Friday, September 15, 2017

Farewell to Our Dear Friend Cassini: Humanity's Wonderfully Awesome Little Space Probe Ends 13-Year "Marvelous Ride" of Amazing Saturnian System Discoveries with "Grand Finale" Plunge

Artist's rendition of Cassini on one of its Grand Finale dives between Saturn and its rings.


Apologies for the timing of this posting. I had really hoped to get this entry up before the end of Cassini's mission but could not finish it last night.

Saturn's rings cast a vast shadow across a belt of the planet while one of its tiny moons creates a mini-solar eclipse in the planet's middle latitudes.


On this sad occasion of the end of the Cassini space probe -- just about 12 hours now since it vaporized in the upper atmosphere of Saturn as a dazzling streak of light and so became part of the mighty ringed world in the grandest act of its Grand Finale, I think it only proper to commemorate the wonderfully awesome little space probe and its 13 years of wondrous discoveries of the Saturnian system. 

A close-up of Saturn's rings with sunlight reflecting off of them in a spectrum of colors. Remember that the rings are water ice and rock.


It was nearly 20 years ago that Cassini was launched from Cape Canaveral on Oct. 15, 1997 atop a Titan IVB rocket, and after a 7-year journey to the outer Solar System that concluded with orbital insertion on June 30 / July 1, 2004, Cassini spent the next 13 years orbiting Saturn -- Lord of the Rings -- and its family of moons.

Saturn in all its ringed glory.


In that time, Cassini and the little planetary probe called Huygens that it deployed to the surface of Titan provided unbelievable discoveries and almost half a million images -- giving humanity incredible discoveries into Saturn and its awesome, intricate, and yet delicate ring system.

Cassini views Saturn's hexagonal cloud formation with its central massive hurricane-like feature centered at the planet's north pole  in an image taken in May 2017.


Cassini also gave us close up views and profound insights into Saturn's sprawling family of approximately five dozen known moons from mighty Titan with its lakes and seas of methane and ethane and tantalizing Enceladus with its geysers of water ice that suggest a hidden liquid water ocean (possibly friendly to life forms as we know them) to the various shepherd moons.

A full disk view of the hazy yellowish ball of Titan.


These shepherd moons some irregular-shaped lumps that "regulate" the behavior of the rings and -- if they are large enough, such as the moon Pan -- create a clear gap in the ring (in the case of Pan, it is the 200-mile wide Encke Gap). The smaller the moonlet, the smaller the gap -- and in some cases, the moonlets are so small that they literally "swim" in the ring material. (The moonlets in some cases orbit at angles ever-so-slightly inclined to the main ring plane.)

Close-up of the density waves within Saturn's A Ring.


Recall that the rings are comprised of an uncountable number of icy pebbles, rocks, and boulders that although 300,000-km wide, orbit in an incredibly narrow plane (thought to be no more than 10 meters or about 30 feet wide). Thus, the thickness-to-width ratio is 1 in 30,000,000. Scaled to a human being, that is far thinner than a piece of hair.

Saturn and its moon of Enceladus with the planet's rings seen nearly edge-on.

In this picture, Enceladus appears to be balanced on the rings. The rings are so thin that in theory, from the exact correct angle, they would in fact appear to vanish -- although the tremendous shadow they cast upon Saturn are clearly visible.


In areas where these tiny moonlets are located, they create a wave-like action of carrying bits of ring material up to 1.5-km (about 1 mile) above and below the ring plane -- and from a certain angle, with the moonlets themselves not visible, it gives a "propeller" like impression. Indeed, NASA refers to them as propellers. An example is the Earhart propeller in Saturn's A ring (yes, it's named for Amelia Earhart) located just next to the Encke Gap (which looks huge in this image):

The Earhart propeller in Saturn's A Ring near the edge of the Encke Gap.


Thus, Saturn's present total of 62 known moons does not include most of these vast number of moonlets including all the "propeller" moonlets.

Below is a picture of the moonlet Daphnis -- counted among the 62 -- and the mini-gap it creates along with the wave-like features in the ring material. In this case, the roughly 8-km wide Daphnis is just large enough to clear a passage and not be concealed as a propeller feature. The caption is excerpted from the what Cassini team wrote for it:

Vertical structures created by the tiny moon Daphnis cast long shadows across the rings as Saturn approached its mid-August 2009 equinox. Daphnis, 8 kilometers (5 miles) across, occupies an inclined orbit within the 42-kilometer (26-mile) wide Keeler Gap in Saturn's outer A ring. Measurements of the shadows ... indicate that the vertical structures range between one-half to 1.5 kilometers tall (about one-third to one mile), making them as much as 150 times as high as the ring is thick.

The main A, B and C rings are only about 10 meters (about 30 feet) thick. Daphnis itself can be seen casting a shadow onto the nearby ring.

Visible light image taken by Cassini narrow-angle camera on May 24, 2009 from approx. 826,000-km (513,000 miles) distance from Daphnis at a Sun-Daphnis-spacecraft (or phase) angle of 121 degrees. Image scale is 5 kilometers (3 miles) per pixel.

