Astronomy: Cassini’s Final Breathtaking Views of Saturn’s Moon Dione

Credit: NASA/JPL-Caltech/Space Science Institute

Credit: NASA/JPL-Caltech/Space Science Institute

Above: NASA’s Cassini spacecraft captured this parting view showing the rough and icy crescent of Saturn’s moon Dione following the spacecraft’s last close flyby of the moon on Aug. 17, 2015. Cassini obtained a similar crescent view in 2005. The earlier view has an image scale about four times higher, but does not show the moon’s full crescent as this view does.

Five visible light (clear spectral filter), narrow-angle camera images were combined to create this mosaic view. The scene is an orthographic projection centered on terrain at 0.4 degrees north latitude, 30.6 degrees west longitude on Dione. An orthographic view is most like the view seen by a distant observer looking through a telescope.

The view was acquired at distances ranging from approximately 37,000 miles (59,000 kilometers) to 47,000 miles (75,000 kilometers) from Dione and at a sun-Dione-spacecraft, or phase, angle of 145 degrees. Image scale is about 1,300 feet (400 meters) per pixel.

North on Dione is up and rotated 34 degrees to the right.

Credit: NASA/JPL-Caltech/Space Science Institute

Credit: NASA/JPL-Caltech/Space Science Institute

Above: Dione hangs in front of Saturn and its icy rings in this view, captured during Cassini’s final close flyby of the icy moon. North on Dione is up. The image was obtained in visible light with the Cassini spacecraft wide-angle camera on Aug. 17, 2015.

The view was acquired at a distance of approximately 45,000 miles (73,000 kilometers) from Dione and at a sun-Dione-spacecraft, or phase, angle of 35 degrees. Image scale is 3 miles (4 kilometers) per pixel.

Credit: NASA/JPL-Caltech/Space Science Institute

Credit: NASA/JPL-Caltech/Space Science Institute

Above: This view from NASA’s Cassini spacecraft looks toward Saturn’s icy moon Dione, with giant Saturn and its rings in the background, just prior to the mission’s final close approach to the moon on August 17, 2015. At lower right is the large, multi-ringed impact basin named Evander, which is about 220 miles (350 kilometers) wide. The canyons of Padua Chasma, features that form part of Dione’s bright, wispy terrain, reach into the darkness at left.

Imaging scientists combined nine visible light (clear spectral filter) images to create this mosaic view: eight from the narrow-angle camera and one from the wide-angle camera, which fills in an area at lower left. The scene is an orthographic projection centered on terrain at 0.2 degrees north latitude, 179 degrees west longitude on Dione. North on Dione is up.

The view was acquired at distances ranging from approximately 106,000 miles (170,000 kilometers) to 39,000 miles (63,000 kilometers) from Dione and at a sun-Dione-spacecraft, or phase, angle of 35 degrees. Image scale is about 1,500 feet (450 meters) per pixel.

The Cassini mission is a cooperative project of NASA, ESA (the European Space Agency) and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA’s Science Mission Directorate, Washington. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colorado.

On the Web:

Cassini-Huygens mission visit http://saturn.jpl.nasa.gov and www.nasa.gov/cassini.

The Cassini imaging team homepage

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Astronomy: Saturn’s Moon Titan Not So Titanic

Credit: NASA/JPL-Caltech/Space Science Institute

Credit: NASA/JPL-Caltech/Space Science Institute

Although Titan (3200 miles or 5150 kilometers across) is the second-largest moon in the solar system, Saturn is still much bigger, with a diameter almost 23 times larger than Titan’s. This disparity between planet and moon is the norm in the solar system.

Earth’s diameter is “only” 3.7 times our moon’s diameter, making our natural satellite something of an oddity. (Another exception to the rule: dwarf planet Pluto’s diameter is just under two times that of its moon.) So the question isn’t why is Titan so small (relatively speaking), but why is Earth’s moon so big?

This view looks toward the anti-Saturn hemisphere of Titan. North on Titan is up. The image was taken with the Cassini spacecraft wide-angle camera on April 18, 2015 using a near-infrared spectral filter with a passband centered at 752 nanometers.

The view was acquired at a distance of approximately 930,000 miles (1.5 million kilometers) from Titan. Image scale is 56 miles (90 kilometers) per pixel.

The Cassini mission is a cooperative project of NASA, ESA (the European Space Agency) and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA’s Science Mission Directorate, Washington. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colorado.

