Thursday, October 12, 2017

Webb and Our Galaxy


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Our galaxy, the Milky Way, contains a supermassive black hole at its core surrounded by a central bulge of old, yellowish stars. Beyond that are bluish spiral arms filled with younger stars, newly forming stars, and dark lanes of dust.

It is just one of billions of galaxies in our universe, but the Milky Way is our galaxy, our home in the universe. The Milky Way contains the closest examples of stars, planets, nebulae, black holes, and other objects that likely reside in every galaxy throughout the cosmos. By studying the Milky Way in the infrared, the Webb Telescope will be able to teach us a great deal about our galaxy and others. Webb will improve our understanding of all stages of star formation — from birth to death and back again to the rise of the next stellar generation. Astronomers know that stars form out of collapsing clouds of gas and dust, but they don't yet know the exact sequence of how stars are born. What triggers a cloud to collapse and a star to begin forming? How much of that mother cloud does a star use up when it forms? How and when do planets begin to form around a newborn star?
At the end of their lives, stars die in a variety of exciting and interesting ways — from gentle exhales of material to violent explosions expelling stellar shrapnel into the galaxy. Many dying stars and stellar corpses are embedded in their ejected material, which shrouds our view in visible light but can be penetrated with Webb's infrared vision. Webb will help us probe and understand both this residual material and the stars that died. It will help astronomers test their theories for how stars end their lives and how the heavier elements forged within these dying stars are recycled into the galactic environment to help create the next generation of stars.
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Omega Centuari is one of the largest globular star clusters residing in the outskirts of our galaxy.

Counting Stars

Webb will help us understand just how many stars there are and how those stars are distributed throughout the galaxy. The most common stars in the Milky Way are "dwarf" stars that are often too dim for Hubble to observe in visible light, but that glow brighter in infrared light. Webb will help astronomers get a firmer grip on just how many of these stars exist, and perhaps help us learn more about them. Knowing how many stars there are of different types also tells us how quickly or efficiently stars formed at various stages in the galaxy's history.
Webb will also study giant stellar swarms called globular clusters, which reside at the outskirts of the Milky Way and contain the oldest stars in the galaxy. Webb will analyze the composition of these ancient stars, and perhaps reveal whether globular clusters formed along with our galaxy or originated somewhere else, and were later absorbed into the galaxy.

Looking Inward and Outward

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This X-ray image shows the region around our galaxy's central supermassive black hole, known as Sagittarius A* (or Sgr A*). Credit: NASA/CXC/Univ. of Wisconsin/Y. Bai et al.

Webb will help us understand what's going on at the very heart of the Milky Way. In our galaxy's core lies a supermassive black hole surrounded by gas, dust, and a densely packed swarm of stars. However, this central black hole does not seem to be consuming as much material as its peers in other galaxies are. Astronomers aren't sure why. Webb’s infrared observations could give us a clearer view of the material and stars near the black hole, and perhaps uncover the reason why our galaxy's black hole is so quiet. Webb's sharp and powerful infrared vision will allow it to peer farther into the Milky Way with greater clarity than infrared telescopes before it — uncovering parts of the galaxy that were once too dim, too distant, or too concealed to study. These investigations will not only help us understand our own Milky Way, but myriad galaxies throughout the universe.

What Is the Center of Our Galaxy Like?



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Hubble's infrared vision reveled more than half a million stars at the core of the Milky Way.
Our solar system resides in one of the spurs off the spiral arms of the Milky Way galaxy, and we tend to think of our experience of the Milky Way as typical. Even in our science fiction films, when the heroes travel between stars, every sky looks the same.
But the Milky Way isn't quite so uniform. If you lived in the center of the Milky Way, you could look up on a sky thick with stars, a thousand to a million times more than we're used to seeing, depending on how close you were to the core. For Earth's inhabitants, the next closest star to our Sun is about 4 light-years away. For our central Milky Way cousins, that star would be around 0.4–0.04 light-years distant.
The center of the Milky Way consists of the region where the galaxy's spiral arm structure has broken down and transformed into a "bulge" of stars, or roughly the inner 10,000 light-years. At its heart — and the dominant force in that area of the galaxy — is a million-solar-mass black hole we call Sagittarius A*.
It would be an inhospitable area for humanity, rife with radiation emanating from a surplus of massive stars and material being torn apart by the black hole. Plus it would take us more than 25,000 years to reach it, even if we could travel close to the speed of light. Fortunately, the Webb telescope is designed to do our exploring of the galactic center for us.
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This simulated image shows a supermassive black hole at the core of a galaxy. The black region in the center represents the event horizon, where no light can escape the massive object's gravitational pull. The black hole's powerful gravity distorts the space around it, stretching light from background stars.

