Saturday, October 31, 2015

Saturn’s Moon Dione

Saturn’s Moon Dione:

Ringside With Dione


Saturn’s moon Dione, with Saturn’s rings visible in the background. Credit: NASA/JPL
Thanks to the Cassini mission, a great deal has been learned about Saturn’s system of moons (aka. the Cronian system) in the past decade. Thanks to the presence of an orbiter in the system, astronomers and space exploration enthusiasts have been treated to a seemingly endless stream of images and data, which in turn has enabled us to learn many interesting things about these moons’ appearances, surface features, composition, and history of formation.

This is certainly true of Saturn’s bright moon of Dione. In addition to being the 15th largest moon in the Solar System, and more massive than all known moons smaller than itself combined, it has much in common with other Cronian satellites – like Tethys, Iapetus and Rhea. This includes being mainly composed of ice, having a synchronous rotation with Saturn, and an unusual coloration between its leading and trailing hemispheres.

Discovery and Naming:Dione was first observed by Italian astronomer Giovanni Domenico Cassini on in 1684 using a large aerial telescope he set up on the grounds of the Paris Observatory. Along with the moons of Iapetus, Rhea and Tethys – which he had discovered in 1671, 1672 and 1684, respectively – he named these moons Sidera Lodoicea (“Stars of Louis”, after his patron, King Louis XIV of France).

These names, however, did not catch on outside of France. By the end of the 17th century, astronomers instead fell into the habit of naming Saturn’s then-known moons as Titan and Saturn I through V, in order of their observed distance from the planet. Being the second most-distant (behind Tethys) Dione came to be known as Saturn II for over a century.



An engraving of the Paris Observatory during Cassini's time. Credit: Public Domain


An engraving of the Paris Observatory during Cassini’s time. Credit: Public Domain
The modern names were suggested in 1847 by John Herschel (the son of famed astronomer William Herschel), who suggested all the moons of Saturn be named after Titans – the sons and daughters of Cronos in the Greek mythology (the equivalent of the Roman Saturn).

In his 1847 publication, Results of Astronomical Observations made at the Cape of Good Hope, he suggested the name Dione, an ancient oracular Titaness who was the wife of Zeus and the mother of Aphrodite. Dione is featured in Homer’s The Iliad, and geological features – such as craters and cliffs – take their names from people and places in Virgil’s Aeneid.

Size, Mass and Orbit:With a mean radius of 561.4 ± 0.4 km and a mass of about 1.0954 × 1021 kg, Dione is equivalent in size to 0.088 Earths and 0.000328 times as massive. It orbits Saturn at an average distance (semi-major axis) of 377,396 km, with a minor eccentricity of 0.0022 – ranging from 376,566 km at periapsis and 378,226 km at apoapsis.

Dione’s semi-major axis is about 2% less than that of the Moon. However, reflecting Saturn’s greater mass, Dione’s orbital period is one tenth that of the Moon (2.736915 days compared to 28). Dione is currently in a 1:2 mean-motion orbital resonance with Saturn’s moon Enceladus, completing one orbit of Saturn for every two orbits completed by Enceladus.



Size comparison between Earth, the Moon, and Saturn's moon Dione. Credit: NASA/JPL/Space Science Institute


Size comparison between Earth, the Moon, and Saturn’s moon Dione. Credit: NASA/JPL/Space Science Institute
This resonance maintains Enceladus’s orbital eccentricity (0.0047) and provides tidal flexing that powers Enceladus’ extensive geological activity (which in turn powers its cryovolcanic jets). Dione has two co-orbital (aka. trojan) moons: Helene and Polydeuces. They are located within Dione’s Lagrangian points, 60 degrees ahead of and behind it, respectively.

Composition and Surface Features:With a mean density of 1.478 ± 0.003 g/cm³, Dione is composed mainly of water, with a small remainder likely consisting of a silicate rock core. Though somewhat smaller and denser than Rhea, Dione is otherwise very similar in terms of its varied terrain, albedo features, and the different between its leading and trailing hemisphere.

Overall, scientists recognize five classes of geological features on Dione – Chasmata (chasms), dorsa (ridges), fossae (long, narrow depressions), craters, and catenae (crater chains). Craters are the most common feature, as with many Cronian moons, and can be distinguished in terms of heavily cratered terrain, moderately cratered plains, and lightly cratered plains.