On the Web:

For more information about the Cassini-Huygens mission 

The Cassini imaging team homepage

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Astronomy: The Week Ahead – Sun 5 Jul to Sat 11 Jul

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Sunday July 05

Today an 18.5-day-old 85% illuminated waning gibbous Moon is at perigee, at 11:54 A.M. This is its closest point to earth in this orbit of our planet. When the Moon is full at perigee, it has been referred to as a Supermoon. See the Moon best today before sunrise. It sets just before 10 a.m. Look carefully and try to see it during the day.

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Monday July 06

This morning in the early sunrise, Earth reaches Aphelion. It will be 1.017 AU (Astronomical Units) from the Sun at 05:59 PDT. This is its farthest point from the Sun in our planet’s elliptical orbit.

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Tuesday July 07

Pluto is at opposition today. The Earth is directly between the dwarf planet and the Sun. So, Pluto rises at sunset today, is up all night, and sets with the next morning’s sunrise. Pluto is in an easy to locate position, right off the tip of the Teaspoon in Sagittarius. The above image shows its position relative to the nearby bright star. Even with this help, Pluto will look like a dim star in a telescope. But, with the New Horizon space probe visiting this distant world on in just one week, this may be a fun target to pursue!

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Wednesday July 08

The Last Quarter Moon occurs at 13:24 today. It will be slightly larger than half illuminated in the early hours when it rises over the eastern horizon in Pisces, below Pegasus. Last Quarter is tacitly the beginning of a dark sky observing window, for deep sky observers, lasting until after New Moon. Get your telescopes out and let us know what you’re looking at!

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Thursday July 09

Before it rose, just after it 1 a.m. this morning, the Moon occulted the seventh planet Uranus. Look at yesterday’s Moon position and you can see how its path to today’s location crossed over the planet. Uranus is in an easy position currently, for star hoppers to get to, just off the two brighter stars in the western string of Pisces. Wait for the moon to move away over the next few nights, and add Uranus to this week’s planetary targets.

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Friday July 10

Did you all enjoy the Venus and Jupiter conjunction last week? It won’t occur again until 2023, so it was really a special treat! Still, the two planets are close together, setting after evening twilight. Continue to watch them move further apart, as Venus approaches Leo’s bright star Regulus, along the Ecliptic, as is Virgo’s Spica.

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Saturday July 11

Due south an hour after sunset, the great globular cluster M4 is easy to view, just off Scorpius’ giant red star Antares. One of the closest globular cluster’s to our position in the Milky Way Galaxy, it is thought to be some 9,000 light years distant. Although it is a Messier object, in the famed French comet hunter’s catalog, it was discovered in 1746 Philippe Loys de Chéseaux. It was the first globular cluster in which individual stars were seen. At magnitude 5.9, this is at the threshold of most people’s sight. Don’t forget to visit nearby Saturn too, if you are in the area.

Happy viewing!

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Astronomy: Icy Tendrils Reaching into Saturn Ring

Credit: NASA/JPL-Caltech/Space Science Institute

Credit: NASA/JPL-Caltech/Space Science Institute

Long, sinuous, tendril-like structures seen in the vicinity of Saturn’s icy moon Enceladus originate directly from geysers erupting from its surface, according to scientists studying images from NASA’s Cassini spacecraft. This result is published online today in a study in the Astronomical Journal, along with additional insights into the nature of the structures.

“We’ve been able to show that each unique tendril structure can be reproduced by particular sets of geysers on the moon’s surface,” said Colin Mitchell, a Cassini imaging team associate at the Space Science Institute in Boulder, Colorado, and lead author of the paper. Mitchell and colleagues used computer simulations to follow the trajectories of ice grains ejected from individual geysers. The geysers, which were discovered by Cassini in 2005, are jets of tiny water ice particles, water vapor and simple organic compounds.

Under certain lighting conditions, Cassini’s wide-view images showing icy material erupting from Enceladus reveal faint, finger-like features, dubbed “tendrils” by the imaging team. The tendrils reach into Saturn’s E ring—the ring in which Enceladus orbits—extending tens of thousands of miles (or kilometers) away from the moon. Since the tendrils were discovered, scientists have thought they were the result of the moon’s geysering activity and the means by which Enceladus supplies material to the E ring. But the ghostly features had never before been traced directly to geysers on the surface.

