Tag: Saturn

Engineers have confirmed that Cassini is now on its final approach to Saturn, with a scheduled dive into the gas giant set for Friday, September 15.

The mission’s final calculations predict loss of contact with the Cassini spacecraft will take place on Sept. 15 at 7:55 a.m. EDT (4:55 a.m. PDT). Cassini will enter Saturn’s atmosphere approximately one minute earlier, at an altitude of about 1,190 miles (1,915 kilometers) above the planet’s estimated cloud tops (the altitude where the air pressure is 1-bar, equivalent to sea level on Earth). During its dive into the atmosphere, the spacecraft’s speed will be approximately 70,000 miles (113,000 kilometers) per hour. The final plunge will take place on the day side of Saturn, near local noon, with the spacecraft entering the atmosphere around 10 degrees north latitude.

We will get Cassini’s last images about 90 minutes after its death.

The Cassini science team released two sets of images taken by the spacecraft of Saturn’s rings.

• Staggering Structure
• Colorful Structure at Fine Scales

The image above, reduced in resolution to show here, is from the second link. As they note,

The pale tan color is generally not perceptible with the naked eye in telescope views, especially given that Saturn has a similar hue.

The material responsible for bestowing this color on the rings—which are mostly water ice and would otherwise appear white—is a matter of intense debate among ring scientists that will hopefully be settled by new in-situ observations before the end of Cassini’s mission.

The different ringlets seen here are part of what is called the “irregular structure” of the B ring. Cassini radio occultations of the rings have shown that these features have extremely sharp boundaries on even smaller scales (radially, or along the direction outward from Saturn) than the camera can resolve here. Closer to Saturn, the irregular structures become fuzzier and more rounded, less opaque, and their color contrast diminishes.

Check out both. They reveal to me that our understanding of these rings remains essentially nil, even after more than a dozen years of study by Cassini.

Cool image time! The Cassini science team has assembled a short movie from 21 images taken by the spacecraft during its August 20th dive between the rings and Saturn. That animation is to the right.

Only two weeks remain in the Cassini mission. The spacecraft dives into Saturn on September 15.

The spacecraft is expected to lose radio contact with Earth within about one to two minutes after beginning its descent into Saturn’s upper atmosphere. But on the way down, before contact is lost, eight of Cassini’s 12 science instruments will be operating. In particular, the spacecraft‘s ion and neutral mass spectrometer (INMS), which will be directly sampling the atmosphere’s composition, potentially returning insights into the giant planet’s formation and evolution. On the day before the plunge, other Cassini instruments will make detailed, high-resolution observations of Saturn’s auroras, temperature, and the vortices at the planet’s poles. Cassini’s imaging camera will be off during this final descent, having taken a last look at the Saturn system the previous day (Sept. 14).

The second link above gives a detailed moment-by-moment breakdown of the final six days.

I would have posted this earlier this week, but my limited internet access made it impossible. Sorry about that.

Cool image time! The Cassini image above, cropped and reduced slightly, shows a close-up view of Saturn’s cloud tops, taken during Cassini’s May 18, 2017 fly-by.

Clouds on Saturn take on the appearance of strokes from a cosmic brush thanks to the wavy way that fluids interact in Saturn’s atmosphere. Neighboring bands of clouds move at different speeds and directions depending on their latitudes. This generates turbulence where bands meet and leads to the wavy structure along the interfaces.

What I see is a much less turbulent storm pattern, when compared with Jupiter. This is not to say that the weather of Saturn is quiet or peaceful, for it certainly cannot be, considering the gas giant’s size and the depth of the atmosphere. Still, this image suggests that the turbulence is less violent here, possibly because Saturn is farther from the Sun and is hit with less solar energy, and because Saturn is smaller and thus produces less of its own internal energy.

Either way, it is beautiful and mysterious, in the way are all such images of alien places.

Cassini is about to begin its last of five orbits of Saturn, before it is sent into the planet’s atmosphere to burn up.

Cassini will make the first of these five passes over Saturn at 12:22 a.m. EDT Monday, Aug. 14. The spacecraft’s point of closest approach to Saturn during these passes will be between about 1,010 and 1,060 miles (1,630 and 1,710 kilometers) above Saturn’s cloud tops.

The spacecraft is expected to encounter atmosphere dense enough to require the use of its small rocket thrusters to maintain stability – conditions similar to those encountered during many of Cassini’s close flybys of Saturn’s moon Titan, which has its own dense atmosphere. “Cassini’s Titan flybys prepared us for these rapid passes through Saturn’s upper atmosphere,” said Earl Maize, Cassini project manager at NASA’s Jet Propulsion Laboratory (JPL) in California. “Thanks to our past experience, the team is confident that we understand how the spacecraft will behave at the atmospheric densities our models predict.”

Cool image time! The image on the right, reduced in resolution to show here, was taken by Cassini in May and looks down at the outer rings of Saturn. The moon Prometheus can also be seen in the large gap between the main rings and the outermost F ring.

