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April 26, 2017

Cassini spacecraft snaps a shot of Earth between Saturn's rings in final 'death dive' – here's the story of its 20-year journey

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NASA’s Cassini spacecraft has had its last close brush with Saturn’s hazy moon Titan and is now beginning its final set of 22 orbits around the ringed planet.

Black and white image of Titan
This unprocessed image of Saturn’s moon Titan was captured by NASA’s Cassini spacecraft during its final close flyby of the hazy, planet-sized moon on April 21, 2017. 

The spacecraft made its 127th and final close approach to Titan on April 21 at 11:08 p.m. PDT (2:08 a.m. EDT on April 22), passing at an altitude of about 608 miles (979 kilometers) above the moon’s surface.

Cassini transmitted its images and other data to Earth following the encounter. Scientists with Cassini’s radar investigation will be looking this week at their final set of new radar images of the hydrocarbon seas and lakes that spread across Titan’s north polar region. The planned imaging coverage includes a region previously seen by Cassini’s imaging cameras, but not by radar. The radar team also plans to use the new data to probe the depths and compositions of some of Titan’s small lakes for the first (and last) time, and look for further evidence of the evolving feature researchers have dubbed the “magic island.”

“Cassini’s up-close exploration of Titan is now behind us, but the rich volume of data the spacecraft has collected will fuel scientific study for decades to come,” said Linda Spilker, the mission’s project scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California.

Black and white image of Titan
This unprocessed image of Saturn’s moon Titan was captured by NASA’s Cassini spacecraft during its final close flyby of the hazy, planet-sized moon on April 21, 2017. 

Gateway to the Grand Finale

The flyby also put Cassini on course for its dramatic last act, known as the Grand Finale. As the spacecraft passed over Titan, the moon’s gravity bent its path, reshaping the robotic probe’s orbit slightly so that instead of passing just outside Saturn’s main rings, Cassini will begin a series of 22 dives between the rings and the planet on April 26. The mission will conclude with a science-rich plunge into Saturn’s atmosphere on Sept. 15.

“With this flyby we’re committed to the Grand Finale,” said Earl Maize, Cassini project manager at JPL. “The spacecraft is now on a ballistic path, so that 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.”

Cassini received a large increase in velocity of approximately 1,925 mph (precisely 860.5 meters per second) with respect to Saturn from the close encounter with Titan.

After buzzing Titan, Cassini coasted onward, reaching the farthest point in its orbital path around Saturn at 8:46 p.m. PDT (11:46 p.m. EDT) on April 22. This point, called apoapse, is where each new Cassini lap around Saturn begins. Technically, Cassini began its Grand Finale orbits at this time, but since the excitement of the finale begins in earnest on April 26 with the first ultra-close dive past Saturn, the mission is celebrating the latter milestone as the formal beginning of the finale.

Black and white image of Titan
This unprocessed image of Saturn’s moon Titan was captured by NASA’s Cassini spacecraft during its final close flyby of the hazy, planet-sized moon on April 21, 2017. 

The spacecraft’s first finale dive will take place on April 26 at 2 a.m. PDT (5 a.m. EDT). The spacecraft will be out of contact during the dive and for about a day afterward while it makes science observations from close to the planet. The earliest time Cassini is scheduled to make radio contact with Earth is 12:05 a.m. PDT (3:05 a.m. EDT) on April 27. Images and other data are expected to begin flowing in shortly after communication is established.

A new narrated, 360-degree animated video gives viewers a sense of what it might be like to fly alongside Cassini as it makes one of its Grand Finale dives.

The Cassini-Huygens mission is a cooperative project of NASA, ESA (European Space Agency) and the Italian Space Agency. NASA’s Jet Propulsion Laboratory, a division of Caltech in Pasadena, manages the mission for NASA’s Science Mission Directorate, Washington. JPL designed, developed and assembled the Cassini orbiter.

Illustrated graphic showing some key numbers for Cassini's Grand Finale. Follow the link for text and details.

The NASA craft has been orbiting Saturn for over a decade and has recorded a wealth of information about the planet and its moons.

Giovanni Domenico Cassini was an Italian mathematician and astronomer born in 1625.He was the first person to notice the division of the rings of Saturn in 1675 and now, over 300 years later, a space probe bearing his name is orbiting the planet.

The Cassini spacecraft was launched by the American space agency on October 15, 1997.

It reached Saturn’s orbit in July, 2004 and has been studying the planet and its moons ever since.

Cassini’s mission is drawing to an end, as NASA prepares to crash it into the surface of Saturn itself. But the craft has, this week, sent back a beautiful new picture of the Earth, glimpsed through Saturn’s rings.

NASA’s Cassini spacecraft captured this view of planet Earth as a point of light between the icy rings of Saturn (Photo: NASA/JPL-Caltech/Space Science Institute)

The fly through Saturn’s rings is part of the last phase of Cassini’s mission.

