Cassini: A parcel of papers published

The Cassini spacecraft ended its mission on Sept. 15, 2017. Image Credit: NASA/JPL-Caltech/Space Science Institute
This natural-color view is a composite of images taken in visible light with the Cassini spacecraft’s narrow-angle camera at a distance of approximately 1.4 million miles (2.3 million kilometers) from Saturn. The Cassini spacecraft ended its mission on Sept. 15, 2017. Image Credit: NASA/JPL-Caltech/Space Science Institute

October 4, 2018 — New research emerging from the final orbits of NASA’s Cassini spacecraft represents a huge leap forward in our understanding of the Saturn system — especially the mysterious, never-before-explored region between the planet and its rings. Some preconceived ideas are turning out to be wrong while new questions are being raised.

Six teams of researchers are publishing their work Oct. 5 in the journal Science, based on findings from Cassini’s Grand Finale. That’s when, as the spacecraft was running out of fuel, the mission team steered Cassini spectacularly close to Saturn in 22 orbits before deliberately vaporizing it in a final plunge into the atmosphere in September 2017.

Knowing Cassini’s days were numbered, its mission team went for gold. The spacecraft flew where it was never designed to fly. For the first time, it probed Saturn’s magnetized environment, flew through icy, rocky ring particles and sniffed the atmosphere in the 1,200-mile-wide (2,000-kilometer-wide) gap between the rings and the cloud tops. Not only did the flight path push the spacecraft to its limits, the new findings illustrate how powerful and agile the instruments were.

Many more Grand Finale science results are to come, but here are some of today’s highlights:

  • Complex organic compounds embedded in water nanograins rain down from Saturn’s rings into its upper atmosphere. Scientists saw water and silicates, but they were surprised to see also methane, ammonia, carbon monoxide, nitrogen and carbon dioxide. The composition of the organics is different from that found on moon Enceladus — and also different from that on moon Titan, meaning there are at least three distinct reservoirs of organic molecules in the Saturn system.
  • For the first time, Cassini saw up close how rings interact with the planet and observed inner-ring particles and gases falling directly into the atmosphere. Some particles take on electric charges and spiral along magnetic-field lines, falling into Saturn at higher latitudes — a phenomenon known as “ring rain.” But scientists were surprised to see that others are dragged quickly into Saturn at the equator. And it’s all falling out of the rings faster than scientists thought — as much as 22,000 pounds (10,000 kilograms) of material per second.
  • Scientists were surprised to see what the material looks like in the gap between the rings and Saturn’s atmosphere. They knew that the particles throughout the rings ranged from large to small. But the sampling in the gap showed mostly tiny, nanometer-sized particles, like smoke, suggesting that some yet-unknown process is grinding up particles.
  • Saturn and its rings are even more interconnected than scientists thought. Cassini revealed a previously unknown electric-current system that connects the rings to the top of Saturn’s atmosphere.
  • Scientists discovered a new radiation belt around Saturn, close to the planet and composed of energetic particles. They found that while the belt actually intersects with the innermost ring, the ring is so tenuous that it doesn’t block the belt from forming.
  • Unlike every other planet with a magnetic field in our Solar System, Saturn’s magnetic field is almost completely aligned with its spin axis. The new data shows a magnetic-field tilt of less than 0.0095 degrees. (Earth’s magnetic field is tilted 11 degrees from its spin axis.) According to everything scientists know about how planetary magnetic fields are generated, Saturn should not have one. It’s a mystery that physicists will be working to solve.
  • Cassini flew above Saturn’s magnetic poles, directly sampling regions where radio emissions are generated. The findings more than doubled the number of direct measurements of radio sources from the planet, one of the few non-terrestrial locations where scientists have been able to study a radio-generation mechanism that is believed to operate throughout the universe.

For the Cassini mission, the science rolling out from Grand Finale orbits more than justifies the calculated risk of diving into the gap — skimming the upper atmosphere and skirting the edge of the inner rings, said Cassini Project Scientist Linda Spilker.

“Almost everything going on in that region turned out to be a surprise,” Spilker said. “That was the importance of going there, to explore a place we’d never been before. And the expedition really paid off — the data is tremendously exciting.”

Analysis of Cassini data from the spacecraft’s instruments will be ongoing for years to come, helping to paint a clearer picture of Saturn.

“Many mysteries remain, as we put together pieces of the puzzle,” Spilker said. “Results from Cassini’s final orbits turned out to be more interesting than we could have imagined.”

The papers published in Science are:

On Oct. 4, as the Science publication embargo lifts, articles describing research complementary to these findings will post online in Geophysical Research Letters (GRL), a journal of the American Geophysical Union (AGU).

Monday, September 11: Monthly Membership Meeting

Image: Artist's concept of Cassini spacecraft at Saturn. Credit: NASA/JPL-Caltech
In this still from the short film Cassini’s Grand Finale, the spacecraft is shown diving between Saturn and the planet’s innermost ring. Credit: NASA/JPL-Caltech

The Cuyahoga Astronomical Association (CAA) will host its monthly meeting at 7:30 PM, Monday, September 11 in the Cleveland Metroparks’ Rocky River Nature Center, North Olmsted. The speaker will be Jay Reynolds who will discuss NASA’s Cassini Mission to Saturn and its finale, set to occur September 15. The program is free and open to the public, no reservations required.

