Raumfahrt - Cassini Grand Finale Around Saturn -Update-4


Cassini skims Saturn's atmosphere

The Cassini probe has begun the final phase of its mission to Saturn. 

The satellite has executed the first of five ultra-close passes of the giant world, dipping down far enough to brush through the top of the atmosphere. 

It promises unprecedented data on the chemical composition of Saturn. 

It also sets the stage for the probe's dramatic end-manoeuvre next month when it will plunge to destruction in the planet's atmosphere. 

Cassini is currently flying a series of loops around Saturn that thread the gap between its atmosphere and its rings. 

Monday's swing-by saw the spacecraft go closer than ever before to the cloud tops - skimming just 1,600km (1,000 miles) above them, at 04:22 GMT (05:22 BST) on Monday. 

This low pass was designed to allow the probe to directly sample the gases of the extended upper-atmosphere. 

Saturn's bulk composition is thought to be about 75% hydrogen with the rest being helium (bar some trace components), explains Nicolas Altobelli, the European Space Agency's Cassini project scientist. 

"It's expected that the heavier helium is sinking down," he told BBC News. "Saturn radiates more energy than it's absorbing from the Sun, meaning there's gravitational energy which is being lost. And so getting a precise measure of the hydrogen and helium in the upper layers sets a constraint on the overall distribution of the material in the interior." 

Cassini will send all the data back to Earth during its next contact on Tuesday. 

Dipping down into the atmosphere should create a drag on the spacecraft, requiring Cassini to use its thrusters to maintain a stable flight configuration and stop itself from tumbling. But the mission's scientists think any buffeting effects ought to be manageable.

They are hopeful that when the post-pass analysis is done, the probe will be permitted to go even lower on the remaining four dip-downs before 15 September's goodbye plunge. 

Artwork: Cassini is running the narrow gap between the top of the planet's atmosphere and the ringsImage copyrightNASA/JPL-CALTECH
Image captionArtwork: Cassini is running the narrow gap between the top of the planet's atmosphere and the rings

The Cassini mission still has some big outstanding questions about Saturn. One of these relates to the length of a day on the planet. 

Researchers know it is roughly 10-and-a-half hours, but they would like a more precise number. 

The solution should come by looking for an offset between the magnetic field and the planet's rotation axis, but frustratingly all the probe's observations to date show these two features to be almost perfectly aligned. 

"All magnetic field theory as we know it requires an offset," said Linda Spilker, the US space agency's Cassini project scientist. 

"To generate a field, you need to keep the currents in the metallic hydrogen layer inside Saturn flowing, and without the offset the thinking is that the field would simply go away. 

"What's going on? Is something shielding our ability to see the offset, or do we simply need a new theory? But without the tilt, without being able to see the tiny wobble, we cannot be more precise about the length of a day." 

Dr Spilker said the mission team would continue to work on the problem. 

Cassini is a joint venture between the US, European and Italian space agencies. They are ending the probe's operations after 20 years because it is running low on fuel and will soon become uncontrollable. 

Scientists want to avoid the possibility of a future collision with Saturn's moons Titan and Enceladus, which could conceivably support simple microbial life. And the only way to prevent that is to deliberately drive the probe to destruction in the atmosphere of the giant planet. 

OrbitsImage copyrightNASA/JPL-CALTECH
Image captionCassini's final orbits (blue) take the probe closer to the planet than it has ever been before

Quelle: BBC


Cloudy Waves (False Color)


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. Saturn’s upper atmosphere generates the faint haze seen along the limb of the planet in this image.


This false color view is centered on 46 degrees north latitude on Saturn. The images were taken with the Cassini spacecraft narrow-angle camera on May 18, 2017 using a combination of spectral filters which preferentially admit wavelengths of near-infrared light. The image filter centered at 727 nanometers was used for red in this image; the filter centered at 750 nanometers was used for blue. (The green color channel was simulated using an average of the two filters.)


The view was obtained at a distance of approximately 750,000 miles (1.2 million kilometers) from Saturn. Image scale is about 4 miles (7 kilometers) per pixel.

Quelle: NASA


Update: 29.08.2017


NASA releases inside-out view of Saturn’s rings

Credit: NASA/JPL-Caltech/Space Science Institute

Continuing to make discoveries in the final weeks of a historic mission at Saturn, NASA’s Cassini spacecraft has recorded a unique time lapse movie looking out toward the planet’s iconic icy rings.

Cassini captured imagery to assemble the movie during an Aug. 20 swing through the gap between Saturn and its rings. Ground controllers set Cassini’s wide-angle camera to take images in a low-resolution mode — 512 x 512 pixels — to allow the spacecraft to gather more photos in a short span of time, according to NASA.

“The entirety of the main rings can be seen here, but due to the low viewing angle, the rings appear extremely foreshortened,” NASA said in a statement. “The perspective shifts from the sunlit side of the rings to the unlit side, where sunlight filters through.”