Dwindling fuel reserves mean NASA will crash the craft into the planet – rather than risking any biological contamination of Saturn’s moons by depositing it there.

Diving between Saturn’s rings will start on April 26 and the little spaceship will be taking pictures of whatever it can as it passes through the giant sheets of icy debris.

The dramatic manoeuvre has inspired a Google doodle that will run on the world’s most famous homepage on April 26, 2017.

It shows an animated Cassini eagerly snapping pictures of Saturn as it passes through the gigantic planet’s rings.

The Cassini Google doodle that will display on April 26 (Photo: Google)

The April 26 dive is the first of 22 planned manoeuvres before it begins the very last part of its mission.

“No spacecraft has ever gone through the unique region that we’ll attempt to boldly cross 22 times,’ said Thomas Zurbuchen, associate administrator for the Science Mission Directorate at NASA Headquarters in Washington.

“What we learn from Cassini’s daring final orbits will further our understanding of how giant planets, and planetary systems everywhere, form and evolve.

“This is truly discovery in action to the very end.”

Once Cassini has made the crossings through the 2,400 kilometre gap, it will transition into its grand finale orbit – taking it through a close flyby of Saturn’s massive moon Titan.

(Photo: Handout)

Then, Titan’s gravity will bend Cassini’s flight and cause its orbit to shrink until it passes between the planet and the inner edges of its rings.

From there it will plunge through the skies on September 15, marking the official termination of the mission.

What’s the story of Cassini?

Following the launch in 1997, Cassini spent seven years flying through the Solar System to get to Saturn. During this time it conducted two flybys of Earth, one of Venus and one of Jupiter.

The craft also released a small probe, Huygens, that landed on Saturn’s moon Titan on January 14, 2005. It’s the first time humans have landed a craft in the outer solar system.

Assembly of the Cassini spacecraft (Photo: NASA/Wikipedia)

Saturn’s moons Enceladus and Titan are believed to be geologically active – and could even support life thanks to vast subsurface oceans. That’s part of the reason that NASA wants to avoid crashing Cassini into either of them.

The Cassini spacecraft (Photo: NASA JPL – Caltech)

Since arriving at Saturn (it travels at 2,040m/s), the 6.8-meter high spacecraft has collected huge amounts of data and snapped many pictures of one of the most fascinating planets in the solar system.

What has Cassini discovered?

In 2013, Cassini captured high resolution images of Saturn lit up by the sun.

The images are the first to show Saturn’s moons and rings, Earth, Venus and Mars all in one shot, with others showing just a few.

It was created by merging 141 shots to give a “natural-colour view” and it sweeps 404,880 miles across Saturn’s system.

(Photo: NASA)

One of the most interesting discoveries was made earlier this month, when Cassini confirmed that hydrogen gas exists on Saturn’s moon Enceladus.

Plunging the craft down over Saturn’s moon Enceladus , NASA was able to capture samples of water plumes erupting from the surface.

Analysis of the samples collected revealed that there was hydrogen gas present in the water that exists under the icy surface.

The presence of hydrogen in the moon’s ocean means that microbes (if any exist there) could use it to obtain energy by combining the hydrogen with carbon dioxide dissolved in the water.

This chemical reaction, known as “methanogenesis” because it produces methane as a byproduct, is at the root of the tree of life on Earth, and could even have been critical to the origin of life on our planet.

NASA’s Cassini probe swoops through the plumes erupting from Enceladus (Photo: NASA)

It’s not conclusive proof of alien life – but the discovery is helping evolve our understanding of Enceladus.

“This is the closest we’ve come, so far, to identifying a place with some of the ingredients needed for a habitable environment,” said Thomas Zurbuchen, associate administrator for NASA’s Science Mission Directorate at Headquarters in Washington.

“These results demonstrate the interconnected nature of NASA’s science missions that are getting us closer to answering whether we are indeed alone or not.”

When – and how – will Cassini die?

Cassini operates too far from the sun to be powered by solar panels. Instead, it draws its power from radioisotope thermoelectric generator (RTG). This is, in effect, a small lump of decaying plutonium-238 that gives off heat and power as it breaks down.

Although the RTG will still have power when Cassini terminates, it won’t have enough to power any further orbits of the massive planet.

(Photo: Press Association)

Instead, Cassini will be piloted directly into the surface of the gas giant on September 15, 2017.

It will keep its antennas firing toward Earth as long as possible, sending back important data and pictures as it moves through the clouds.

“This planned conclusion for Cassini’s journey was far and away the preferred choice for the mission’s scientists,” said Linda Spilker, Cassini project scientist at NASA’s Jet Propulsion Laboratory.

“Cassini will make some of its most extraordinary observations at the end of its long life.”

“It’s a thrilling final chapter for our intrepid spacecraft, and so scientifically rich that it was the clear and obvious choice for how to end the mission.”

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