Because the team was able to show that tendril structures of different shapes correspond to different sizes of geyser particles, the team was able to zero in on the sizes of the particles forming them. They found the tendrils are composed of particles with diameters no smaller than about a hundred thousandth of an inch, a size consistent with the measurements of E-ring particles made by other Cassini instruments.

As the researchers examined images from different times and positions around Saturn, they also found that the detailed appearance of the tendrils changes over time. “It became clear to us that some features disappeared from one image to the next,” said John Weiss, an imaging team associate at Saint Martin’s University in Lacey, Washington, and an author on the paper.

The authors suspect that changes in the tendrils’ appearance likely result from the cycle of tidal stresses—squeezing and stretching of the moon as it orbits Saturn—and its control of the widths of fractures from which the geysers erupt. The stronger the tidal stresses raised by Saturn at any point on the fractures, the wider the fracture opening and the greater the eruption of material. The authors will investigate in future work whether this theory explains the tendrils’ changing appearance.

There is even more that can be extracted from the images, the scientists say. “As the supply lanes for Saturn’s E ring, the tendrils give us a way to ascertain how much mass is leaving Enceladus and making its way into Saturn orbit,” said Carolyn Porco, team leader for the imaging experiment and a coauthor on the paper. “So, another important step is to determine how much mass is involved, and thus estimate how much longer the moon’s sub-surface ocean may last.” An estimate of the lifetime of the ocean is important in understanding the evolution of Enceladus over long timescales.

Because of its significance to the investigation of possible extraterrestrial habitable zones, Enceladus is a major target of investigation for the final years of the Cassini mission. Many observations, including imaging of the plume and tendril features, and thermal observations of the surface of its south polar geyser basin, are planned during the next couple of years.

The Cassini-Huygens mission is a cooperative project of NASA, ESA (European Space Agency) and the Italian Space Agency. NASA’s Jet Propulsion Laboratory in Pasadena, California, manages the mission for the agency’s Science Mission Directorate in Washington. JPL is a division of the California Institute of Technology in Pasadena. The Cassini imaging operations center is based at the Space Science Institute in Boulder, Colorado.

On the Web:

For more information about Cassini, visit:
www.nasa.gov/cassini
http://saturn.jpl.nasa.gov

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Astronomy: The Week Ahead – Mon 23 Feb thru Sat 28 Feb

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Monday February 23

Today Saturn is at western quadrature. The earth and Saturn are at a 90-degree angle from each other relative to the Sun. Quadrature occurs at 5:38 A.M. PST.

Saturn will culminate (reach its highest point above the horizon) at sunrise today. This means Saturn is visible in the night sky for almost half the dark hours, rising at 01:05 and fading into the sunrise which occurs at 06:47.

As the Earth catches up in our orbit, Saturn will be visible in the night sky during more dark hours.

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Tuesday February 24

Crash Test: Have you been watching the Venus and Mars show the last several days? Mars was just above Venus last week, and has now moved below. Which planet is moving faster across our sky?

The two are just under 1 and 1/2 degrees apart from our viewpoint, but separated by 0.815 AU (Astronomical Units) in actual distance. An AU is the measure of the Earth’s average distance from the sun, approximately 93 million miles.

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Wednesday February 25

Today’s first quarter Moon is 1 degree north of the giant red star Aldebaran, in Taurus. They form a striking pair set above the giant Orion, and its bright stars Betelgeuse and Rigel. Early in the evening, you can also see Jupiter to the east and the pair of Venus and Mars to the west.

Today also marks Neptune’s conjunction. It is on the opposite side of the sun from the earth.

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Thursday February 26

Everyone loves a two-for-one deal, and here’s one to start your day with.

The constellation Hercules is up in the east before sunrise. You can make out its shape above the bright star Vega (in Lyra). It contains two great globular clusters, M13 and M92. They are easy to find in binoculars using the “keystone” shape of Hercules’ body. M13 is 2/3rd along one side of the keystone. Using the other side of the keystone, hop twice that distance to a star, then go about 1/3rd the way back toward the top star in the keystone. Both giant globular clusters are about 25,000 light years distant.

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Friday February 27

The summer constellation Scorpius is up nicely in the morning skies. Saturn is in the same field of view as the multiple star Nu Scorpii. Antares burns red as the heart of the scorpion. The stinger is easy to imagine as the stars Shaula and Lesath.

Nearby are the binocular objects M6 and M7, two lovely bright open clusters in the heart of the Milky Way.