Sleeping Giant

Our central, supermassive black hole is relatively quiet when compared to its counterparts in other galaxies, flaring only occasionally with X-rays and infrared light as objects fall into it. It could be that there's simply not that much material around Sagittarius A*. Webb will investigate our strangely calm central black hole, providing a more accurate measurement of its mass, as well as how much material is falling into it, and when. Furthermore, the mass of our black hole ranges on the low end of normal for galaxies of our size. Webb will examine why that is and the relationship between a black hole and the matter surrounding it.
While Webb helps reveal why we have the kind of black hole that we do, it'll also be shedding light on central, supermassive black holes in other galaxies. Active galactic nuclei (AGN) are a type of extremely bright galaxy core seemingly fueled by powerful black holes actively gobbling large amounts of material. Astronomers would like to know what, exactly, AGN are and if they are triggered by events occurring in the centers of galaxies or by mergers between galaxies.
Webb's investigations of our own black hole and the relationship between black holes and galaxy evolution could help solve a cosmic chicken-and-egg problem: Did black holes come first and galaxies form around them, did galaxies form first and develop black holes, or did the galaxies and black holes develop together?

How Are Stars Born?



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Hubble captured the Eagle Nebula in visible light (left) and infrared light (right) in 2015. While the visible-light image shows opaque clouds, infrared light penetrates gas and dust, revealing both stars behind the nebula and those hidden away inside the pillar.

Stars form from collapsing clouds of gas and dust. It's a process that occurred in our distant past and continues to take place today. Astronomers can train their telescopes on giant clouds of hydrogen gas in our own galaxy and find knots of denser, colder gas and dust that are in the process of giving rise to stars. But these dust-thick regions of starbirth are often dark and opaque. The Pillars of Creation in the Eagle Nebula, depicted in one of Hubble's most famous images, is a stellar nursery, but what we see of it looks like a dense cloud.
In 2015, Hubble revisited the Eagle Nebula to create two new images of that famous region — one capturing visible light, and the other near-infrared.
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This young cluster of about 3,000 stars in our Milky Way is called Westerlund 2 and contains some of the galaxy's hottest, brightest, and most massive stars. Hubble's infrared vision pierced dust around this stellar nursery to reveal the dense concentration of stars in the central cluster.

The pictures illustrate the striking difference between what visible-light telescopes like Hubble see, and what infrared telescopes like Webb will show us. In the near-infrared image, the clouds are transformed into ghostly outlines and hidden stars blaze forth from both within and beyond. Newborn stars shine dramatically from within the cloud. Infrared light, unlike visible light, travels through dust clouds. And cameras that can capture it can see through such clouds as though they were nearly invisible. Furthermore, Webb's cameras will detect the infrared glow of the dust and gas itself, allowing us to learn what it's made of, how hot and dense it is, and what chemical processes have affected it. These abilities will make Webb a critical tool for learning just how star formation works within those dusty depths.

Seeing Stars

For instance, astronomers know collapsing clouds have a point of no return, when they become so dense and so cold that they cannot hold themselves together against collapse. Above this threshold stars form, below it they don't. But the gas drifting between stars isn't dense enough by itself to trigger star formation. Is star formation triggered mainly by shockwaves from exploding stars, or the pressure created by radiation and stellar winds from massive stars — or can those processes get in the way of the collapse? Could star formation begin with the collision and accumulation of sparse pockets of dust and gas? Do stars compete for material in the cloud, or do they form mostly in isolation? Does the entire cloud collapse into stars at the same time, or do stars form in groups? With its powerful infrared sensitivity and resolution, Webb will be able to peer into star-forming regions across the entire Milky Way galaxy, where previous infrared telescopes were limited to dust clouds within our own galactic neighborhood. Webb will collect a wide array of examples to give astronomers plenty of star-formation regions to compare.

Dwarfing the Giants

What Makes Brown Dwarfs Unique?