The sequence shows Cassini crossing the relatively thin ring plane as it zipped by Saturn at a speed of some 76,000 mph (122,000 kilometers per hour) relative to the planet’s cloud tops.

“On the sunlit side, the grayish C ring looks larger in the foreground because it is closer; beyond it is the bright B ring and slightly less-bright A ring, with the Cassini Division between them,” NASA said. “The F ring is also fairly easy to make out.”

Running low on fuel, Cassini is set to end its nearly 20-year mission Sept. 15 with a guided plunge into Saturn’s atmosphere. The probe will disintegrate as it descends into the deeper layers of the atmosphere, succumbing to increasing aerodynamic pressures.

Composed mostly of blocks and particles of ice, Saturn’s rings are one of the major targets for Cassini’s scientific instruments during the mission’s final months. Scientists hope to learn the mass of the rings, a figure that should yield an estimate of their age and origin.

Saturn itself is also a focus during Cassini’s grand finale.

Measurements of Saturn’s gravity and magnetic fields could help scientists study the planet’s interior structure, and Cassini’s final five passes near the planet are taking the spacecraft into the outermost reaches of the atmosphere, allowing its instruments to take direct samples.

Cassini will beam back atmospheric data in real-time during its final descent into Saturn next month until the probe’s control thrusters are unable to keep the craft’s antenna locked on Earth.

Launched from Cape Canaveral in October 1997, the spacecraft spent much of its time at Saturn making observations of the planet’s numerous moons. Cassini dropped a European Space Agency lander to Titan’s surface for a parachute-assisted landing in 2005, and found evidence that Enceladus harbors a global ocean of water buried beneath a veneer of ice.

NASA officials want to end the mission in a guided manner while the spacecraft is still healthy to avoid the chance an uncontrolled Cassini could crash into Titan or Enceladus, potentially spoiling habitats that might be prime for finding alien microbial life.

Since April, Cassini has made repeated passes between Saturn and its rings. Two more flybys are planned before the final dive Sept. 15.

Quelle: SN


Update: 30.08.2017


NASA’s Cassini spacecraft nears a fiery, brutal end, when it will plunge into Saturn

After 13 years of observing Saturn, its rings and its myriad moons, NASA’s Cassini spacecraft is less than three weeks away from a fiery, brutal end.

Early in the morning on Sept. 15 the aging spacecraft will hurl itself into Saturn’s atmosphere at speeds of more than 75,000 mph.

It’s a deliberate death plunge from which it has no hope of returning.

Within three minutes of diving into Saturn’s tenuous upper layers, the two-story-tall spacecraft will be torn apart.

In the end, Cassini will become part of the very planet it has studied for more than a decade.

“I like to say it’s going out in a blaze of glory,” said Linda Spilker of the Jet Propulsion Laboratory, the project scientist for the mission. “It will be trailblazing until the very last second.”

Earth-bound astronomers will keep a close eye on the planet at the moment of Cassini’s death to see whether they can detect a small flare as the spacecraft burns up like a meteor in the Saturnian sky.

But they don’t expect to see much.

“The mass of Cassini is so small compared to the mass of the planet,” Spilker said. “And the plunge is happening on Saturn’s day side.”

She added that there is no fear that Cassini’s death dive will contaminate the ringed giant. Molecules from the spacecraft will quickly spread out across the planet, which is big enough to hold more than 700 Earths.

Still, Cassini’s suicide mission is not all gloom and doom. The spacecraft will venture deeper into Saturn’s atmosphere than ever before, collecting brand new data and beaming it back to Earth up until the last seconds of its life.

“We are reconfiguring the spacecraft and turning Cassini into an atmospheric probe,” said Earl Maize, the mission’s program manager, who also works at JPL.

Cassini could make it as far as 120 miles into the Saturnian atmosphere before all its instruments cease to function, mission engineers said.

“I find great comfort that Cassini will continue teaching us until the very last second on Sept. 15,” said Curt Niebur, Cassini program scientist based at NASA’s headquarters in Washington.

The flagship mission launched in 1997 and entered the Saturnian system in 2004.

Over the last 13 years it has discovered plumes of water ice on the moon Enceladus, confirmed the presence of methane and ethane lakes and rivers on Titan, watched the seasons change on Saturn, discovered six new moons and revealed complexities in the planet’s rings that had never been seen before.

“We’ve rewritten the textbooks on Saturn,” Spilker said. “Literally, there are so many new books coming out.”

Although the spacecraft’s instruments continue to work flawlessly, its gas tank is nearing empty. Mission planners decided to crash Cassini into Saturn to avoid any risk of contaminating Enceladus or Titan, two of Saturn’s moons that could harbor the ingredients necessary for life.

Beginning in April, Cassini began a series of orbits that took it speeding through the gap between the planet and its rings for the first time. This has allowed scientists to address new questions including the age of the rings and how quickly the planet’s interior is spinning.

By Sept. 15, the spacecraft will have completed 22 of these orbits to collect new data even as its end looms.

“Who knows what new mysteries the next two weeks will bring?” Spilker said.