Tag: Cassini

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.