Friday, April 28, 2017

An Astronomical Feast of Saturnalia: NASA JPL Video and Vox.com Reposting of Cassini's Grand Finale - An Overview; Images from Cassini's First Successful Pass Between Saturn and Its Rings


The best 3 minutes 40 seconds you'll spend today: NASA JPL overview of the fearlessly awesome little Cassini probe's "Grand Finale" act at Saturn.

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Below are two articles reposted in full from Vox.com with images taken from the articles along with some additional relevant pictures / photographs. (I've moved a few of the images to break up the text and made some additions / tweaks to the captions, as well.)

The Cassini spacecraft's dive in between Saturn's rings, explained

The spacecraft begins its "grand finale" before crashing into the gas giant later this year.

Updated by Brian Resnick@B_resnickbrian@vox.com Apr 26, 2017, 1:51pm EDT

Source here.

An artist's rendition of what Cassini’s crash into Saturn will look like. NASA/JPL-Caltech

Note: Yes, the daytime sky of Saturn just above the clouds appears blue -- it's still Rayleigh scattering. (Mars is odd because it is a hybrid of Rayleigh and Mie scattering.)

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The Cassini spacecraft is going where no ship has gone before: On Wednesday, it begins a series of dives into the space between Saturn and its magnificent rings. The maneuver -- a series of 22 orbits that will bring Cassini increasingly closer to Saturn's surface before crashing into it -- is called the spacecraft's "grand finale." And to mark this final journey, Cassini is being honored with a Google Doodle.


Over its last 13 years in orbit, Cassini has had an amazing run studying Saturn and its moons. Here's what the spacecraft has taught us so far -- and why its final mission may be its most spectacular yet.

In its last days, Cassini keeps generating fascinating insights

Colorized image of Titan in front of Saturn's rings.

Cassini -- named after the 17th-century astronomer Giovanni Cassini -- launched from Cape Canaveral in October 1997 in collaboration with the European Space Agency. When it launched, we were still a few months away from Bill Clinton's damning "I did not have sexual relations with that woman" remark. Harry Potter had not yet been published in the United States.

From there, it took Cassini and the Huygens lander (destined to touch down on the moon Titan) seven years to reach Saturn. Once it arrived, it started to make impressive discoveries.

On Titan, Cassini and Huygens revealed surprisingly Earthlike geographic features and great lakes of liquid natural gas on the moon's surface that outweigh all the oil and gas reserves on Earth. Cassini found evidence of an underground ocean on the moon Enceladus. It learned how new moons could form out of Saturn's rings.

Colorized image of the surface of Titan taken by Huygens; source here.

And it has taken detailed, beautiful photographic surveys of the planet's rings and surface features.

NASA/JPL/Space Science Institute; Saturn and its mighty rings silhouetted by the eclipsed distant Sun.

Nearing the end of its life, Cassini is still producing scientific discoveries at a fast clip.

Earlier in April, NASA announced that the spacecraft had found the most compelling evidence yet that the ocean underneath Enceladus could contain life.

Previously, the Cassini spacecraft has observed jets of water containing organic chemicals streaming from Enceladus. This latest finding adds a key ingredient for life to the mix: hydrogen. The presence of hydrogen in the jets makes NASA scientists suspect there are geothermal geysers on Enceladus's ocean floor. Like the geothermal vents deep within Earth's oceans, these could be home to microbes that use the chemical energy of hydrogen and carbon dioxide to produce methane and energy for life.

Above: Jets of water shooting from the surface of Enceladus, as seen by Cassini.

Now Cassini is beginning a series of harrowing orbits that bring it into the space between Saturn and its rings -- a region no spacecraft has been before. When Cassini is in the inner rings, it will finally be able to take the measurements that will aid in calculations to determine the mass of the rings.

Why NASA is diving into the space between Saturn and its rings

Density waves in Saturn's A Ring as seen in a close up image taken by Cassini in 2004 spanning about 220-km; source here.

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On Wednesday, Cassini begins a maneuver that is unprecedented in the history of spaceflight: It's adjusting its trajectory to bring it inside the 1,500-mile-wide gap between Saturn and its rings for 22 orbits.

This is what that dive will look like from Cassini's perspective:

In this looped animated gif, we see Saturn from the perspective of Cassini -- swooping "down" onto the planet at 77,000 mph (21.4 miles per second) relative to it for a pass that takes Cassini inside the orbit of its inner rings and barely 1,900 miles above the cloud tops. As Saturn recedes after the first pass, we can see the distant Sun.

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In these illustrations (below), the blue lines represent each of the 22 orbits getting closer and closer to the atmosphere of the giant planet. The red line represents the final orbit, which will end with Cassini crashing into Saturn's atmosphere.

NASA / JPL

In this space, Cassini will be able to take new measurements to better determine the total mass of Saturn's rings. NASA already knows the mass of Saturn plus its rings. Getting closer to the planet will allow Cassini to take its mass without factoring in the rings. That information will help scientists better understand how the rings formed (which in turn can help them understand how all the planets formed from rings of material around the sun).

The orbits will also produce the closest-ever observations of Saturn's clouds -- yielding incredible images.

NASA / JPL

It will be a thrilling journey, but also a perilous one. NASA has saved the ring-grazing orbits for Cassini's finale in part because they are dangerous. The orbits will bring Cassini close to debris and rocks that could take it offline.