Most of the small moon’s surface is in darkness due to the viewing geometry here. Cassini was positioned behind Saturn and Prometheus with respect to the sun, looking toward the moon’s dark side and just a bit of the moon’s sunlit northern hemisphere.

Also visible here is a distinct difference in brightness between the outermost section of Saturn’s A ring (left of center) and rest of the ring, interior to the Keeler Gap (lower left).

The image clearly shows the gravitational influence of the moon on the outer ring. As Prometheus orbits past the F ring its mass creates waves through the ring’s materials.

Cool image time. One of the images released by the Cassini science team this week when it announced the first results from the spacecraft’s weekly dives between Saturn and its innermost rings was a short video made from 137 images taken on its first dive on April 26, 2017.

It is absolutely worthwhile to view this video. It begins at Saturn’s north pole, looking down into what appears to be a bottomless vortex, and continues south to almost the equator. Along the way the movie captures what seems to be innumerable horizontal bands across the gas giant’s surface. Not only are do we see the major bands that have been observed from Earth for centuries, there are bands within bands, and bands within those bands. Like a fractal it appears that the deeper you go, the more horizontal jet streams you see.

Like Juno at Jupiter, the mysteries of a gas giant like Saturn is overwhelming. This is a big and very active planet. We understand almost nothing about its weather systems, its atmosphere, and its interior. And this glimpse by Cassini is only that, a mere glimpse. When Cassini’s mission ends in September, it will leave us a treasure trove of knowledge. It will also leave us a much larger library of unanswered questions, all of which will remain unanswered until we can return, decades hence, with new probes..

The first results from Cassini’s weekly dives between Saturn and its innermost rings have now been released.

The big surprise so far is the lack of a tilt to Saturn’s magnetic field.

Based on data collected by Cassini’s magnetometer instrument, Saturn’s magnetic field appears to be surprisingly well-aligned with the planet’s rotation axis. The tilt is much smaller than 0.06 degrees — which is the lower limit the spacecraft’s magnetometer data placed on the value prior to the start of the Grand Finale.

This observation is at odds with scientists’ theoretical understanding of how magnetic fields are generated. Planetary magnetic fields are understood to require some degree of tilt to sustain currents flowing through the liquid metal deep inside the planets (in Saturn’s case, thought to be liquid metallic hydrogen). With no tilt, the currents would eventually subside and the field would disappear.

Any tilt to the magnetic field would make the daily wobble of the planet’s deep interior observable, thus revealing the true length of Saturn’s day, which has so far proven elusive.

They also have gotten lots of much better images of the planet’s cloud tops.

In 2010 NASA extended the Cassini mission orbiting Saturn for seven years to the planet’s next solstice, so that the spacecraft could observe Saturn, its moons, and its rings, for one full season of its 28 year orbit.

Today, Cassini reached that target.

NASA’s Cassini spacecraft still has a few months to go before it completes its mission in September, but the veteran Saturn explorer reaches a new milestone today. Saturn’s solstice — that is, the longest day of summer in the northern hemisphere and the shortest day of winter in the southern hemisphere — arrives today for the planet and its moons. The Saturnian solstice occurs about every 15 Earth years as the planet and its entourage slowly orbit the sun, with the north and south hemispheres alternating their roles as the summer and winter poles.

The article provides a detailed review of all the changes that have occurred during this long time period.

Cool image time! The latest image releases from Cassini, taken during its recent close fly-by campaign of the rings of Saturn, focus on the propeller features produced in the rings by larger ring objects. The image on the right, reduced to show here, reveals dozens of propellers of all shapes and sizes.

The original discovery of propellers in this region in Saturn’s rings … was made using several images taken from very close to the rings during Cassini’s 2004 arrival at Saturn. Those discovery images were of low resolution and were difficult to interpret, and there were few clues as to how the small propellers seen in those images were related to the larger propellers Cassini observed later in the mission….

This image, for the first time, shows swarms of propellers of a wide range of sizes, putting the ones Cassini observed in its Saturn arrival images in context. Scientists will use this information to derive a “particle size distribution” for propeller moons, which is an important clue to their origins.

The parallel pattern of rings in the center of the image is a series of density waves in the ring structure, caused by an interaction with one of Saturn’s larger nearby moons.

They have also released the best view we can now expect of a propeller by Cassini.

This is the third and final propeller to be targeted for a close flyby observation during Cassini’s ring-grazing orbits (the period from Nov. 2016 to April 2017 when Cassini’s orbit passed just outside the main rings). …Because propellers are seen in the outermost parts of the main rings, the ring-grazing orbits provided Cassini’s best opportunity to see them up close.

Cassini is now diving between the rings and the planet, so the propellers are farther away.

