Category Archives: Cassini

Transit of Dione

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This is the moon Dione seen against a background of Saturn, what is known as a transit.  The image gives a good perspective of just how large Saturn is. The view is from 2.4 million km / 1.4 million miles from Cassini to Saturn and Dione is 1123 km / 698 miles across.

Transits are very important in astronomy and are used to precisely determine orbital parameters of planets and their atmospheres. We also see planetary transits from Earth of Venus and Mercury against the Sun.

We will have a transit of Mercury next year: 16 May 2016 but will have to wait until 11 December 2117 for the next transit of Venus.

NASA has a couple of fun transit sites, the first for Mercury and the second for Venus.

Click the image for a large version and check out the far rings, almost can make out perturbations.

Image Credit: NASA/JPL-Caltech/Space Science Institute

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Dione

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Here are a couple more images from Cassini’s final flyby of the Saturn moon Dione.

dionelandscapeThe image above was taken as Cassini was heading towards the moon. The impact basin on the lower right is called Evander and is about 350 km / 220 miles wide.

On the left you can see the bright Padua Chasma, reaching into the left side of the moon.

To the right we have a very nice close up of Dione from just 750 km / 470 miles giving a resolution of 45 meters per pixel. Click the thumbnail to see the bigger version, the picture gives a great sense of the icy make-up of the moon. North is down in this view.

mage Credit: NASA/JPL-Caltech/Space Science Institute

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Nearing Dione

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As Cassini neared the Saturn moon Dione it captured this image of the moon in front of Saturn’s rings.

When the image was taken Cassini was 158,000 km / 98,000 miles away on 17 August. We see the southern part of the moon in the image – north is up.

This is the final flyby of the moon for Cassini.

Image Credit: NASA/JPL-Caltech/Space Science Institute

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The Last at Dione

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Cassini took this image of the Saturn moon Dione in the last flyby of the moon.

The main mission for Cassini ended in June 2008. The Cassini Equinox Mission was the first mission extention and ran until September 2010. The current (and second) mission extention called the Solstice Mission is scheduled to end in September 2017.

So keeping the end of mission in mind this image really is from the last flyby of the moon.

The original caption from Cassini:
While not bursting with activity like its sister satellite Enceladus, the surface of Dione is definitely not boring. Some parts of the surface are covered by linear features, called chasmata, which provide dramatic contrast to the round impact craters that typically cover moons.

The bright network of fractures on Dione (698 miles or 1123 kilometers across) was seen originally at poor resolution in Voyager images and was labeled as “wispy terrain.” The nature of this terrain was unclear until Cassini showed that they weren’t surface deposits of frost, as some had suspected, but rather a pattern of bright icy cliffs among myriad fractures. One possibility is that this stress pattern may be related to Dione’s orbital evolution and the effect of tidal stresses over time.

This view looks toward the trailing hemisphere of Dione. North on Dione is up. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on April 11, 2015.

The view was acquired at a distance of approximately 68,000 miles (110,000 kilometers) from Dione. Image scale is 2,200 feet (660 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.

Image Credit: NASA/JPL-Caltech/Space Science Institute

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Saturn’s Atmosphere

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The Cassini spacecraft took this image with an infrared filter on its camera.

This particular filter is sensitive to infrared wavelengths absorbed by methane. There isn’t a lot of methane in Saturn’s atmosphere but there is enough to make a difference in how much light is reflected by different clouds.

The darker areas are showing clouds lower in the atmosphere and scientists think the atomosphere is descending. The brighter areas are higher altitude clouds and it is belived the atmosphere is rising. So we get some sense of the vertical movements of atmospheric gasses.

The filter used admits wavelengths in the near-infrared centered at 890 nanometers.

Cassini was 1.5 million km / 930,000 miles from Saturn when it took this image.