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Saturday February 28

The Moon now dominates the evening sky, growing in waxing gibbous phase and 82% illuminated tonight. Its brightness will obliterate our view of the winter Milky Way running between Orion and Gemini. Only the brightest stars and Jupiter will be visible in the nearby constellations. Look for Procyon in Canis Minor – it shines at magnitude 0.37. Can you spot it with a bright moon so nearby?

Happy viewing!

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Sunday Reader: Think Rings of Saturn – Only Bigger

If your eyes could only see the color red, this is how Saturn's rings would look. Many Cassini color images, like this one, are taken in red light so scientists can study the often subtle color variations of Saturn's rings. These variations may reveal clues about the chemical composition and physical nature of the rings. For example, the longer a surface is exposed to the harsh environment in space, the redder it becomes. Putting together many clues derived from such images, scientists are coming to a deeper understanding of the rings without ever actually visiting a single ring particle. This view looks toward the sunlit side of the rings from about 11 degrees above the ringplane. The image was taken in red light with the Cassini spacecraft narrow-angle camera on Dec. 6, 2014. The view was acquired at a distance of approximately 870,000 miles (1.4 million kilometers) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 27 degrees. Image scale is 5 miles (8 kilometers) per pixel. Phot courtesy of NASA/JPL-Caltech/Space Science Institute

If your eyes could only see the color red, this is how Saturn’s rings would look.
Many Cassini color images, like this one, are taken in red light so scientists can study the often subtle color variations of Saturn’s rings. These variations may reveal clues about the chemical composition and physical nature of the rings. For example, the longer a surface is exposed to the harsh environment in space, the redder it becomes. Putting together many clues derived from such images, scientists are coming to a deeper understanding of the rings without ever actually visiting a single ring particle.
This view looks toward the sunlit side of the rings from about 11 degrees above the ringplane. The image was taken in red light with the Cassini spacecraft narrow-angle camera on Dec. 6, 2014.
The view was acquired at a distance of approximately 870,000 miles (1.4 million kilometers) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 27 degrees. Image scale is 5 miles (8 kilometers) per pixel.
Photo courtesy of NASA/JPL-Caltech/Space Science Institute

If you think you’ve seen big rings in our own solar system, think again. Say hello to J1407b

When the University of Rochester’s Eric Mamajek tells other astronomers about the object he and his colleagues discovered about 430 light-years from Earth, they tend to be skeptical—very skeptical. And no wonder: What he’s found is a giant ring system, sort of like Saturn’s, but some 200 times bigger, circling what may be an exoplanet between ten and 40 times the size of Jupiter. If you put these rings in our own Solar System, they’d stretch all the way from the Earth to the Sun, a distance of 93 million miles (150 km). And what’s more, there’s evidence that the rings are sculpted by at least one exomoon—something that also happens at Saturn, but not remotely on this scale.

“It took us a year even to convince ourselves of what we were seeing,” says Mamajek, whose paper is based on a new analysis of observations taken back in 2007 by the SuperWASP planet search project. At the time, the observations seemed to make no sense: when a planet passes in front of a star, you usually see a dip in starlight that lasts for up to a few hours. In this case, the starlight dimmed for two months.

It wasn’t a steady dip, either. The star would fade, then brighten, then fade again, in a way that made no sense at all. When Mamajek and his group stumbled on the data in 2010, he says, “I took a printout of the light curve, put it on the wall, and stared at it for a week.” Crazy as it seemed, the most plausible explanation was a giant ring system with gaps like Saturn’s that let more or less light through at different times during the passage. “It’s the same indirect way the rings of Uranus were discovered in 1977,” he says.

The planet itself doesn’t show up in the observations, but that could be explained if the ring system is slightly off-center as it moves in front of the star. You can see how this works in an animation put together by Mamajek’s collaborator Matthew Kenworthy, of the University of Leiden, in the Netherlands.

The star which the new planet orbits is thought to be very young—about 16 million years, compared with our own Solar System’s 4.6 billion. If the scientists are right about what they’re seeing, the mammoth ring system will get smaller over time as the outer bands condense into moons. “That’s what you see in [our] Solar System,” says Kenworthy. “You have rings tucked in close to the planets and moons further out. So presumably we’re seeing the intermediate step.”