Cool image time! The image on the right, reduced and cropped to show here, was taken by Cassini in January as it was preparing to head into its final 22 orbital dives between Saturn and its rings. The image shows the gas giant’s north polar, with all its magnificent features. As the press release notes, the north pole was in shadow for the first part of Cassini’s mission, and only in recent years has finally moved into sunlight so that the spaceship can see and image it.

Although the sunlight falling on the north pole of Saturn is enough to allow us to image and study the region, it does not provide much warmth. In addition to being low in the sky (just like summer at Earth’s poles), the sun is nearly ten times as distant from Saturn as from Earth. This results in the sunlight being only about 1 percent as intense as at our planet.

This view looks toward Saturn from about 31 degrees above the ring plane. The image was taken with the Cassini spacecraft wide-angle camera on Jan. 22, 2017 using a spectral filter which preferentially admits wavelengths of near-infrared light centered at 939 nanometers.

Be sure to look at the full image.

The Cassini science team has released a movie compiled from images taken during the spacecraft’s first dive between Saturn and its rings.

I have embedded the movie below the fold. It cuts off before the spacecraft’s closest approach because they took a very conservative approach to this fly-by, turning the vehicle so that its antenna dish protected it. This prevented the taking of the best images. The second dive, that occurred yesterday, will have better images.
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The uncertainty of science: The data from Cassini’s first dive through the gap between Saturn and its innermost ring suggests that the space there has much less material than predicted.

Cassini’s Radio and Plasma Wave Science (RPWS) instrument was one of two science instruments with sensors that poke out from the protective shield of the antenna (the other being Cassini’s magnetometer). RPWS detected the hits of hundreds of ring particles per second when it crossed the ring plane just outside of Saturn’s main rings, but only detected a few pings on April 26. When RPWS data are converted to an audio format, dust particles hitting the instrument’s antennas sound like pops and cracks, covering up the usual whistles and squeaks of waves in the charged particle environment that the instrument is designed to detect. The RPWS team expected to hear a lot of pops and cracks on crossing the ring plane inside the gap, but instead, the whistles and squeaks came through surprisingly clearly on April 26.

“It was a bit disorienting — we weren’t hearing what we expected to hear,” said William Kurth, RPWS team lead at the University of Iowa, Iowa City. “I’ve listened to our data from the first dive several times and I can probably count on my hands the number of dust particle impacts I hear.” The team’s analysis suggests Cassini only encountered a few particles as it crossed the gap — none larger than those in smoke (about 1 micron across).

The second dive through the rings will occur tomorrow, with data sent down the next day. Because the gap is emptier than expected, they will not have to protect the spacecraft so thoroughly, and will thus have more instruments gathering data during the fly through.

The Cassini spacecraft has successfully survived its close flyby of Saturn, diving between the rings and the top of the planet’s cloud tops.

As it dove through the gap, Cassini came within about 1,900 miles (3,000 kilometers) of Saturn’s cloud tops (where the air pressure is 1 bar — comparable to the atmospheric pressure of Earth at sea level) and within about 200 miles (300 kilometers) of the innermost visible edge of the rings.

Cassini is beaming down its images and data now, with the images to be available later today.

Cool image time! The image on the right, cropped and reduced to show here, captures the same propeller feature in Saturn’s A ring, with the top image showing the sunlight side and the bottom image the side away from the Sun.

Cassini scientists have been tracking the orbit of this object for the past decade, tracing the effect that the ring has upon it. Now, as Cassini has moved in close to the ring as part of its ring-grazing orbits, it was able to obtain this extreme close-up view of the propeller, enabling researchers to examine its effects on the ring. These views, and others like them, will inform models and studies in new ways going forward.

…The propeller’s central moonlet would only be a couple of pixels across in these images, and may not actually be resolved here. The lit-side image shows that a bright, narrow band of material connects the moonlet directly to the larger ring, in agreement with dynamical models. That same thin band of material may also be obscuring the moonlet from view.

Lengthwise along the propeller is a gap in the ring that the moonlet has pried open. The gap appears dark on both the lit and unlit sides. Flanking the gap near the moonlet are regions of enhanced density, which appear bright on the lit side and more mottled on the unlit side.

The scale of the two images is slightly different, 0.33 and 0.44 miles, as they were taken as Cassini was zipping past during one of its ring-grazing orbits on February 21.

To my eye, the effect here faintly resembles a wake produced by a boat in water, except that the wake moves in opposite directions on opposite sides of the tiny moonlet as it plows through the A ring.

Cool image time! The photo at the right, reduced to show here, was taken by Cassini on October 28, 2016 as it was shifting its orbit to prepare for the spacecraft’s last year at Saturn, where it will make repeated dives down past and even inside the gas giant’s rings. As they note at the link,

No Earth-based telescope could ever capture a view quite like this. Earth-based views can only show Saturn’s daylit side, from within about 25 degrees of Saturn’s equatorial plane. A spacecraft in orbit, like Cassini, can capture stunning scenes that would be impossible from our home planet.

It is interesting to compare this image with this July image. Comparing the two provides a sense of Cassini’s travels.