Here is a close up image of Daphnis and its mini-gap:

Close-up of Saturn's moon of irregular lump of a moon Daphnis.


In all, Cassini took several hundred thousand images of Saturn, its rings, and its system of moons. The Cassini mission page contains all of them to include raw (unprocessed) ones and also videos. Here are the links:

The main Cassini mission control webpage (link embedded):

Raw Images

Since late April, Cassini has made 22 orbits in and out of the region between Saturn and its rings -- a zone never before traversed by human-made object -- and made detailed measurements that are being used to estimate the mass and age of the rings.

Now -- following a gravity nudge from Titan (a "final kiss" NASA dubbed it) on September 11, 2017 -- Cassini is on a collision course with Saturn.

Cassini Final Plunge information graphic. Click on image for larger version.

In the, Cassini managed to hang on to its communications link to Earth an extra 30 seconds-- with the final signal received by NASA's Deep Space Network antenna complex in Canberra, Australia at 1155GMT (7:55AM EDT / 4:55AM PDT) 15 September 2017.

The actual demise of the space probe occurred one hour and 23 minutes earlier. (Saturn, if you will, is 83 light minutes from Earth.)


To be clear, Cassini is expected to disintegrate about 900 miles above the cloud tops of Saturn -- well into the upper atmosphere of Saturn -- because the space craft's tremendous velocity of about 76,000-mph means even the tenuous number of molecules it is expected to strike at that altitude will be enough to destroy the space probe.

A full view schematic of Cassini's Grand Finale set of orbits between April 26, and September 15, 2017.


Before continuing, I just want to add my personal Cassini memory (in a manner of speaking). Cassini's power source are 72 marshmallow-sized plutonium pellets encased in iridium and graphite containers that were designed to withstand an explosion at launch or fiery reentry into Earth's atmosphere.

Animated gif of one of Cassini's 22 Grand Finale dives from above the orbital plane of Saturn to below it.


Using this fuel source caused some controversy precisely because of a fear that an explosion at launch or fiery reentry would cause radioactive plutonium to spew into Earth's atmosphere. There was even a dumb protest at Lafayette Square by the White House in mid-1997. I remember this guy with a microphone warning of the dangers of this launch because of plutonium.

Artist's rendition of Cassini on its final Grand Finale passage -- making its rendezvous with its destiny to become part of Saturn itself.


There weren't many people listening to him (there was also some other protest ongoing), but I was very irritated by him and told him that "this is why the left is a joke in this country." I also told him that when Cassini reached Saturn and provided amazing discoveries, would be admit he was wrong? He said something about irradiated milk.

Artist's conception of Cassini's Grand Finale plunge as it encounters the resistance of Saturn's upper atmosphere.

(Yes, the sky is blue on Saturn above the cloud tops -- since the same Rayleigh scattering occurs there as it occurs on Earth and dust seems limited.)


An animated gif showing Cassini during the last moments of its dive into the Saturnian atmosphere -- its exterior heating up to incandescence -- and as the gallant little probe turns into a shooting star in the sky of Saturn.


Anyway, I've decided to repost the AP article by Marcia Dunn that was published on Sept. 12th (and carried by multiple news outlets). It is interspersed with some additional pictures.

NASA's Saturn - orbiting Cassini spacecraft faces fiery finish
By Marcia Dunn
September 12, 2017

Source here.

CAPE CANAVERAL, Fla. (AP) -- After a 20-year voyage, NASA’s Cassini spacecraft is poised to dive into Saturn this week to become forever one with the exquisite planet.

There's no turning back: Friday it careens through the atmosphere and burns up like a meteor in the sky over Saturn.

A view of Saturn seen well above its orbital plane with the planet's bulk casting a shadow on its vast ring system.


NASA is hoping for scientific dividends up until the end. Every tidbit of data radioed back from Cassini will help astronomers better understand the entire Saturnian system -- rings, moons and all.

The only spacecraft ever to orbit Saturn, Cassini spent the past five months exploring the uncharted territory between the gaseous planet and its dazzling rings. It's darted 22 times between that gap, sending back ever more wondrous photos.

Saturn with Titan (upper right) and Tethys (lower left) visible, Cassini image, Jan. 30, 2008.


On Monday, Cassini flew past jumbo moon Titan one last time for a gravity assist -- a final kiss goodbye, as NASA calls it, nudging the spacecraft into a deliberate, no-way-out path.

During its final plunge early Friday morning, Cassini will keep sampling Saturn’s atmosphere and beaming back data, until the spacecraft loses control and its antenna no longer points toward Earth. Descending at a scorching 76,000 mph (122,000 kph), Cassini will melt and then vaporize.

Saturn's moon of Mimas is visible above the planet's rings.


It should be all over in a minute.

"The mission has been insanely, wildly, beautifully successful, and it's coming to an end," said NASA program scientist Curt Niebur. "I find great comfort in the fact that Cassini will continue teaching us up to the very last second."

Nice colorized image of Titan in front of Saturn.


Telescopes on Earth will watch for Cassini's burnout nearly a billion miles (1.6 billion kilometers) away. But any flashes will be hard to see given the time -- close to high noon at Saturn -- and Cassini's minuscule size against the solar system's second largest planet.