The heavily cratered terrain has numerous craters greater than 100 km (62 mi) in diameter, whereas the plains areas tend to have craters less than 30 km (19 mi) in diameter (with some areas being more heavily cratered than others).



This global map of Dione, a moon of Saturn, shows dark red in the trailing hemisphere, which is due to radiation and charged particles from Saturn's intense magnetic environment. Credit: NASA/JPL/Space Science Institute


Global map of Dione, showing dark red in the trailing hemisphere (left), which is due to radiation and charged particles from Saturn’s. Credit: NASA/JPL/Space Science Institute
Much of the heavily cratered terrain is located on the trailing hemisphere, with the less cratered plains areas present on the leading hemisphere. This is the opposite of what many scientists expected, and suggests that during the period of Heavy Bombardment, Dione was tidally locked to Saturn in the opposite orientation.

Because Dione is relatively small, it is theorized that an impact large enough to cause a 35 km crater would have been sufficient to spin the satellite in the opposite direction. Because there are many craters larger than 35 km (22 mi), Dione could have been repeatedly spun during its early history. The pattern of cratering since then and the leading hemisphere’s bright albedo suggests that Dione has remained in its current orientation for several billion years.

Dione is also known for its differently colored leading and trailing hemispheres, which are similar to Tethys and Rhea. Whereas its leading hemisphere is bright, its trailing hemisphere is darker and redder in appearance. This is due to the leading hemisphere picking up material from Saturn’s E-Ring, which is fed by Enceladus’ cryovolcanic emissions.

Meanwhile, the trailing hemisphere interacts with radiation from Saturn’s magnetosphere, which causes organic elements contained within its surface ice to become dark and redder in appearance.



Dione's trailing hemisphere, showing the patches of "whispy terrain". Credit: NASA/JPL


Dione’s trailing hemisphere, pictured by the Cassini orbiter, which shows its patches of “wispy terrain”. Credit: NASA/JPL
Another prominent feature is Dione’s “wispy terrain“, which covers its trailing hemisphere and is composed entirely of high albedo material that is also thin enough as to not obscure the surface features beneath. The origin of these features are unknown, but an earlier hypothesis suggested that that Dione was geologically active shortly after its formation, a process which has since ceased.

During this time of geological activity, endogenic resurfacing could have pushed material from the interior to the surface, with streaks forming from eruptions along cracks that fell back to the surface as snow or ash. Later, after the internal activity and resurfacing ceased, cratering continued primarily on the leading hemisphere and wiped out the streak patterns there.

This hypothesis was proven wrong by the Cassini probe flyby of December 13th, 2004, which produced close-up images. These revealed that the ‘wisps’ were, in fact, not ice deposits at all, but rather bright ice cliffs created by tectonic fractures (chasmata). During this flyby, Cassini also captured oblique images of the cliffs which showed that some of them are several hundred meters high.

Atmosphere:Dione also has a very thin atmosphere of oxygen ions (O+²), which was first detected by the Cassini space probe in 2010. This atmosphere is so thin that scientists prefer to call it an exosphere rather than a tenuous atmosphere. The density of molecular oxygen ions determined from the Cassini plasma spectrometer data ranges from 0.01 to 0.09 per cm3 .



Crescent Dione from Cassini, October 11, 2005. The crater near the limb at top is Alcander, with larger crater Prytanis adjacent to its left. At lower right, several of the Palatine Chasmata fractures are visible, one of which can be seen bisecting the smaller craters Euryalus (right) and Nisus. NASA / Jet Propulsion Laboratory / Space Science Institute


Dione viewed by Cassini on October 11th, 2005, showing the Alcander crater (top) and the larger Prytanis crater to its left. Credit: NASA/JPL/SSI
Unfortunately, the prevalence of water molecules in the background (from Saturn’s E-Ring) obscured detection of water ice on the surface, so the source of oxygen remains unknown. However, photolysis is a possible cause (similar to what happens on Europa), where charged particles from Saturn’s radiation belt interact with water ice on the surface to create hydrogen and oxygen, the hydrogen being lost to space and the oxygen retained.

Exploration:Dione was first imaged by the Voyager 1 and 2 space probes as they passed by Saturn on their way to the Outer Solar System in 1980 and 1981, respectively. Since that time, the only probe to conduct a flyby or close-up imaging of Dione has been the Cassini orbiter, which conducted five flybys of the moon between 2005 and 2015.