Image credit: NASA/JPL-Caltech/Space Science Institute

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Giant Saturn

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The two moons against a backdrop of Saturn and the ring plane are:

Mimas – about 200,000 km / 124,000 miles from Saturn with an orbital period of about 23 hours,
and
Dione – about 377,400 km (234,000 miles) from Saturn with an orbital period of about 2.7 Earth days (~ 65 hours).

From the Cassini site:

Thanks to the illumination angle, Mimas (right) and Dione (left) appear to be staring up at a giant Saturn looming in the background.

Although certainly large enough to be noticeable, moons like Mimas (246 miles or 396 kilometers across) and Dione (698 miles or 1123 kilometers across) are tiny compared to Saturn (75,400 miles or 120,700 kilometers across). Even the enormous moon Titan (3,200 miles or 5,150 kilometers across) is dwarfed by the giant planet.
This view looks toward the unilluminated side of the rings from about one degree of the ring plane. The image was taken with the Cassini spacecraft wide-angle camera on May 27, 2015 using a spectral filter which preferentially admits wavelengths of near-infrared light centered at 728 nanometers.

The view was obtained at a distance of approximately 634,000 miles (one million kilometers) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 85 degrees. Image scale is 38 miles (61 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.

Credit: NASA/JPL-Caltech/Space Science Institute

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Reddish Arcs on Tethys

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The Cassini spacecraft took these intriguing images. What are those red arcs on Saturns moon Tethys? Keep in mind these are color enhanced and taken through four different filters, including infrared and ultraviolet. Still they are pretty surprising. The press release explains the image and shares a couple of possible explanations. It seems like it could also be a combination of two of the ideas below, ices being contaminated by outgassing.

The press release from the Cassini site:

Unusual arc-shaped, reddish streaks cut across the surface of Saturn’s ice-rich moon Tethys in this enhanced-color mosaic. The red streaks are narrow, curved lines on the moon’s surface, only a few miles (or kilometers) wide but several hundred miles (or kilometers) long. The red streaks are among the most unusual color features on Saturn’s moons to be revealed by Cassini’s cameras.

Continue reading

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Mother and Daughter

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From the Cassini page:

In Greek mythology, Dione was the daughter of Tethys, so we should perhaps not be surprised to see the two eponymous moons together.

In reality, the moons Tethys (660 miles or 1062 kilometers across) and Dione (698 miles or 1123 kilometers across) are not mother and daughter in any sense. They are perhaps more like sisters since scientists believe that they formed out of the same disk around an early Saturn.

Dione in this image is the upper moon, while Tethys is the lower.

This view looks toward the anti-Saturn side of Dione. North on Dione is up. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on April 4, 2015.

The view was obtained at a distance of approximately 1.5 million miles (2.4 million kilometers) from Dione. Image scale is 9 miles (14 kilometers) per pixel.

Credit: NASA/JPL-Caltech/Space Science Institute

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Pluto Seen from Cassini

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As New Horizons made its close approach, the Cassini spacecraft in orbit around Saturn was able to take this image showing Pluto.  Hard to tell which is Pluto?  Yes I agree, it is just about centered in the frame, click the image for an annotated version.

The distance to Pluto at the time was 3.9 billion km / 2.4 billion miles and the resolution of the image is too low to be able to see other members of the Pluto family.

You will note in the annotated version there are four stars labeled too. Those stars have a magnitude of between 11 and 12.

New Pluto pictures starting later today.

Image Credit:  NASA/JPL-Caltech/Space Science Institute

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Titan and Saturn

While we wait for data from New Horizons, here is a lovely Cassini image of Saturn and Titan. This image was acquired from 1.5 million km / 930,000 miles looking toward the anti-Saturn hemisphere of Titan.

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The image gives us a nice sense of scale, Titan at 5,150 km / 3,200 miles across (and the second largest moon in our solar system) has a diameter 23 times smaller than its parent planet Saturn.

Things are much different in the Plutonian system, Charon is just over half the diameter of Pluto and many people think of the pair as binary planets.

Image Credit: NASA/JPL-Caltech/Space Science Institute

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