Cassini’s swan song: 13-year Saturn mission ends Friday

Prepare to say goodbye to Cassini.

In the early morning hours of September 15, NASA’s 13-year mission exploring Saturn and its moons will come to an end as the spacecraft deliberately dives into Saturn’s atmosphere and plunges itself into the planet.

Even then, Cassini will transmit new data about the planet’s composition as long as its antenna remains pointed toward Earth, with the assist from small thrusters. No spacecraft has ever been so close to Saturn.

“You can think of Cassini as the first Saturn probe,” said Linda Spilker, Cassini project scientist.

Europa Enceladus

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The Ion and Neutral Mass Spectrometer will act as the “nose” of the spacecraft, directly sampling the composition and structure of the atmosphere – something that can’t be done from orbit, said Hunter Waite, team lead for the spectrometer.

It will also investigate the “ring rain” phenomenon discovered by NASA’s Voyager mission in the early 1980s, where it appeared that the rings were raining down material on the planet and causing changes in the atmosphere. The spectrometer will attempt to investigate what material is from the rings and what material is part of the atmosphere.

But contact will quickly be lost once the spacecraft enters Saturn’s atmosphere at a high speed. About two minutes later, Cassini will burn and disintegrate completely – any traces of it will melt due to the heat and high pressure of the giant planet’s hostile atmosphere.

This will likely happen around 6:30 a.m. Eastern Time for the spacecraft, but given the time it takes for the signal to reach Earth, we will receive those last bits of data just before 8 a.m. – long after Cassini is “gone.”

“The spacecraft’s final signal will be like an echo,” said Earl Maize, Cassini project manager. “It will radiate across the solar system for nearly an hour and a half after Cassini itself has gone.”

Tethys is one of Saturn's larger icy moons. This photo was snapped by NASA's Cassini spacecraft in November 2016 from a distance of approximately 228,000 miles (367,000 kilometers).

A bright disruption in Saturn's narrow F ring suggests it may have been disturbed recently by the interaction of a small object embedded in the ring itself. They are hard to see, but their handiwork reveals their presence, and scientists use the Cassini spacecraft to study these stealthy sculptors of the F ring.

Saturn's moons Tethys, Enceladus and Mimas are shown in this image taken by the Cassini spacecraft on December 3, 2015. Tethys is above the rings, Enceladus is below the rings in the center of the image, and Mimas is below and to the left. Cassini has been exploring Saturn and its moons since 2004. The mission is scheduled to end in September 2017.

Saturn's icy moon Dione, with giant Saturn and its rings in the background, was captured in this mosaic of images just prior the Cassini spacecraft's final close approach to the moon on August 17, 2015. Scientists combined nine visible light images to create the mosaic. Cassini was at distances ranging from approximately 106,000 miles (170,000 kilometers) to 39,000 miles (63,000 kilometers) from Dione when the images were taken.

NASA's Cassini spacecraft has spotted mysterious reddish streaks on the surface of Saturn's icy moon Tethys. The red streaks are only a few miles wide but several hundred miles long. The images were taken in April. Scientists aren't sure what's causing the streaks.

This mosaic of Saturn's moon Mimas was created from images taken by Cassini in February 2010. A recent study indicates the moon may contain a liquid water ocean.

Cassini glided high above Saturn in October 2013 to capture this 36-image mosaic of the ringed planet. The colors of the planet appear natural, just as the human eye would see them.

Plumes of water ice and vapor shoot up from the surface of Saturn's moon Enceladus in this two-image mosaic taken by Cassini in November 2009. Analysis by NASA scientists indicated that water can reach the Saturnian moon's surface.

A small, bright blip can be seen on the outermost edge of Saturn's rings in this image taken in April 2013. The bump in the smooth ring structure is an icy object that could provide clues to how Saturn's moons formed.

This false-color image of Saturn's north polar storm looks like a giant red rose surrounded by green foliage. Measurements indicate the storm's eye is a staggering 1,250 miles across with cloud swirling as fast as 330 mph.

Saturn's rings cast a narrow shadow on its surface in this image taken in August 2009.

The scars of time and space mark the surface of Saturn's moon Enceladus. Saturn has at least 62 moons in its orbit.

The Saturn-facing side of Enceladus is illuminated by light bouncing off the planet. Plumes of water ice can be seen streaming off the moon's southern pole.

Saturn's moon Rhea is seen from approximately 174,181 miles away in this March 2013 image.