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Credit: NASA/JPL-Caltech
Twinkle, twinkle, little star
How I wonder what you are
It’s not easy to tell a star from a planet when you look up at the night sky. Ancient astronomers noted that some lights moved across the sky, while others appeared to remain in a fixed position. The Greeks, picking up the work of these earlier scientists, called such a travelling point of light planēs – wanderer. We still call them planets today.
But other than orbiting around a star, what makes a planet a planet? As telescopes become more sophisticated and we learn more about the universe, the less some old definitions make sense. We now know that some planets are rocky, like Earth, while others are so-called gas giants, like Jupiter.
We also know that our middle-aged Sun is one type of a variety of stars, classified by their phase in a lifecycle we are still in the process of understanding. A star shines by producing its own light from nuclear fusion in its dense, hot core. Planets shine—to our eyes on Earth—by reflecting the light of stars.
These were the simple, sharp definitions of stars and planets until the discovery of a brown dwarf in 1995. Theorized as early as the 1960s, this new type of celestial body blurred the line between star and planet, requiring an exciting re-thinking of the universe.
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An artist’s depiction of the relative sizes of the Sun, a low mass star, a brown dwarf, Jupiter, and the Earth. Credit: NASA/JPL-Caltech/UCB
Despite their name, brown dwarfs can be up to 70 times more massive than gas giants like Jupiter. Brown dwarfs form like stars do, by the contraction of gas that collapses into a dense core under the force of its own gravity, whereas planets form from the accumulation of leftover debris from these stellar births. However, brown dwarfs do not have enough mass for their cores to burn nuclear fuel and radiate starlight. This is why they are sometimes referred to as “failed stars.” They are smaller and cooler than the Sun, and have complex planet-like outer atmospheres, including clouds and molecules such as H2O. Astronomers now disagree on whether some “free-floating” bodies detected in space – not orbiting a star, but also not shining like a star – should be called planets or brown dwarfs. A brown dwarf atmosphere is easier to study than that of an exoplanet, which is typically obscured in the blinding light of its parent star. But to study brown dwarfs you first have to find them. Their dim light makes this difficult, and eventually the visible light left over from their birth fades completely beyond the red end of the visible spectrum, and they emit only infrared light.
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Credit: STScI
Difficult to detect, brown dwarfs hint at the many undiscovered wonders the universe still holds, hidden for centuries beyond the bounds of visible light. Much of the mass that holds the universe together with its gravity has thus far been undetectable, and is known as dark matter. In 2019, the James Webb Space Telescope will continue the work of NASA’s Hubble Space Telescope and infrared Spitzer Space Telescope in probing the furthest and “darkest” regions of the universe. Webb will see farther and in higher resolution, with unprecedentedly powerful infrared cameras and spectrographs. When Hubble launched, the only planets we knew of were those in our own Solar System. There were no images of brown dwarfs. Webb will take a detailed look at the atmospheres of brown dwarfs and exoplanets, determining their temperatures and chemical compositions. Do the traditional boundaries between planets and stars still make sense? Once purely philosophical, these questions now loom large in science. With infrared observations, the Webb Telescope will add to our understanding of the universe’s ongoing evolution, and the place of Earth and our Solar System within that bigger picture.

Friday, September 29, 2017

Elon Musk is aiming to land spaceships on Mars in 2022

Elon Musk just unveiled more of his grand plan for colonizing Mars.

 

The hard-charging tech mogul said his rocket company, SpaceX, aims to land at least two cargo ships on the Red Planet in 2022 in order to place power, mining and life support systems there for future flights.
That's just five years from now.
"That's not a typo -- although it is aspirational," Musk said Friday during a presentation at the International Astronautical Congress in Australia. Ships carrying crews would arrive in 2024, he added.


To hit those deadlines, SpaceX plans to start building the first spaceship by the middle of next year, he said. The billionaire entrepreneur does have a track record of setting ambitious time frames for SpaceX, and failing to meet them.
Musk revealed more details on the spacecraft -- the BFR, or Big Falcon Rocket, which inside the company is nicknamed the "Big F--king Rocket." If he has his way, Mars colonizers will eventually travel in style in the BFR, which will accommodate around 100 people spread out over 40 cabins, and include large common areas and an entertainment system.

SpaceX has figured out a way to pay for the costly missions, according to Musk, but he shied away from giving specific numbers.
The company thinks it can make enough money from its current business of launching satellites and servicing the International Space Station to finance its Mars ambitions. Musk's goal is to make the BFR reusable, which would significantly bring down the cost of launches.