This is an artist's conception of the extrasolar ring system circling the young giant planet or brown dwarf J1407b with Saturn and its ring system to scale (in upper right quadrant). The rings are shown eclipsing the young sun-like star J1407, as they would have appeared in early 2007. The best fit model is consistent with a system of at least 30 rings, and there are gaps where satellites ("exomoons") may have already formed. Image courtesy Ron Miller/University of Rochester

This is an artist’s conception of the extrasolar ring system circling the young giant planet or brown dwarf J1407b with Saturn and its ring system to scale (in upper right quadrant). The rings are shown eclipsing the young sun-like star J1407, as they would have appeared in early 2007. The best fit model is consistent with a system of at least 30 rings, and there are gaps where satellites (“exomoons”) may have already formed.
Image courtesy Ron Miller/University of Rochester

It all seems familiar, except for the ring system’s size, which is unprecedented—and which is the reason other astronomers are waiting to be convinced. “I agree with the authors that it’s appropriate to consider an interpretation based on rings,” says Eric Ford, an expert on exoplanets at Penn State. The idea that the outer parts would condense into moons relatively quickly, however, means that we’re seeing the rings at their full extent during a very narrow window of existence—the sort of coincidence that scientists don’t love to see. “Whenever your explanation involves catching something during a phase that won’t last very long,” Ford says, “it’s a little concerning.”

Much of the doubt could be erased if astronomers could see the rings pass by again on another orbit around the star. Unfortunately, that hasn’t happened: they’ve got only the single passage back in 2007, meaning the exoplanet is on a relatively long orbit. “We think it’s at least ten or 15 years,” says Kenworthy.

They don’t know for sure, though, and since it’s tough to keep big telescopes aimed at this one star hoping for another passage, the astronomers have recruited members of the high-end amateur group, the American Association of Variable Star Observers, to monitor the situation. They’re also going back through digitized versions of old images from observatories around the world, looking for evidence of other stars that faded mysteriously for a while without explanation. “Now that we know what we’re looking for,” Mamajek says, “we might find that there are lots of them out there.”

They might, that is, if they’re really seeing rings. “I keep telling people, ‘if you can think of a better explanation, please let me know,’” Mamajek says, and he means it. So far, he has no takers. “The signal is very strong,” says Harvard’s David Kipping, who is doing his own search for exomoons, “and its difficult to believe the instrument could misbehave on such a huge scale. I think many of us find the signal interesting,” he says. That, by itself, is enough to keep the astronomy community looking.

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For the Love of…..Saturn!

Lots of Waving Going On!

Lots of Waving Going On!

This mosaic is made up of 1400+ images of Earthlings waving at Saturn, in celebration of the July 19 Cassini spacecraft photo shoot of Earth and Saturn’s rings.

SATURN will disappear into the sunset twilight in late October, only to re-emerge in morning skies in mid-November.

On the Web:

See a larger image of this cool mosaic http://bit.ly/17OoqOX

See Cassini’s July 19 images http://bit.ly/16Blp5x

More on observing Saturn in 2013http://1.usa.gov/1871fyg

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A Splendor Seldom Seen

Aside

Image

NASA’s Cassini–Huygens spacecraft has delivered a glorious view of Saturn, taken while the spacecraft was in Saturn’s shadow. The cameras were turned toward Saturn and the sun so that the planet and rings are backlit. (The Sun is behind the planet, which is shielding the cameras from direct sunlight.) In addition to the visual splendor, this special, very-high-phase viewing geometry lets scientists study ring and atmosphere phenomena not easily seen at a lower phase.

Since images like this can only be taken while the sun is behind the planet, this beautiful view is all the more precious for its rarity. The last time Cassini captured a view like this was in Sept. 2006, when it captured a mosaic processed to look like natural color, entitled “In Saturn’s Shadow.” In that mosaic, planet Earth put in a special appearance, making “In Saturn’s Shadow” one of the most popular Cassini images to date. Earth does not appear in this mosaic as it is hidden behind the planet.

Also captured in this image are two of Saturn’s moons: Enceladus and Tethys. Both appear on the left side of the planet, below the rings. Enceladus is closer to the rings; Tethys is below and to the left.

This view looks toward the non-illuminated side of the rings from about 19 degrees below the ring plane.

Images taken using infrared, red and violet spectral filters were combined to create this enhanced-color view. The images were obtained with the Cassini spacecraft wide-angle camera on Oct. 17, 2012 at a distance of approximately 500,000 miles (800,000 kilometers) from Saturn. Image scale at Saturn is about 30 miles per pixel (50 kilometers per pixel).

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA’s Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colo.

(Photo courtesy of NASA)

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