Rhea's surface is pockmarked with craters from billions of years of impacts. The moon is Saturn's second-largest, with a diameter of 949 miles.

Saturn's largest moon, Titan, has a diameter of 3,200 miles. It looks like a fuzzy orange ball because of its atmosphere.

Though it's the largest moon orbiting Saturn, Titan is dwarfed by Saturn itself.

Saturn has a small moon called Dione orbiting about 234,000 miles away. That's about the same distance Earth is from its moon.

The surface of Dione is seen in this May 2012 image.

Tethys, top left, is dwarfed by Saturn as it orbits the planet, though scientists think the moon is much larger than Saturn's ring system.

The Odysseus Crater spans 280 miles across the northern hemisphere of Tethys.

The small bright dot seen in the bottom right is not another Saturn moon. It's Earth. The distance between Saturn and our planet is constantly changing because both are constantly in motion. When they are closest together during their orbits, Saturn is 746 million miles away from Earth. At its farthest, they are just over a billion miles apart. See Cassini's top 10 discoveries about Saturn

Saturn and its moons

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Why the dramatic ending?

Cassini had its closest approach with Saturn’s moon Titan on Monday, dubbed a “goodbye kiss” by the mission’s engineers because it provides the gravity assist that sends it on its final encounter with Saturn.

Mission scientists and operators are giving Cassini this fiery send-off on purpose. While many other options were considered – such as “parking” the spacecraft in orbit around Saturn – they didn’t want to risk Cassini colliding with any of Saturn’s moons.

Cassini data and observations revealed that while seemingly inhospitable to us, two of Saturn’s moons, Enceladus and Titan, could be potentially habitable for some form of life. And NASA didn’t want to risk contaminating the moons or any future studies of the moonswith Earth particles. Although Cassini has been in space for 20 years – seven spent traveling to Saturn, 13 within the Saturn system – microbes from Earth could still viably exist on the spacecraft without air, water or protection from radiation.

UNDATED: This undated NASA handout shows Saturn's moon, Titan, in ultraviolet and infrared wavelengths. The Cassini spacecraft took the image while on its mission to. gather information on Saturn, its rings, atmosphere and moons. The different colors represent various atmospheric content on Titan. (Photo by NASA via Getty Images)

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While the mission itself is ending, the data and observations provided by Cassini will provide new details about the planet, its unique rings and moons for decades to come.

Cassini’s grand finale actually began in April, with a series of dives between Saturn’s rings, close to the planet and its moons, providing unprecedented insight. This is another reason the mission scientists decided on Cassini’s particular end-game. The final dive on Friday is a dramatic conclusion to this unique, long and scientifically valuable goodbye.

Cassini's top 10 discoveries about Saturn —

To celebrate the 10th anniversary of Cassini's Saturn orbit insertion, NASA has listed the top 10 things we wouldn't know if the spaceship hadn't traveled to the ringed planet. Also: Cassini images of Saturn and its moons.

Cassini's top 10 discoveries about Saturn —

10. First complete view of the north polar hexagon and discovery of giant hurricanes at both of Saturn's poles. Saturn's polar regions have surprised scientists with the presence of a long-lived hexagonal-shaped jet stream in the north and hurricane-like storms at both poles. The driving forces of each remain a mystery.

Cassini's top 10 discoveries about Saturn —

9. Mystery of the dual bright-dark surface of the moon Iapetus solved. The origin of Iapetus' two-faced surface has been a mystery for more than 300 years. The Cassini spacecraft solved the puzzle, showing that dark, reddish dust in Iapetus' orbital path is swept up and lands on the leading face of the moon.

Cassini's top 10 discoveries about Saturn —

8. Study of prebiotic chemistry on Titan. Titan's atmosphere is the most chemically complex in the solar system. Here, bodies of liquid near Titan's north pole can be seen.

Cassini's top 10 discoveries about Saturn —

7. Vertical structures in the rings imaged for the first time. Once about every 15 years, the sun shines on the edge of the ring plane and northern and southern sides of the rings receive little sunlight. Cassini measured the thick, long shadows from this rare event to determine the heights of structures within the rings.

Cassini's top 10 discoveries about Saturn —

6. Radio-wave patterns shown not to be tied to Saturn's interior rotation as previously thought. Saturn emits radio waves known as Saturn Kilometric Radiation. A similar radio wave pattern was measured at Jupiter to deduce the length of that planet's day. However, it was discovered that the variation in radio waves controlled by the planet's rotation is different in the northern and southern hemispheres. The northern and southern rotational variations also appear to change with the Saturnian seasons and the hemispheres have actually swapped rates. Saturn's length of day is still not known.