The Cassini spacecraft will make its final approach to the giant planet Saturn this Friday, ending an extremely productive seven-year mission. This encounter will be like no other. This time, Cassini will dive into the planet’s atmosphere, sending science data for as long as its small thrusters can keep the spacecraft’s antenna pointed at Earth. Soon after, Cassini will burn up and disintegrate like a meteor.

In addition to being a research astronomer who teaches at Cleveland State University, Reynolds is CAA’s observatory director. He frequently appears on Cleveland television, hosting a show about astronomy on WKYC, Channel 3.

Following the program, the club’s monthly membership meeting will convene.

Best views yet

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

Newly released images showcase the incredible closeness with which NASA’s Cassini spacecraft, now in its “Ring-Grazing” orbits phase, is observing Saturn’s dazzling rings of icy debris.

The views are some of the closest-ever images of the outer parts of the main rings, giving scientists an eagerly awaited opportunity to observe features with names like “straw” and “propellers.” Although Cassini saw these features earlier in the mission, the spacecraft’s current, special orbits are now providing opportunities to see them in greater detail. The new images resolve details as small as 0.3 miles (550 meters), which is on the scale of Earth’s tallest buildings.

Cassini is now about halfway through its penultimate mission phase — 20 orbits that dive 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 finale plunge is scheduled for April 26.

For now, the veteran spacecraft is shooting past the outer edges of the rings every week, gathering some of its best images of the rings and moons. Already Cassini has sent back the closest-ever views of small moons Daphnis and Pandora.

Some of the structures seen in recent Cassini images have not been visible at this level of detail since the spacecraft arrived at Saturn in mid-2004. At that time, fine details like straw and propellers — which are caused by clumping ring particles and small, embedded moonlets, respectively — had never been seen before. (Although propellers were present in Cassini’s arrival images, they were actually discovered in later analysis, the following year.)

Cassini came a bit closer to the rings during its arrival at Saturn, but the quality of those arrival images (examples: 1, 2, 3) was not as high as in the new views. Those precious few observations only looked out on the backlit side of the rings, and the team chose short exposure times to minimize smearing due to Cassini’s fast motion as it vaulted over the ring plane. This resulted in images that were scientifically stunning, but somewhat dark and noisy.

In contrast, the close views Cassini has begun capturing in its ring-grazing orbits (and soon will capture in its Grand Finale phase) are taking in both the backlit and sunlit side of the rings. Instead of just one brief pass lasting a few hours, Cassini is making several dozen passes during these final months.

“As the person who planned those initial orbit-insertion ring images — which remained our most detailed views of the rings for the past 13 years — I am taken aback by how vastly improved are the details in this new collection,” said Cassini Imaging Team Lead Carolyn Porco, of Space Science Institute, Boulder, Colorado. “How fitting it is that we should go out with the best views of Saturn’s rings we’ve ever collected.”

Cassini saves daring maneuvers and observations for last

Cassini ring-grazing orbits. Image credit: NASA/JPL-Caltech
This graphic shows the closest approaches of Cassini’s final two orbital phases. Ring-grazing orbits are shown in gray (at left); Grand Finale orbits are shown in blue. The orange line shows the spacecraft’s Sept. 2017 final plunge into Saturn. Image credit: NASA/JPL-Caltech

NASA’s Saturn-orbiting Cassini spacecraft has made its first close dive past the outer edges of Saturn’s rings since beginning its penultimate mission phase on Nov. 30.

Cassini crossed through the plane of Saturn’s rings on Dec. 4 at 8:09 AM EST at a distance of approximately 57,000 miles (91,000 kilometers) above Saturn’s cloud tops. This is the approximate location of a faint, dusty ring produced by the planet’s small moons Janus and Epimetheus, and just 6,800 miles (11,000 kilometers) from the center of Saturn’s F ring.

About an hour prior to the ring-plane crossing, the spacecraft performed a short burn of its main engine that lasted about six seconds. About 30 minutes later, as it approached the ring plane, Cassini closed its canopy-like engine cover as a protective measure.

“With this small adjustment to the spacecraft’s trajectory, we’re in excellent shape to make the most of this new phase of the mission,” said Earl Maize, Cassini project manager at NASA’s Jet Propulsion Laboratory, Pasadena, California.

A few hours after the ring-plane crossing, Cassini began a complete scan across the rings with its radio science experiment to study their structure in great detail.

“It’s taken years of planning, but now that we’re finally here, the whole Cassini team is excited to begin studying the data that come from these ring-grazing orbits,” said Linda Spilker, Cassini project scientist at JPL. “This is a remarkable time in what’s already been a thrilling journey.”

Cassini’s imaging cameras obtained views of Saturn about two days before crossing through the ring plane, but not near the time of closest approach. The focus of this first close pass was the engine maneuver and observations by Cassini’s other science instruments. Future dives past the rings will feature some of the mission’s best views of the outer regions of the rings and small, nearby moons.

Each of Cassini’s orbits for the remainder of the mission will last one week. The next pass by the rings’ outer edges is planned for Dec. 11. The ring-grazing orbits — 20 in all — will continue until April 22, when the last close flyby of Saturn’s moon Titan will reshape Cassini’s flight path. With that encounter, Cassini will leap over the rings, making the first of 22 plunges through the 1,500-mile-wide (2,400-kilometer) gap between Saturn and its innermost ring on April 26.