"We're going out in a blaze of glory"

This artist's concept shows an over-the-shoulder view of Cassini making one of its grand finale dives over Saturn. NASA / JPL

Come September 15, Cassini will crash into Saturn, having spent all of its fuel. But the death dive isn't just for fireworks. If the spacecraft doesn’t plunge into Saturn’s atmosphere, it runs the risk of potentially contaminating one of the planet's moons with debris and microbes from Earth.

Curious wave-like patterns in Saturn's otherwise incredibly smooth rings created by the tiny moonlet of Daphnis (diameter just 5-miles and casting a shadow).

As Phil Plait explained here, Daphnis orbits at a slight angle to the rings -- alternating "above" and "below" them, pulling the pebble-to-boulder-sized icy, rocky bits that comprise the rings. Daphnis creates the Keeler Gap within the A Ring.

And there's no turning back: "The spacecraft is now on a ballistic path," Earl Maize, a Cassini project manager, said in a press statement, meaning that the spacecraft's path is shaped mostly by gravity, not by thrusters. "Even if we were to forgo future small course adjustments using thrusters, we would still enter Saturn’s atmosphere on Sept. 15 no matter what."

Artist's conception of what it looks like "within" Saturn's rings. The rings are believed to be incredibly "flat" -- with a width no more than a few dozen meters "thick" versus 200,000-km across.

Keep in mind that any stray bit of rock/ice that collides with Cassini on any one of its remaining 21 "Grand Finale" orbits could knock it out of commission before the final plunge.

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Cassini's dramatic finale is also a last chance to squeeze some more insights out of the 20-year-old spacecraft. As it descends into Saturn’s atmosphere, "several of the instruments will be on," including the mass spectrometer, Preston Dyches, a NASA spokesperson, says. This instrument essentially can "sniff" the atmosphere and determine the chemical compounds it's composed of.

Saturn's strange little moon of Mimas appears to hover above the planet's awesome ring system.

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On April 12, days before it made its final flyby of Titan, Cassini captured this incredible image of Earth shining through Saturn's rings, as if to remind us of how far it's come since beginning its journey. From Saturn, we're just a tiny bright speck in the darkness.

When Cassini finally goes offline in September, it will die doing what it's been doing all along: exploring.

Earth as a speck of light seen through the rings of Saturn across 870 million miles (1.4 billion kilometers) as captured by Cassini at 10:41PM PDT April 12, 2017 / 1:41AM EDT April 13, 2017.

As noted here, although far too small to be visible in the image, the part of Earth facing Cassini at the time was the southern Atlantic Ocean.

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Here we can see Earth's Moon (smaller, dimmer speck to the left) in this zoomed in / cropped image.

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Screenshot from this strange YouTube video short of Saturn "plunging" into the Sun -- making its approach to Earth (here about 1 million km away). The dark spot on Saturn is the Earth's shadow. (Obviously, this cataclysmic scenario would obliterate the Earth - Moon system.)

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Photos: what Cassini saw as it dived in between Saturn and its rings

The spacecraft survived its first "grand finale" dive. These pictures prove it.

On Wednesday, April 26, the Cassini spacecraft did something extraordinary: It slipped through the gap between Saturn and its rings, becoming the first spacecraft ever to explore this region.

The trip took it closer to the top of Saturn's atmosphere than any spacecraft had been before. And, yes, there are pictures.

On Thursday, NASA released these unprocessed images from Cassini. (Unprocessed means dust and other photographic artifacts are still in the shot.) What they represent is extraordinary: photographs taken just 1,900 miles above Saturn's atmosphere, while traveling at a speed of 77,000 mph relative to Saturn.

Here you can see a cyclone spinning in Saturn's atmosphere:


These two images (below) show banding and cloud features in Saturn's atmosphere.

Not bad at all for a 20-year-old camera 800 million-plus miles away.


Cassini's camera didn't have its color filters turned on for this pass, so we won't be seeing close-up, full-color images. "We were moving too fast to be able to take multiple filters over the same place on the planet," Preston Dyches, a NASA spokesperson, says. "When the spacecraft is really close to just Saturn like that, it's not possible to remain pointed at the same location long enough to snap a red, green, and blue Image." (NASA usually can combine the red, green, and blue images for a full-color representation.)

In any case, it's still thrilling to get shots this close.


The images come to us from Cassini's "grand finale" -- a series of 22 orbits bringing the spacecraft inside the 1,500-mile-wide gap between Saturn and its rings. The photos are only a small part of the mission. Another objective is to take new measurements to better determine the total mass of Saturn's rings.

NASA already knows the mass of Saturn plus its rings. Getting closer to the planet will allow Cassini to take its mass without factoring in the rings. That information will help scientists better understand how the rings formed (which in turn can help them understand how all the planets formed from rings of material around the sun).

After the 22 orbits, Cassini will crash into Saturn's atmosphere. Read more about Cassini's "grand finale" here.

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As a concluding image ...

A favorite image of mine: Saturn as it would appear in the daytime sky if at the distance of the Moon; source here (yes, another Yeti Dynamics production).

In reality, being that close, Earth would be uninhabitable for life given it would be deep inside Saturn's magnetosphere. Furthermore, so deep within the gravity well of far more massive Saturn, Earth would also be quickly tidally-locked to it so that even if our planet had a tolerable atmosphere, the climate would be totally incapable of supporting life as it now exists.

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OK, that's all for now. Please see previous entry for a brief update.

--Regulus

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