocky, frigid worlds far beyond Neptune. "While study of `Oumuamua's colors shows that this body shares characteristics with both Kuiper Belt objects and organic-rich comets and trojan asteroids," said Meech, "its hyperbolic orbit says it comes from far beyond." 

"We are continuing to observe this unique object," added Hainaut, "and we hope to more accurately pin down where it came from and where it is going next on its tour of the galaxy. And now that we have found the first interstellar rock, we are getting ready for the next ones!"

Astronomers estimate that an interstellar asteroid similar to `Oumuamua passes inside the orbit of Earth several times year, but they are faint and hard to spot, so they have been missed up until now. It is only recently that survey telescopes, such as Pan-STARRS, are powerful enough to have a chance to discover them. "Our successful follow-up observations are a model for the future - especially when the next major survey telescope, LSST, comes on line," added Meech. 

This research is presented in a paper entitled "A brief visit from a red and extremely elongated interstellar asteroid", by K. Meech et al., appears in the journal Nature on November 20, 2017.

 

This animation shows the interstellar asteroid 1I/2017 (`Oumuamua). Observations with Gemini, ESO's Very Large Telescope, CFHT, and others have shown that this unique object is dark, reddish in colour and highly elongated.
This video is available in a variety of formats here.
Credit: ESO/M. Kornmesser
Gemini Observatory image of interstellar object
Gemini South telescope color composite image of `Oumuamua (center).

This composite was produced by combining 192 images obtained through three visible and two near-infrared filters totaling 1.6 hours of integration on October 27 at the Gemini South telescope. Processing removes the background stars. The field of view represents a patch of sky 5,000 km (~3,100 miles) on a side at the distance of `Oumuamua. Visible colors for the image were assigned to each filter as follows:
g (398-552 nm) = blue
r (562-692 nm) = green
i (706-850 nm) = yellow
z (830-925 nm) = orange
Y (970-1070 nm) = red
While assigning visible colors to filtered images is somewhat subjective, the resulting color of `Oumuamua in optical light is similar to the hue of some of the moons of outer planets in our Solar System, and possibly indicates a similar composition (a combination of minerals, carbon, iron, and organic compounds). Color composite produced by Travis Rector, University of Alaska Anchorage, using Gemini South GMOS data obtained and processed by Meech et. al. 
Credit: Gemini Observatory/AURA/NSF
This animation (annotated) shows the path of the interstellar asteroid 1I/2017 (`Oumuamua) through the Solar System.
This video is available in a variety of formats here
Credit: ESO/M. Kornmesser, L.Calcada. Music: Azul Cobalto 
light curve of interstellar object
'Oumuamua's Varying Brightness

This plot shows how the interstellar asteroid `Oumuamua varied dramatically in brightness during three days in October 2017. The large range of brightness - about a factor of ten (2.5 magnitudes) - is due to the very elongated shape of this unique object, which rotates every 7.3 hours. The different coloured dots represent measurements through different filters, covering the visible and near-infrared part of the spectrum. The dotted line shows the light curve expected if `Oumuamua were an ellipsoid with a 1:10 aspect ratio, the deviations from this line are probably due to irregularities in the object's shape or surface albedo. 
Credit: ESO/K. Meech et al.
 

 

Quelle: University of Hawaii 

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Meet 'Oumuamua, the interstellar asteroid, before it's gone for good

Amina Khan

Astronomers have spotted an unprecedented interplanetary traveler: an asteroid-like object that came from far beyond our own solar system.

The discovery of ‘Oumuamua, described in the journal Nature, marks the first time that researchers have identified an interstellar object — and it could mean that there are many more already in our solar system just waiting to be found.

The fast-moving space rock, known formally as 1I/2017 U1, was spotted Oct. 19 by Robert Weryk of the Institute for Astronomy using the Pan-STARRS1 telescope system, which is hunting for near-Earth objects that could be hazardous to our planet. After looking back in the archive, astronomers soon realized their cameras had already picked up 1I/2017 U1 the night before. More telescopes quickly joined in the fray.

“Even with a little bit of data, this … was really unusual,” said lead author Karen Meech, a planetary astronomer at the Institute for Astronomy.