The first close flyby took place on October 11th, 2005, at a distance of 500 km (310 mi), followed by another on April 7th, 2010, (again at a distance of 500 km). A third flyby was performed on December 12th, 2011, and was the closest, at an distance of 99 km (62 mi). The fourth and fifth flybys took place on June 16th and August 17th, 2015, at a distance of 516 km (321 mi) and 474 km (295 mi), respectively.



In addition to obtaining images of Cassini’s cratered and differently-colored surface, the Cassini mission was also responsible for detecting the moon’s tenuous atmosphere (exosphere). Beyond that, Cassini also provided scientists with new evidence that Dione could be more geologically active than previously predicted.

Based on models constructed by NASA scientists, it is now believed that Dione’s core experiences tidal heating, which increases the closer it gets to Saturn. Because of this, scientists also believe that Dione may also have a liquid water ocean at its core-mantle boundary, thus joining moons like Enceladus, Europa and others in being potential environments where extra-terrestrial life could exist.

This, as well as Dione’s geological history and the nature of its surface (which could be what gives rise to its atmosphere) make Dione a suitable target for future research. Though no missions to study the moon are currently being planned, any mission to the Saturn system in the coming years would likely include a flyby or two!

We have many great articles on Dione and Saturn’s moons here at Universe Today. Here is one about Cassini’s first flyby, its closest flyby, it’s possible geological activity, its canyons, and its wispy terrain.

Universe Today also has an interview with Dr. Kevin Grazier, a member of the Cassini-Huygens mission.





About 

Matt Williams is the Curator of the Guide to Space for Universe Today, a a regular contributor to HeroX, a science fiction author, and a Taekwon-Do instructor. He lives with his family on Vancouver Island in beautiful BC.

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Charon’s Twin ‘Star Wars’ Craters Are Distinctly Different; New Horizons Continues Toward KBO

Charon’s Twin ‘Star Wars’ Craters Are Distinctly Different; New Horizons Continues Toward KBO:



This composite image is based on observations from the New Horizons Ralph/LEISA instrument made at 10:25 UT (6:25 a.m. EDT) on July 14, 2015, when New Horizons was 50,000 miles (81,000 kilometers) from Charon. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.


This composite image is based on observations from the New Horizons Ralph/LEISA instrument made at 10:25 UT (6:25 a.m. EDT) on July 14, 2015, when New Horizons was 50,000 miles (81,000 kilometers) from Charon. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.
Just like Luke and Leia, two craters named for the Star Wars twins (Skywalker and Organa) have many similarities. They look about the same size and shape, and appear to have been created at the same time, and therefore are about the same age. But instruments on the New Horizons spacecraft detected one major difference: Organa and its surrounding area are laced with ammonia.

“Why are these two similar-looking and similar-sized craters, so near to each other, so compositionally distinct?” asked Will Grundy, who leads the New Horizons Composition team. “We have various ideas when it comes to the ammonia in Organa. The crater could be younger, or perhaps the impact that created it hit a pocket of ammonia-rich subsurface ice. Alternatively, maybe Organa’s impactor delivered its own ammonia.”


Both craters are roughly 5 kilometers (3 miles) in diameter, with similar appearances, such as bright rays of ejecta. One apparent difference is that Organa has a central region of darker ejecta, though from the map created with data from New Horizons’ Ralph/LEISA instrument, it appears that the ammonia-rich material extends beyond this dark area.

The nearby Skywalker crater, however, shows an infrared spectrum that is similar to the rest of Charon’s craters and surface, with features mostly dominated by ordinary water ice.

“This is a fantastic discovery,” said Bill McKinnon, deputy lead for the New Horizons Geology, Geophysics and Imaging team. “Concentrated ammonia is a powerful antifreeze on icy worlds, and if the ammonia really is from Charon’s interior, it could help explain the formation of Charon’s surface by cryovolcanism, via the eruption of cold, ammonia-water magmas.”

The New Horizons team is informally naming features after various sci-fi characters. So maybe – like their Star Wars namesakes – the craters Skywalker and Organa actually are different ages, as students at the University of Leicester calculated in a paper published earlier this year. The students said that Leia would be about 2 years old than Luke because of relative velocity time dilation – which describes the bending of spacetime due to differences in speed. Their different journeys through space in various craft would change how fast they are aging.

