Posted on by Nancy Atkinson

Unprecedented Views of Saturn’s Rings as Cassini Dances Death Spiral

This image shows a region in Saturn's outer B ring. NASA's Cassini spacecraft viewed this area at a level of detail twice as high as it had ever been observed before. And from this view, it is clear that there are still finer details to uncover. Credit: NASA/JPL-Caltech/Space Science Institute

As the Cassini spacecraft moves ever closer to Saturn, new images provide some of the most-detailed views yet of the planet’s spectacular rings. From its “Ring-Grazing” orbit phase, Cassini’s cameras are resolving details in the rings as small as 0.3 miles (550 meters), which is on the scale of Earth’s tallest buildings.

On Twitter, Cassini Imaging Team Lead Carolyn Porco called the images “outrageous, eye-popping” and the “finest Cassini images of Saturn’s rings.”

Project Scientist Linda Spilker said the ridges and furrows in the rings remind her of the grooves in a phonograph record.

These images are giving scientists the chance to see more details about ring features they saw earlier in the mission, such as waves, wakes, and things they call ‘propellers’ and ‘straw.’

This Cassini image features a density wave in Saturn’s A ring (at left) that lies around 134,500 km from Saturn. Density waves are accumulations of particles at certain distances from the planet. This feature is filled with clumpy perturbations, which researchers informally refer to as “straw.” Credit: NASA/JPL-Caltech/Space Science Institute

As of this writing, Cassini just started the 10th orbit of the 20-orbit ring-grazing phase, which has the spacecraft diving past the outer edge of the main ring system. The ring-grazing orbits began last November, and will continue until late April, when Cassini begins its grand finale. During the 22 finale orbits, Cassini will repeatedly plunge through the gap between the rings and Saturn. The first of these plunges is scheduled for April 26.

The spacecraft is actually close enough to the ‘F’ ring that occasionally tenuous particle strike Cassini, said project scientist Linda Spilker, during a Facebook Live event today.

“These are very small and tenuous, only a few microns in size,” Spilker said, “like dust particles you’d see in the sunlight. We can actually ‘hear’ them hitting the spacecraft in our data, but these particles are so small, they won’t hurt Cassini.”

I talked with Spilker about ring particles for my book “Incredible Stories From Space:”

Spilker has envisioned holding a ring particle in her hand. What would it look like?

“We have evidence of the particles that have an icy core covered with fluffy regolith material that is very porous,” she said, “and that means the particle can heat up and cool down very quickly compared to a solid ice cube.”

The straw features are caused by clumping ring particles and the propellers are caused by small, embedded moonlets that creates propeller shaped wakes in the rings.

The wavemaker moon, Daphnis, is featured in this view, taken as NASA’s Cassini spacecraft made one of its ring-grazing passes over the outer edges of Saturn’s rings on Jan. 16, 2017. This is the closest view of the small moon obtained yet. Daphnis is 5 miles or 8 kilometers across. Credit: NASA/JPL-Caltech/Space Science Institute

This stunning view of the moon Daphnis shows the moon interacting with the ring particles, creating waves in the rings around it.

A close-up of Saturn and its rings. Assembled using raw uncalibrated RGB filtered images taken by the Cassini spacecraft on January 18 2017. Credit:
NASA/JPL-Caltech/SSI/image editing by Kevin M. Gill

“These close views represent the opening of an entirely new window onto Saturn’s rings, and over the next few months we look forward to even more exciting data as we train our cameras on other parts of the rings closer to the planet,” said Matthew Tiscareno, a Cassini scientist who studies Saturn’s rings at the SETI Institute, Mountain View, California. Tiscareno planned the new images for the camera team.

Further reading: JPL, CICLOPS

Posted on by Nancy Atkinson

Cassini Is About To Graze Saturn’s Rings In Mission Endgame

A lovely view of Saturn and its rings as seen by the Cassini spacecraft on Aug. 12, 2009. Credit: NASA/JPL-Caltech/Space Science Institute.

There is a Twitter-bot that randomly tweets out “NOOOOOOOO Cassini can’t be ending!” (with varying amounts of “O’s”). @CassiniNooo represents the collective sigh of sadness and consternation felt by those of us who can’t believe the the historic and extensive Cassini mission will be over in just a matter of months.

And next week is the beginning of the end for Cassini.