The plutonium on board will be the last thing to go. The dangerous substance was encased in super-dense iridium as a safeguard for Cassini’s 1997 launch and has been used for electric power to run its instruments. Project officials said once the iridium melts, the plutonium will be dispersed into the atmosphere. Nothing -- not even traces of plutonium -- should escape Saturn’s deep gravity well.

A hazy, crescent Titan with sunlight visible through this large moon's dense, frigid atmosphere.


The whole point of this one last exercise -- dubbed the Grand Finale -- is to prevent the spacecraft from crashing into the moons of Enceladus (ehn-SEHL'-uh-duhs) or Titan. NASA wants future robotic explorers to find pristine worlds where life might possibly exist, free of Earthly contamination.

It's inevitable that the $3.9 billion U.S. - European mission is winding down. Cassini's fuel tank is almost empty, and its objectives have been accomplished many times over since its 2004 arrival at Saturn following a seven-year journey.

Close-up of Mimas.
No, it's not the Death Star from Star Wars. That's just the Herschel Crater (about 140-km across).


The leader of Cassini's imaging team, planetary scientist Carolyn Porco, already feels the loss.

"There's another part of me that's just, 'It's time. We did it.' Cassini was so profoundly, scientifically successful," said Porco, a visiting scholar at the University of California, Berkeley. "It's amazing to me even, what we were able to do right up until the end."

Photomosaic of Iapetus in true color taken by Cassini, December 31, 2004.


Until Cassini, only three spacecraft had ventured into Saturn's neighborhood: NASA's Pioneer 11 in 1979 and Voyager 1 and 2 in the early 1980s. Those were just flybys, though, and offered fleeting glances. And so Cassini and its traveling companion, the Huygens (HOY'-gens) lander, actually provided the first hard look at Saturn, its rings and moons. They are named for 17th-century astronomers, Italian Giovanni Domenico Cassini and Dutch Christiaan Huygens, who spotted Saturn's first moon, Titan. The current count is 62.

Close-up of the spine-line equatorial ridge running across much of Iapetus.


Cassini discovered six moons -- some barely a mile or two across -- as well as swarms of moonlets that are still part of Saturn's rings.

Geysers of ice and water being shot into space from the south polar region of Enceladus.

All told, Cassini has traveled 4.9 billion miles (7.9 billion kilometers) since launch, orbited Saturn nearly 300 times and collected more than 453,000 pictures and 635 gigabytes of scientific data.

The European Space Agency's Huygens lander -- which hitchhiked all the way to Saturn aboard Cassini -- still rests on Titan. It parachuted down in 2005, about six months after Cassini arrived at Saturn, and relayed data for more than an hour from the moon’s frigid surface.

Still believed intact, Huygens remains the only spacecraft to actually land in one of our outer planetary systems.

Other than Titan's size -- about as big as Mercury -- little was known about Saturn's biggest and haze-covered moon before Cassini and Huygens showed up. They revealed seas and lakes of methane and ethane at Titan -- the result of rainfall -- and provided evidence of an underground ocean, quite possibly a brew of water and ammonia.

Left: Surface image of Titan from Huygens.


Animated image of an icy plume ejected from Enceladus filmed as Cassini shifted executed a looping maneuver around the small moon.


Over at the little moon Enceladus, Cassini unveiled plumes of water vapor spewing from cracks at the south pole. These geysers are so tall and forceful that they actually blast icy particles into one of Saturn's rings. Thanks to Cassini, scientists believe water lies beneath the icy surface of Enceladus, making it a prime spot to look for traces of potential life.

Close-up view of Enceladus.

There could be a liquid ocean under that cracked ice -- one that has conditions suitable for harboring life.


"Enceladus has no business existing and yet there it is, practically screaming at us, 'Look at me. I completely invalidate all of your assumptions about the solar system.'" Niebur said. "It's an amazing destination."

A zoomed in and cropped image of Earth and the Moon as seen through Saturn's ring, 1:41AM EDT April 13, 2017.

Cassini was 870 million miles (1.4 billion kilometers) away from Earth when the image was taken. Although far too small to be visible in the image, the part of Earth facing Cassini at the time was the southern Atlantic Ocean. Source here.


Looking the other way: Uranus as seen from Saturn by Cassini, April 11, 2014. Info here.


That's precisely why scientists didn't want to risk Cassini crashing into it, said program manager Earl Maize at NASA's Jet Propulsion Laboratory in Pasadena, California.

"The book is not complete. There's more to come" from exploring the planets, Maize said. "But this has been a marvelous ride."


The giant hurricane-like storm at the center of the hexagon region at the geographic North Pole of Saturn.

To be clear, as a gas giant, Saturn really has no "surface" in the sense that Earth and the other terrestrial rocky planets do.


Here are several more news article links:

From the Washington Post: NASA's Cassini spacecraft will crash into Saturn -- its final screaming success. The Post carried the above AP story as well here.

Enceladus sets behind Saturn in one of the last sequence of images taken by Cassini shortly before end-of-mission, Sept. 13, 2017.


Cassini will always live on.

OK, that's all for now. My next planned entry will be tomorrow (Saturday) night.


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