But we digress…



A new map of Pluto's 'heart.' This image released on October 29, 2015, provides fascinating new details to help the science team map the informally named Krun Macula (the prominent dark spot at the bottom of the image) and the complex terrain east and northeast of Pluto's "heart" (Tombaugh Regio). Pluto's north pole is on the planet's disk at the 12 o'clock position of this image. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute


A new map of Pluto’s ‘heart.’ This image released on October 29, 2015, provides fascinating new details to help the science team map the informally named Krun Macula (the prominent dark spot at the bottom of the image) and the complex terrain east and northeast of Pluto’s “heart” (Tombaugh Regio). Pluto’s north pole is on the planet’s disk at the 12 o’clock position of this image. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
Meanwhile, as New Horizons continues to send back more imagery and data, the spacecraft’s hydrazine-fueled thrusters completed the third of four maneuvers to direct the spacecraft towards an ancient and distant Kuiper Belt Object named 2014 MU69.

As we explained in our previous article, the four maneuvers are designed change New Horizons’ path to send it toward a close encounter with the KBO on Jan. 1, 2019. Even though the New Horizons spacecraft hasn’t officially been approved to do this flyby as an extended mission, the team is taking advantage of being able to do the maneuvers early, thereby saving fuel.

The science team hopes to bring the spacecraft even closer to MU69 than it came to Pluto this summer, which was approximately 7,750 miles (12,500 kilometers)

The fourth and final KBO targeting maneuver is scheduled for next week, Nov. 4, 2015.

Another image released this week from the New Horizons team:



This image was made just 15 minutes after New Horizons' closest approach to Pluto on July 14, 2015, as the spacecraft looked back at Pluto toward the sun. The wide-angle perspective of this view shows the deep haze layers of Pluto's atmosphere extending all the way around Pluto, revealing the silhouetted profiles of rugged plateaus on the night (left) side. The image was taken with New Horizons' Multi-spectral Visible Imaging Camera (MVIC) from a distance of 11,000 miles (18,000 kilometers) to Pluto. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute


This image was made just 15 minutes after New Horizons’ closest approach to Pluto on July 14, 2015, as the spacecraft looked back at Pluto toward the sun. The wide-angle perspective of this view shows the deep haze layers of Pluto’s atmosphere extending all the way around Pluto, revealing the silhouetted profiles of rugged plateaus on the night (left) side. The image was taken with New Horizons’ Multi-spectral Visible Imaging Camera (MVIC) from a distance of 11,000 miles (18,000 kilometers) to Pluto. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
In September, the New Horizons team released a stunning but incomplete image of Pluto’s crescent. Thanks to new processing work by the science team, New Horizons is releasing the entire, breathtaking image of Pluto.

Alex Parker, one of the science team members who worked on the image said on Twitter, “The haze over Pluto’s dark limb were frustratingly run through with instrumental artifacts. This version is my latest destripe and denoise.” He also noted a few things: look closely, and you can see background stars behind Pluto. Additionally, look at Pluto’s shadowed limb:

Another wonderful thing about that new image: the bright haze gives us a look at how bumpy Pluto's shadowed limb is! pic.twitter.com/hUSGIZB8Bw
— Alex Parker (@Alex_Parker) October 29, 2015
Sources: NASA, Johns Hopkins U.





About 

Nancy Atkinson is currently Universe Today's Contributing Editor. Previously she served as UT's Senior Editor and lead writer, and has worked with Astronomy Cast and 365 Days of Astronomy. Nancy is also a NASA/JPL Solar System Ambassador.

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Antarctic Analemma

Antarctic Analemma: APOD: 2015 September 23 - Antarctic Analemma



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2015 September 23


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Antarctic Analemma
Image Credit & Copyright: Adrianos Golemis
Explanation: Does the Sun return to the same spot on the sky every day? No. A better and more visual answer to that question is an analemma, a composite image taken from the same spot at the same time over the course of a year. The featured weekly analemma was taken despite cold temperatures and high winds near the Concordia Station in Antarctica. The position of the Sun at 4 pm was captured on multiple days in the digital composite image, believed to be the first analemma constructed from Antarctica. The reason the image only shows the Sun from September to March is because the Sun was below the horizon for much of the rest of the year. In fact, today being an equinox, the Sun rises today at the South Pole after a six month absence and won't set again until the next equinox in March, baring large atmospheric refraction effects. Conversely, today the Sun sets at the North Pole after half a year of continuous daylight. For all of the Earth in between, though, the equinox means that today will have a nighttime and daytime that are both 12 hours long.