‘Oumuamua quickly revealed itself to be new to the neighborhood. It had a highly eccentric path — one that didn’t appear to be a closed orbit at all. It was also coming in from above the plane of the solar system, and its path showed that it hadn’t passed close to any of the known planets.

“Its orbit shape means it is not gravitationally bound to our solar system,” Meech said. “Now the sun certainly influenced its orbit because it passed close to it … [but] had this passed much farther from the sun, it could have effectively gone through our solar system on a straight path with no change.”

‘Oumuamua appears to be long and thin, like a cigar, about a quarter-mile long and a tenth as wide. Because it dramatically brightened by a factor of 10 and them dimmed on a regular cycle, astronomers could tell that it spun like a pencil placed flat on a table. When the narrow shaft faced Earth, it looked dim. When its long body faced Earth, offering up more surface area to reflect sunlight, it looked brighter. The object completed one of these rotations every 7.3 hours.

“We don’t have anything in the solar system that’s got this shape,” Meech said.

‘Oumuamua did not have a tail of material outgassing from its body, the way a comet does, so the object may be an interstellar asteroid.

Asteroids are the building blocks of our solar system that never quite made it into a planet — which means their contents have not been changed for billions of years. These space fossils can shed light on what our early solar system looked like.

But a space fossil from another solar system? That would give an unprecedented peek at the planetary building blocks around distant stars.

“It’s our first opportunity to look at a piece of material from another solar system up close,” Meech said. “That made it really exciting. You want to turn every telescope possible onto it and try and characterize it as best you can.”

‘Oumuamua appears to have a reddish hue, much like the material in our own solar system. That could mean that, like many of our own asteroids, it’s also covered in organic, carbon-rich material.

The asteroid was named after the Hawaiian term for a messenger or scout. But the interstellar visitor is already leaving town, speeding away at about 85,700 mph from the sun, as of Monday. It passed the Martian orbit around Nov. 1 and will blow past Jupiter’s this coming May and Saturn’s in January of 2019, headed toward the Pegasus constellation. This hasn’t left scientists much time to get an in-depth, up-close look.

The good news is that this asteroid’s appearance hints that such strange objects are far more common than scientists thought — and now that they now what to look for, they’ll be better prepared to study the next one, Meech said.

Quelle: Los Angeles Times

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Solar System’s First Interstellar Visitor Dazzles Scientists

Astronomers recently scrambled to observe an intriguing asteroid that zipped through the solar system on a steep trajectory from interstellar space—the first confirmed object from another star. 

Artist’s concept of interstellar asteroid 1I/2017 U1 (‘Oumuamua)
Artist’s concept of interstellar asteroid 1I/2017 U1 (‘Oumuamua) as it passed through the solar system after its discovery in October 2017. The aspect ratio of up to 10:1 is unlike that of any object seen in our own solar system.
Credits: European Southern Observatory/M. Kornmesser

Now, new data reveal the interstellar interloper to be a rocky, cigar-shaped object with a somewhat reddish hue. The asteroid, named ‘Oumuamua by its discoverers, is up to one-quarter mile (400 meters) long and highly-elongated—perhaps 10 times as long as it is wide. That aspect ratio is greater than that of any asteroid or comet observed in our solar system to date. While its elongated shape is quite surprising, and unlike asteroids seen in our solar system, it may provide new clues into how other solar systems formed.

The observations and analyses were funded in part by NASA and appear in the Nov. 20 issue of the journal Nature. They suggest this unusual object had been wandering through the Milky Way, unattached to any star system, for hundreds of millions of years before its chance encounter with our star system.

“For decades we’ve theorized that such interstellar objects are out there, and now – for the first time – we have direct evidence they exist,” said Thomas Zurbuchen, associate administrator for NASA’s Science Mission Directorate in Washington. “This history-making discovery is opening a new window to study formation of solar systems beyond our own.” 

Immediately after its discovery, telescopes around the world, including ESO’s Very Large Telescope in Chile and other observatories around the world were called into action to measure the object’s orbit, brightness and color. Urgency for viewing from ground-based telescopes was vital to get the best data. 

Combining the images from the FORS instrument on the ESO telescope using four different filters with those of other large telescopes, a team of astronomers led by Karen Meech of the Institute for Astronomy in Hawaii found that ‘Oumuamua varies in brightness by a factor of ten as it spins on its axis every 7.3 hours. No known asteroid or comet from our solar system varies so widely in brightness, with such a large ratio between length and width. The most elongated objects we have seen to date are no more than three times longer than they are wide.   