He envisions the rockets enabling SpaceX to eventually establish a lunar base, which he dubbed "Moon Base Alpha."
"It's 2017, we should have a lunar base by now," he said. "What the hell's going on?"


Musk suggested the BFR could eventually also be used to clean up space by gobbling up old satellites and other junk orbiting the earth.
And it may come in handy closer to home. Musk said the rockets could fly people from city to city on Earth in incredibly short time spans, such as from New York to Shanghai in 39 minutes.
Related: Elon Musk wants to fly you anywhere in the world in less than an hour 


SpaceX isn't the only company with an eye on Mars, though.
Just hours before Musk took the stage in Australian city of Adelaide on Friday, Lockheed Martin (LMT) touted its plans for a "Mars Base Camp" -- a mobile space habitat that it's developing for NASA. The system would work in tandem with Orion, the spacecraft NASA is developing for crewed missions to deep space.
Lockheed said the structure could be assembled at NASA's Deep Space Gateway -- a structure the agency is developing that would live in space between Earth and the Moon.
Related: Branson! Musk! Bezos! The billionaire space race throwdown
The company says it hopes to build and send the structure to the Mars in about 10 years.
Unlike SpaceX's plan, NASA and Lockheed aren't looking to colonize Mars. Their project is focused on executing experimental missions in which highly trained astronauts would visit the planet and ultimately return to Earth.
Aerospace giant Boeing (BA) has also said it wants the first person to set foot on Mars to get there on one of its rockets.

Saturday, September 16, 2017

Cassini's Greatest Hits: Best Photos of Saturn and Its Moons

The Cassini spacecraft spent just over 13 years in the Saturn system, studying this massive, gaseous planet, its rings and its moons. Thanks to gravity assists from Saturn's moon Titan, the probe was able to change its orbit around the planet many times, and view the planet from various angles. In total, the probe sent back more than 450,000 images of the Saturn system. That's a lot to sift through, but we think these are some of the most spectacular images Cassini took of Saturn.


1. End of an era



Saturn's largest moon, Titan, can be seen beyond the rings

2. Rings and Waves

Cassini captured this view of a wave structure in Saturn's rings, known as the Janus 2:1 spiral density wave. Resulting from the same process that creates spiral galaxies, spiral density waves in Saturn's rings are much more tightly wound

3. Ring Shadows



The shadow of Saturn's rings appear as a thin band at the equator in this image that was taken by Cassini as Saturn approached its August 2009 equinox.

4. Planetary portrait

Credit: NASA/JPL/Space Science Institute
Cassini snapped this photo as it rested in Saturn's shadow, producing a stunning image where the planet's inner rings, seven of its moons and Earth can be seen.

5. Rainbow on the rings


Cassini's spacecraft offered unprecedented views of Saturn and its moons, including this photo of a rainbow on the planet's rings.

6. Colorful pole


This spectacular, vertigo-inducing, false-color image from NASA Cassini mission highlights the storms at Saturn's north pole. The angry eye of a hurricane-like storm appears dark red.

7. Unique image


In a rare wide-angle camera image from Cassini, Saturn's rings and Earth (seen here as a bright speck) share space.

8. Titan's seas


This near-infrared, color view shows the sun glinting off of Titan's north polar seas.

9.Venus through the Rings

Peering over the shoulder of Saturn, through its rings, and across interplanetary space, the Cassini spacecraft spies the bright, cloudy planet Venus

10. Trio of moons


Cassini observes three of Saturn's moons set against the darkened night side of the planet. Seen here are Rhea, closest to Cassini, Enceladus to right of Rhea, and Dione, to the left of Rhea.

11. Geysers on Enceladus

This image from Cassini, one that was acquired in a survey conducted by the spacecraft's imaging science team, shows the geyser basin at the south pole of Enceladus.

12. Light and shadow

Capturing the interplay between light and shadow, NASA's Cassini spacecraft looks toward the night side of Saturn, where sunlight reflected off the rings has dimly illuminated what would otherwise be the dark side of the planet.

13. Dione

Saturn's pale, icy moon Dione is enriched by the tranquil gold and blue hues of Saturn in the distance. The horizontal stripes near the bottom of the image are Saturn's rings.

14. Farewell, Mimas


On Jan. 30, 2017, Cassini bid so long to Saturn's "Death Star"-like moon Mimas.