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Tomorrow's picture: dark markings on the sky

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LDN 988 and Friends

LDN 988 and Friends:

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2015 September 24
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Explanation: Stars are forming in dark, dusty molecular cloud LDN 988. Seen near picture center some 2,000 light-years distant, LDN 988 and other nearby dark nebulae were cataloged by Beverly T. Lynds in 1962 using Palomar Observatory Sky Survey plates. Narrowband and near-infrared explorations of the dark nebula reveal energetic shocks and outflows light-years across associated with dozens of newborn stars. But in this sharp optical telescopic view, the irregular outlines of LDN 988 and friends look like dancing stick figures eclipsing the rich starfields of the constellation Cygnus. From dark sky sites the region can be identified by eye alone. It's part of the Great Rift of dark nebulae along the plane of the Milky Way galaxy known as the Northern Coalsack.

M31 versus M33

M31 versus M33: APOD: 2015 September 26 - M31 versus M33



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2015 September 26


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M31 versus M33
Image Credit & Copyright: Malcolm Park (North York Astronomical Association)
Explanation: Separated by about 14 degrees (28 Full Moons) in planet Earth's sky, spiral galaxies M31 at left, and M33 are both large members of the Local Group, along with our own Milky Way galaxy. This narrow- and wide-angle, multi-camera composite finds details of spiral structure in both, while the massive neighboring galaxies seem to be balanced in starry fields either side of bright Mirach, beta star in the constellation Andromeda. Mirach is just 200 light-years from the Sun. But M31, the Andromeda Galaxy, is really 2.5 million light-years distant and M33, the Triangulum Galaxy, is also about 3 million light years away. Although they look far apart, M31 and M33 are engaged in a gravitational struggle. In fact, radio astronomers have found indications of a bridge of neutral hydrogen gas that could connect the two, evidence of a closer encounter in the past. Based on measurements, gravitational simulations currently predict that the Milky Way, M31, and M33 will all undergo mutual close encounters and potentially mergers, billions of years in the future.

Tomorrow's picture: SuperMoonSunday

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Total Lunar Eclipse over Waterton Lake

Total Lunar Eclipse over Waterton Lake: APOD: 2015 September 28 - Total Lunar Eclipse over Waterton Lake



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2015 September 28
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Total Lunar Eclipse over Waterton Lake
Image Credit & Copyright: Yuichi Takasaka / TWAN / www.blue-moon.ca
Explanation: Recorded in 2014 April, this total lunar eclipse sequence looks south down icy Waterton Lake from the Waterton Lakes National Park in Alberta, Canada, planet Earth. The most distant horizon includes peaks in Glacier National Park, USA. An exposure every 10 minutes captured the Moon's position and eclipse phase, as it arced, left to right, above the rugged skyline and Waterton town lights. In fact, the sequence effectively measures the roughly 80 minute duration of the total phase of the eclipse. Around 270 BC, the Greek astronomer Aristarchus also measured the duration of lunar eclipses - though probably without the benefit of digital clocks and cameras. Still, using geometry, he devised a simple and impressively accurate way to calculate the Moon's distance, in terms of the radius of planet Earth, from the eclipse duration. This modern eclipse sequence also tracks the successive positions of Mars, above and right of the Moon, bright star Spica next to the reddened lunar disk, and Saturn to the left and below.

Gallery: Last night's total supermoon eclipse
Tomorrow's picture: lightning eclipse

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Supermoon Total Lunar Eclipse and Lightning Storm

Supermoon Total Lunar Eclipse and Lightning Storm:

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2015 September 29


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Explanation: What's more rare than a supermoon total lunar eclipse? How about a supermoon total lunar eclipse over a lightning storm. Such an electrifying sequence was captured yesterday from Ibiza, an island in southeastern Spain. After planning the location for beauty, and the timing to capture the entire eclipse sequence, the only thing that had to cooperate for this astrophotographer to capture a memorable eclipse sequence was the weather. What looked to be a bother on the horizon, though, turned out to be a blessing. The composite picture features over 200 digitally combined images from the same location over the course of a night. The full moon is seen setting as it faded to red in Earth's shadow and then returned to normal. The fortuitous lightning is seen reflected in the Mediterranean to the right of the 400-meter tall rocky island of Es Vedra. Although the next total eclipse of a large and bright supermoon will occur in 2033, the next total eclipse of any full moon will occur in January 2018 and be best visible from eastern Asia and Australia.