“This unusually big variation in brightness means that the object is highly elongated: about ten times as long as it is wide, with a complex, convoluted shape,” said Meech. We also found that it had a reddish color, similar to objects in the outer solar system, and confirmed that it is completely inert, without the faintest hint of dust around it.”

These properties suggest that ‘Oumuamua is dense, comprised of rock and possibly metals, has no water or ice, and that its surface was reddened due to the effects of irradiation from cosmic rays over hundreds of millions of years. 

A few large ground-based telescopes continue to track the asteroid, though it’s rapidly fading as it recedes from our planet. Two of NASA’s space telescopes (Hubble and Spitzer) are tracking the object the week of Nov. 20. As of Nov. 20, ‘Oumuamua is travelling about 85,700 miles per hour (38.3 kilometers per second) relative to the Sun. Its location is approximately 124 million miles (200 million kilometers) from Earth -- the distance between Mars and Jupiter – though its outbound path is about 20 degrees above the plane of planets that orbit the Sun. The object passed Mars’s orbit around Nov. 1 and will pass Jupiter’s orbit in May of 2018. It will travel beyond Saturn’s orbit in January 2019; as it leaves our solar system, ‘Oumuamua will head for the constellation Pegasus.

Observations from large ground-based telescopes will continue until the object becomes too faint to be detected, sometime after mid-December. NASA’s Center for Near-Earth Object Studies (CNEOS) continues to take all available tracking measurements to refine the trajectory of 1I/2017 U1 as it exits our solar system. 

This remarkable object was discovered Oct. 19 by the University of Hawaii’s Pan-STARRS1 telescope, funded by NASA’s Near-Earth Object Observations (NEOO) Program, which finds and tracks asteroids and comets in Earth’s neighborhood. NASA Planetary Defense Officer Lindley Johnson said, “We are fortunate that our sky survey telescope was looking in the right place at the right time to capture this historic moment. This serendipitous discovery is bonus science enabled by NASA’s efforts to find, track and characterize near-Earth objects that could potentially pose a threat to our planet.”  

Preliminary orbital calculations suggest that the object came from the approximate direction of the bright star Vega, in the northern constellation of Lyra. However, it took so long for the interstellar object to make the journey – even at the speed of about 59,000 miles per hour (26.4 kilometers per second) -- that Vega was not near that position when the asteroid was there about 300,000 years ago. 

While originally classified as a comet, observations from ESO and elsewhere revealed no signs of cometary activity after it slingshotted past the Sun on Sept. 9 at a blistering speed of 196,000 miles per hour (87.3 kilometers per second). 

The object has since been reclassified as interstellar asteroid 1I/2017 U1 by the International Astronomical Union (IAU), which is responsible for granting official names to bodies in the solar system and beyond. In addition to the technical name, the Pan-STARRS team dubbed it ‘Oumuamua (pronounced oh MOO-uh MOO-uh), which is Hawaiian for “a messenger from afar arriving first.” 

Astronomers estimate that an interstellar asteroid similar to ‘Oumuamua passes through the inner solar system about once per year, but they are faint and hard to spot and have been missed until now. It is only recently that survey telescopes, such as Pan-STARRS, are powerful enough to have a chance to discover them. 

“What a fascinating discovery this is!” said Paul Chodas, manager of the Center for Near-Earth Object Studies at NASA’s Jet Propulsion Laboratory, Pasadena, California. “It’s a strange visitor from a faraway star system, shaped like nothing we’ve ever seen in our own solar system neighborhood.”

Scientists were surprised and delighted to detect --for the first time-- an interstellar asteroid passing through our solar system. Additional observations brought more surprises: the object is cigar-shaped with a somewhat reddish hue. The asteroid, named ‘Oumuamua by its discoverers, is up to one-quarter mile (400 meters) long and highly-elongated—perhaps 10 times as long as it is wide. That is unlike any asteroid or comet observed in our solar system to date, and may provide new clues into how other solar systems formed.
Credits: NASA JPL
Quelle: NASA

 

 

 

 

 

 

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