15. Mind the gap


Saturn's moon Pan is seen in this color view as it sweeps through the so-called Encke Gap in Saturn's rings. As the lemon-shaped moon orbits Saturn, it always keeps its long axis pointed along a line toward the planet


16. Shadowy moon


Jagged-looking shadows stretch away from vertical structures of ring material created by the moon Daphnis in this image taken as Saturn approached its August 2009 equinox.

17. Titan and Mimas


Cassini captures a mutual event between Titan and Mimas in front of a backdrop of Saturn's rings. This image was snapped shortly before Saturn's largest moon passed in front of and occulted the small moon Mimas.

18. Dione's closeup


This view of Saturn's moon Dione was taken during a close flyby on June 16, 2015. It was Cassini's fourth targeted flyby of Dione.

19. Moon's gravity


The gravity of the potato-shaped Prometheus periodically creates streamer channels in Saturn's rings. The moon's handiwork can be seen in the dark channels in this image taken by the Cassini spacecraft.
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20. Dione in transit


Saturn's moon Dione crosses the face of the giant planet in this view, a phenomenon astronomers call a transit.

21. Icy Enceladus


On Oct. 14, 2015, the Cassini spacecraft snapped a ethereal image of Enceladus' icy north pole

22. Ligeia Mare


This image, from the Radar instrument aboard the Cassini spacecraft, shows the evolution of a transient feature in the large hydrocarbon sea named Ligeia Mare on Saturn's moon Titan.

23. Herschel Crater


Shadows cast across Mimas' defining feature, Herschel Crater, provide an indication of the size of the crater's towering walls and central peak.

24. Polar Hexagon


These natural color views compare the appearance of Saturn's north-polar region in June 2013 and April 2017. In both views, Saturn's polar hexagon dominates the scene.

Friday, September 15, 2017

In Photos: Cassini Mission Ends with Epic Dive into Saturn

Before NASA's Cassini spacecraft began the "grand finale" phase of its mission at Saturn, it took one last photo of the giant planet and its ring system from afar



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Cassini's Last Photo

Credit: NASA/JPL-Caltech/Space Science Institute

RIP, Cassini: Historic Mission Ends with Fiery Plunge into Saturn

NASA received its last data transmission from the Cassini spacecraft at 4:55:46 a.m. PDT (7:55:46 a.m. EDT, 1146 GMT) today (Sept. 15), before losing contact with the probe as it hurtled into Saturn's atmosphere. It was a fiery grand finale  for the probe, which spent 13 years orbiting the ringed planet. NASA officials expect that Cassini broke apart about 45 seconds after that final transmission, due to the intense friction and heat generated by the fall.

"I hope you're all ... deeply proud of this amazing accomplishment," Earl Maize, the Cassini program manager, said to the mission team after the spacecraft signal was lost. "Congratulations to you all. This has been an incredible mission, an incredible spacecraft, and you're all an incredible team. I'm going to call this the end of mission."

The Cassini spacecraft has plunged into Saturn, sending back its final communications before burning up in the ringed planet's atmosphere.
Credit: NASA/JPL-Caltech 
  The final stream of data from Cassini was received at NASA's Jet Propulsion Laboratory (JPL) in southern California. The spacecraft communicated with Earth via the Deep Space Network, a series of telescopes around the world that keep contact with spacecraft that fly beyond the moon. The Deep Space Network is managed from JPL.
During Cassini's final moments, mission scientists and team members watched anxiously as data continued to come in from the spacecraft as it hurtled through Saturn's atmosphere. The signal was lost when Cassini could no longer keep its antenna pointed at Earth, due to the intense friction created by its fall through the atmosphere. Maize said he anticipated that the probe would completely break apart about 45 seconds later. The team members stood and applauded somberly when Maize announced end of mission.

"This is a historic moment, and I think the mood reflects that," Morgan Cable, a research scientist at JPL, said of the event. "This is a celebration of an amazing mission and incredible legacy."