Seasonal Streaks Point to Recent Flowing Water on Mars

Seasonal Streaks Point to Recent Flowing Water on Mars:

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2015 September 30


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Explanation: What creates these changing streaks on Mars? Called Recurring Slope Linea (RSL), these dark features start on the slopes of hills and craters but don't usually extend to the bottom. What's even more unusual is that these streaks appear to change with the season, appearing fresh and growing during warm weather and disappearing during the winter. After much study, including a recent chemical analyses, a leading hypothesis has emerged that these streaks are likely created by new occurrences of liquid salty water that evaporates as it flows. The source for the briny water is still unclear, with two possibilities being condensation from the Martian atmosphere and underground reservoirs. An exciting inference is that if these briny flows are not too salty, they may be able to support microbial life on Mars even today. The featured image of a hill inside Horowitz Crater was investigated by instruments aboard the robotic Mars Reconnaissance Orbiter that has been returning data from Mars since 2006.

Eclipsed in Southern Skies

Eclipsed in Southern Skies:

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2015 October 1


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Explanation: This stunning panorama in southern skies was recorded on the colorful night of September 27/28 from Carnegie Las Campanas Observatory. A diffuse glow and dark rifts of the central Milky Way hang over domes of the twin 6.5 meter Magellan telescopes. But most eye-catching is the deep red glow of the Moon. Immersed in Earth's shadow during the much anticipated perigee-total-lunar eclipse, the Moon's surface reflects the light of sunsets and sunrises scattered and refracted into the planet's cone-shaped umbra. Along with the dramatic hue of the eclipsed Moon, other colors of that night captured by the sensitive digital camera include the red and green shades of atmospheric airglow. Viewers can also spot the Andromeda Galaxy below the Moon, seen as a tiny smudge through the reddish airglow and lights along the horizon. The Magellanic Clouds, satellite galaxies of the Milky Way, join in at the far left of the full panorama frame.

Charon: Moon of Pluto

Charon: Moon of Pluto: APOD: 2015 October 2 - Charon: Moon of Pluto



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2015 October 2


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Charon: Moon of Pluto
Image Credit: NASA, Johns Hopkins Univ./APL, Southwest Research Institute
Explanation: A darkened and mysterious north polar region informally known as Mordor Macula caps this premier high-resolution portrait of Charon, Pluto's largest moon. Captured by New Horizons near its closest approach on July 14, the image data was transmitted to Earth on September 21. The combined blue, red, and infrared data is processed to enhance colors, following variations in surface properties with a resolution of about 2.9 kilometers (1.8 miles). In fact, Charon is 1,214 kilometers (754 miles) across, about 1/10th the size of planet Earth but a whopping 1/2 the diameter of Pluto itself. That makes it the largest satellite relative to its planet in the solar system. This remarkable image of Charon's Pluto-facing hemisphere shows a clearer view of an apparently moon-girdling belt of fractures and canyons that seems to separate smooth southern plains from varied northern terrain.

Tomorrow's picture: four in a row

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A Blue Blood Moon

A Blue Blood Moon:

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2015 October 3


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Explanation: This sharp telescopic snapshot caught late September's Harvest Moon completely immersed in Earth's dark umbral shadow, at the beginning of a total lunar eclipse. It was the final eclipse in a tetrad, a string of four consecutive total lunar eclipses. A dark apparition of the Full Moon near perigee, this total eclipse's color was a deep blood red, the lunar surface reflecting light within Earth's shadow filtered through the lower atmosphere. Seen from a lunar perspective, the reddened light comes from all the sunsets and sunrises around the edges of a silhouetted Earth. But close to the shadow's edge, the limb of the eclipsed Moon shows a distinct blue hue. The blue eclipsed moonlight is still filtered through Earth's atmosphere though, originating as rays of sunlight pass through layers high in the upper stratosphere, colored by ozone that scatters red light and transmits blue.