On November 30, Cassini will begin a phase of the mission that the science team calls “Cassini’s Ring-Grazing Orbits,” as the spacecraft will start skimming past the outer edge of the rings, coming within – at times — 4,850 miles (7,800 kilometers) of the rings.

“The scientific return will be incredible,” Linda Spilker, Cassini project scientist, told me earlier this year. “We’ll be studying things we just couldn’t do any other place.”

Between November 30, 2016 and April 22, 2017 Cassini will circle high over and under the poles of Saturn, diving every seven days for a total of 20 times through the unexplored region at the outer edge of the main rings.

During the close passes, Cassini’s instruments will attempt to directly sample the icy ring particles and molecules of faint gases that are found close to the rings. Cassini will also capture some of the best high-resolution images of the rings, and garner the best views ever of the small moons Atlas, Pan, Daphnis and Pandora, which orbit near the rings’ outer edges.

During the first two ring-grazing orbits, the spacecraft will pass directly through an extremely faint ring produced by tiny meteors striking the two small moons Janus and Epimetheus. Later ring crossings in March and April will send the spacecraft through the dusty outer reaches of the F ring.

“Even though we’re flying closer to the F ring than we ever have, … there’s very little concern over dust hazard at that range,” said Earl Maize, Cassini project manager at JPL.

Of course, the ultimate ‘endgame’ is that Cassini will plunge into Saturn with its “Grand Finale,” ending the mission on September 15, 2017. Since Cassini is running out of fuel, destroying the spacecraft is necessary to ensure “planetary protection,” making sure any potential microbes from Earth that may still be attached to the spacecraft don’t contaminate any of Saturn’s potentially habitable moons.

This graphic illustrates the Cassini spacecraft's trajectory, or flight path, during the final two phases of its mission. The view is toward Saturn as seen from Earth. The 20 ring-grazing orbits are shown in gray; the 22 grand finale orbits are shown in blue. The final partial orbit is colored orange. Image credit: NASA/JPL-Caltech/Space Science Institute
This graphic illustrates the Cassini spacecraft’s trajectory, or flight path, during the final two phases of its mission. The view is toward Saturn as seen from Earth. The 20 ring-grazing orbits are shown in gray; the 22 grand finale orbits are shown in blue. The final partial orbit is colored orange. Image credit: NASA/JPL-Caltech/Space Science Institute

To prepare for the Grand Finale, Cassini engineers have been slowly adjusting the spacecraft’s orbit since January of this year, doing maneuvers and burns of the engine to bring Cassini into the right orbit so that it can ultimately dive repeatedly through the narrow gap between Saturn and its rings, before making its mission-ending plunge. During some of those final orbits, Cassini will pass as close as 1,012 miles (1,628 kilometers) above the cloudtops of Saturn.

One question for Cassini’s engineering team is how much fuel is actually left in the tank for Cassini’s main engines, which do the majority of the work for orbit adjustments. Each time they’ve used the main engines this past year, the team has held their breath, hoping there is enough fuel.

One final burn of the main engine remains, on December 4. This maneuver is important for fine-tuning the orbit and setting the correct course to enable the remainder of the mission, said Maize.

“This will be the 183rd and last currently planned firing of our main engine,” he said. “Although we could still decide to use the engine again, the plan is to complete the remaining maneuvers using thrusters,” said Maize.

A montage of images from Cassini of various moons and the rings around Saturn. Credit: NASA/JPL-Caltech/Space Science Institute
A montage of images from Cassini of various moons and the rings around Saturn. Credit: NASA/JPL-Caltech/Space Science Institute

When I visited with Maize and Spilker earlier this year, Spilker wistfully said that they had begun to experience some of the “lasts” of the mission — the final flyby of Enceladus and other moons. And there’s one big “last” coming up: on Nov. 29, 2016, Cassini will come within 6,800 miles (11,000 km) of Titan, the final flyby of this eerily Earthlike but yet totally alien moon.

This final flyby, named Flyby T-125 has two primary goals: Mapmaking of Titan’s surface, and enabling the change in Cassini’s orbit to begin the end of the mission. But it also might be the most daring and thrilling part of Cassini’s nearly 20-year mission.

But still ….. NOOOOOO!

Keep track of Cassini’s latest endeavours at the Cassini website