Cassini's top 10 discoveries about Saturn —

5. Studies of the great northern storm of 2010-2011. In 2010 Cassini got a front-row seat to a massive storm that disrupted Saturn's relatively tranquil atmosphere. The largest temperature increases recorded for any planet were measured. Molecules never before seen in Saturn's upper atmosphere were detected. The storm diminished shortly after its head collided with its tail, a little less than a year after it began. Saturn's rings are seen as the thin blue line in this image, due to the filters used to show methane absorption. The rings are outside the atmosphere, and therefore are not affected by methane absorption.

Cassini's top 10 discoveries about Saturn —

4. Titan revealed as Earth-like world with rain, rivers, lakes and seas. Titan is the only known place in the solar system, other than Earth, that has stable liquid on its surface. Rather than water, its lakes are made of liquid ethane and methane.

Cassini's top 10 discoveries about Saturn —

3. Saturn's rings revealed as active and dynamic. Cassini's mission allowed scientists to observe changes in Saturn's dynamic ring system, including what could be the birth of a moon. The disturbance, visible in the lower left of this image, is thought to be an icy body migrating out of the ring, believed to be part of the process required to form a moon.

Cassini's top 10 discoveries about Saturn —

2. Icy plumes on Saturn's moon Enceladus. Here, the plumes can be seen back-lit by the sun. The discovery was such a surprise that the mission was completely reshaped to get a better look. Evidence of water-based ice in the plume excited scientists further, as life as we know it relies on water.

Cassini's top 10 discoveries about Saturn —

1. The Huygens probe makes first landing on a moon in the outer solar system. The Huygens probe's historic landing in 2005 was the most distant to date. In addition to providing this photo, the probe found the moon to be similar to Earth before life evolved, with methane rain, erosion, drainage channels and dry lake beds.

Cassini's top 10 discoveries about Saturn

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What Cassini taught us and what’s next

Inspired to learn more after flybys of Saturn by NASA’s Voyager missions, the Cassini mission was designed to be an international effort that united NASA, the European Space Agency (ESA) and the Italian Space Agency.

It is known as the Cassini-Huygens mission because it delivered ESA’s Huygens probe to Titan – the “first descent and landing on a world in the outer solar system,” according to NASA.

UNDATED: This undated NASA handout shows Saturn's moon, Titan, in ultraviolet and infrared wavelengths. The Cassini spacecraft took the image while on its mission to. gather information on Saturn, its rings, atmosphere and moons. The different colors represent various atmospheric content on Titan. (Photo by NASA via Getty Images)

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The Cassini mission, which arrived at the Saturn system in 2004, has been extended twice and will finally use up the last of its rocket propellant this week.

In the end, Cassini will have witnessed half of a Saturn year. A year on Saturn equals nearly 30 Earth years. When Cassini arrived, the northern hemisphere of Saturn was emerging from winter. As seasons on Saturn last about seven Earth years each, Cassini was just able to witness summer in the northern hemisphere before the mission ends.

Over the years, Cassini has revealed new insight about Saturn, its rings and how they operate, the complexities of Saturn’s moons, the history of the solar system and planet formation and even the other places in our solar system where life might exist – ocean worlds. Cassini has collected 450,000 images using a visible light camera.

When Cassini arrived, it witnessed a giant storm circling the planet for nine months. We learned there are 3-D structures in the rings. Serendipitous observations showed that icy jets erupt from Enceladus. And Titan not only has seas and lakes of liquid ethane and methane, but an atmosphere of chemicals that rain down, forming a unique chemistry that could lead to life.

Related article 'Jet' disrupts one of Saturn's rings

“Cassini has transformed our thinking in so many ways, but especially with regard to surprising places in the solar system where life could potentially gain a foothold,” said Thomas Zurbuchen, associate administrator for NASA’s Science Mission Directorate, in a statement.

And the spacecraft itself, as well as its instruments, are informing future missions that NASA has planned, like NASA’s Europa Clipper mission to explore Jupiter’s icy moon, launching in the 2020s.

“Cassini has enabled those future missions to be possible,” said Jim Green, NASA’s director of planetary science.

Intrigued by Cassini’s discoveries, scientists have submitted concepts for future “spacecraft to drift on the methane seas of Titan and fly through the Enceladus plume to collect and analyze samples for signs of biology” that are currently under consideration, according to NASA.