The Sombrero Galaxy in Infrared

The Sombrero Galaxy in Infrared: APOD: 2015 October 4 - The Sombrero Galaxy in Infrared



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2015 October 4


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The Sombrero Galaxy in Infrared
Image Credit: R. Kennicutt (Steward Obs.) et al., SSC, JPL, Caltech, NASA
Explanation: This floating ring is the size of a galaxy. In fact, it is a galaxy -- or at least part of one: the photogenic Sombrero Galaxy, one of the largest galaxies in the nearby Virgo Cluster of Galaxies. The dark band of dust that obscures the mid-section of the Sombrero Galaxy in optical light actually glows brightly in infrared light. The above image, digitally sharpened, shows the infrared glow, recently recorded by the orbiting Spitzer Space Telescope, superposed in false-color on an existing image taken by NASA's Hubble Space Telescope in optical light. The Sombrero Galaxy, also known as M104, spans about 50,000 light years across and lies 28 million light years away. M104 can be seen with a small telescope in the direction of the constellation Virgo.

Tomorrow's picture: Orion, over and under

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Orion Over and Under Tibet

Orion Over and Under Tibet:

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2015 October 5
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Explanation: This night was so serene you could see Orion rise downwards. The unusual spectacle was captured in this single-exposure image, featuring a deep sky around the famous constellation of Orion that appeared both above -- and reflected in -- a peaceful lake in the Gyirong Valley of Tibet, China. Taken last year at this time, the three belt stars of Orion can be seen lined up almost vertically above and below the Himalayan Mountains. The complex Orion Nebula can be seen to the belt stars' right, while the red-glowing circular structure surrounding Orion is Barnard's Loop. Also, the bright red star Betelgeuse is doubly visible on the image left, while bright blue Rigel appears twice on the image right. Familiar Orion is becoming increasingly visible as Winter (Summer) descends on the Northern (Southern) hemisphere.

La Palma Eclipse Sequence

La Palma Eclipse Sequence:

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2015 October 7
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Explanation: At left, a dramatic image sequence follows late September's total lunar eclipse above a rugged landscape and sea of clouds from the Canary island of La Palma. Composited in a circular fisheye projection, the brightness of the Full Perigee Moon changes drastically in transition from outside the total eclipse phase compared to its dim glow during the 72 minute long totality. At right, a single frame captures the dark red lunar disk in a moment during the total eclipse phase, the Moon deep within Earth's shadow. In fact, the size of the eclipsed Moon image at right approximately illustrates the relative size of Earth and Moon, when compared to the circular projection of the eclipse sequence.

M83: The Thousand Ruby Galaxy

M83: The Thousand Ruby Galaxy: APOD: 2015 October 8 - M83: The Thousand Ruby Galaxy



Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.


2015 October 8


See Explanation. Clicking on the picture will download the highest resolution version available.



M83: The Thousand-Ruby Galaxy
Image Credit: Subaru Telescope (NAOJ), Hubble Space Telescope,
European Southern Observatory - Processing & Copyright: Robert Gendler
Explanation: Big, bright, and beautiful, spiral galaxy M83 lies a mere twelve million light-years away, near the southeastern tip of the very long constellation Hydra. Prominent spiral arms traced by dark dust lanes and blue star clusters lend this galaxy its popular name, The Southern Pinwheel. But reddish star forming regions that dot the sweeping arms highlighted in this sparkling color composite also suggest another nickname, The Thousand-Ruby Galaxy. About 40,000 light-years across, M83 is a member of a group of galaxies that includes active galaxy Centaurus A. In fact, the core of M83 itself is bright at x-ray energies, showing a high concentration of neutron stars and black holes left from an intense burst of star formation. This sharp composite color image also features spiky foreground Milky Way stars and distant background galaxies. The image data was taken from the Subaru Telescope, the European Southern Observatory's Wide Field Imager camera, and the Hubble Legacy Archive.

Tomorrow's picture: moon slide slim

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Stardust in Perseus

Stardust in Perseus:

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2015 October 10


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Explanation: This cosmic expanse of dust, gas, and stars covers some 6 degrees on the sky in the heroic constellation Perseus. At upper left in the gorgeous skyscape is the intriguing young star cluster IC 348 and neighboring Flying Ghost Nebula. At right, another active star forming region NGC 1333 is connected by dark and dusty tendrils on the outskirts of the giant Perseus Molecular Cloud, about 850 light-years away. Other dusty nebulae are scattered around the field of view, along with the faint reddish glow of hydrogen gas. In fact, the cosmic dust tends to hide the newly formed stars and young stellar objects or protostars from prying optical telescopes. Collapsing due to self-gravity, the protostars form from the dense cores embedded in the dusty molecular cloud. At the molecular cloud's estimated distance, this field of view would span almost 90 light-years.