    In Cassini's final months and days, scientists and the public alike have voiced their affection for the space probe and the incredible discoveries it made.
"[I'm] feeling the love, if I may be so corny," Maize said when asked about the public outpouring. "It's just very heartening. Because it's part of what we try to do — to extend everybody out to Saturn. It's not [just for] scientists in the ivory tower; it's for humanity. And so for everybody to get on the ride … it is just phenomenal."
Cassini's descent into Saturn was intentional. The spacecraft was rapidly running out of fuel, after spending nearly 20 years in space, and NASA scientists decided to make use of the mission's inevitable conclusion. By crashing into Saturn, Cassini had the opportunity to see what the planet's upper atmosphere is made of, and that's the data that the probe sent back to Earth during its final few moments of life. The probe took its last images of the Saturn system yesterday (Sept. 14), and transmitted those images back to Earth the same day, ahead of its plunge.

Members of the Cassini team and other JPL employees watch the final minutes of the Cassini mission, next to a full-scale model of the spacecraft.
Credit: Calla Cofield/Space.com 
  During its 13-year tenure at Saturn, Cassini captured breathtaking images of the ringed planet, revealing swirling storms and a hexagonal jet stream swirling around Saturn's north pole. The probe saw strange features in the planet's ring system, found evidence of meteors crashing through the rings in the past, and watched as the planet's many moons caused the rings to change and evolve.
The spacecraft discovered new moons around Saturn; the planet has 53 named moons and another nine unnamed moons, and there are many more small objects that might one day be confirmed as moons. Cassini found geysers erupting from the surface of the large, icy moon Enceladus. Further study of the geysers has since indicated that Enceladus' subsurface water ocean might have conditions suitable for life. Cassini revealed new details about the strange surface of the moon Titan, which is dotted with liquid methane lakes, rivers and oceans.

This is the last image taken by NASA's Cassini spacecraft before it dove into Saturn's atmosphere. Cassini captured this view on Sept. 14, 2017 at 12:59 p.m. PDT (3:59 p.m. EDT; 19:59 GMT). It shows the location where the spacecraft would enter the planet's atmosphere hours later.
Credit: NASA/JPL-Caltech/Space Science Institute 
  "We left the world informed but still wondering," Maize said during a news conference Wednesday (Sept. 13). "I could not ask for more."
The $3.26 billion Cassini-Huygens mission  — a joint effort by NASA, the European Space Agency and the Italian Space Agency — launched in 1997 and arrived at the Saturn system in 2004. In 2005, the Huygens lander dropped onto the surface of Saturn's largest moon, Titan, revealing the hidden world beneath its opaque, orange atmosphere. The Cassini orbiter's initial mission was meant to last until 2008 but was extended twice, stretching the spacecraft's life to 2017

The Cassini team cheers, hugs and cries after receiving the final signal from Cassini that indicated the mission had come to an end with the spacecraft's disintegration in Saturn's atmosphere.
Credit: Joel Kowski/NASA/UPI/Newscom 
"One of the greatest legacies of the mission is not just the scientific discoveries it makes, and what you learn about, but the fact that you make discoveries that are so compelling that you have to go back," said Mike Watkins, director of JPL. "We will go back and fly through the geysers of Enceladus, we will go back and look at Titan, because the Cassini findings are just groundbreaking."

Watch Cassini’s Saturn dive live with mission control

Join NASA scientists as they bid farewell to Cassini.



Saturn backlit by the Sun, as seen by Cassini in 2013. Earth can be seen as a few bright pixels below the right hand side of the rings.
Saturn backlit by the Sun, as seen by Cassini in 2013. Earth can be seen as a few bright pixels below the right hand side of the rings.
NASA / JPL-Caltech / SSI
Just before 10:00pm (AEST) this evening, 15 September 2017, NASA’s Cassini spacecraft will end its 20-year mission of discovery as a fireball screaming through the skies of Saturn.

Cassini’s final Saturn plunge approaches

Since leaving Earth in October 1997, Cassini has seen Saturn and its rings up close, landed a probe on the icy crust of Titan, detected water spraying from the cryovolcanoes of Enceladus, and changed our understanding of the Solar System.
You can watch Cassini’s final act along with NASA scientists in the live stream from mission control below. The live broadcast will begin at 9 pm AEST (11am GMT, 7am EDT).
It’s expected that the signal from Cassini will cut out around 55 minutes later as the probe begins to burn up as it descends into the atmosphere. The spectacular flame-out is to ensure that no terrestrial life – in the form of possible stowaway microbes and the like – stands a chance of contaminating Saturn or its moons.
It takes about 80 minutes for signals to reach Earth from Saturn. By the time the broadcast starts, Cassini will already be gone.