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After 13 years spent orbiting Saturn, the Cassini spacecraft’s mission comes to an end

SCIENCE

The last days of Saturn's admirer

Cassini has buzzed around Saturn's planetary namesake for the past 13 years and, in the process, utterly transformed scientists' understanding of the solar system's most visually spectacular world . Its mission comes to an end this week

With this view, Cassini captured one of its last looks at Saturn and its main rings from a distance. Cassini has been orbiting Saturn for 13 years, nearly half of a Saturnian year, but that journey is nearing its end. The entire north pole is bathed in the continuous sunlight of summer in this photo.

In ancient tradition, Saturn was the god of time, an agricultural deity whose harvesting scythe eventually took on a different connotation in the sense that all good things must eventually come to an end. This week, that proverb is proving especially apt for Cassini, the intrepid spacecraft that has buzzed around Saturn's planetary namesake for the past 13 years and, in the process, utterly transformed scientists' understanding of the solar system's most visually spectacular world.

First envisioned by the U.S. space agency, NASA, more than three decades ago, Cassini was launched in the fall of 1997 and arrived at Saturn in 2004, becoming the first spacecraft ever to orbit the enigmatic ringed planet and spy on its many unusual moons. Since then, it has fulfilled its promise to a remarkable degree, notching discovery upon discovery and rewriting the textbooks along the way. For many, it rates as the most successful planetary mission in history.

On Friday morning, Cassini's mission will finally come to an end. With fuel for manoeuvring and control running out, flight controllers were faced with a tough decision. Even after 13 years in space, there's a good chance that Cassini, named after a 17th-century astronomer who studied Saturn, could be transporting bacteria from Earth. To avoid the possibility that it could one day contaminate one of Saturn's moons (particularly Enceladus, which, thanks to Cassini, scientists have realized could have some native life of its own), the mission is ending with a suicide dive into the atmosphere of Saturn. There, it will burn up as Saturn's powerful gravity draws it in and friction builds. Torn apart atom by atom, Cassini will become one with the planet it has illuminated so definitively.

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"There's no backing out now," said Joan Stupik, a guidance and control engineer with NASA's Jet Propulsion Laboratory in Pasadena, Calif. On Monday, Ms. Stupik was part of the mission team that directed Cassini to make a close pass of Saturn's giant moon, Titan. The moon's gravity altered the probe's trajectory just enough to put it on a collision course with the planet.

"That's what sealed our fate," she told The Globe and Mail.

Between now and then, scientists are scrambling to capture Cassini's final bursts of data, including an anticipated measurement of Saturn's atmospheric composition as it disappears into the planet's impenetrable clouds, it's antenna pointed toward Earth until the end. For those who have been with the mission for most of their careers, receiving Cassini's final readings will mark a bittersweet moment.

"It's been an irreplaceably valuable experience for me," said Jonathan Lunine, a planetary scientist and professor at Cornell University who was selected to be a member of Cassini's science team 27 years ago.

"Once the week is over, it's all going to be fine. It's just going through it that's going to be a little bit tough," he added.

But what is most important is that Cassini has left an unparalleled legacy of images and data from a world that has proven timeless in its capacity to elicit wonder.


A planet for all seasons

A false-color view of Saturn’s clouds.

A vast, gaseous orb encircled by a stunning series of concentric rings, Saturn has always been a showstopper for astronomers. By looking at the planet from every possible angle over many years, Cassini demonstrated that Saturn's visual richness includes great diversity and change. When the spacecraft arrived in July, 2004, Saturn was illuminated from below, marking the winter solstice for the northern hemisphere. Since then, Cassini has witnessed close to one half of Saturn's 30-year cycle of seasons. During that time, summer has returned to the north, triggering dramatic ammonia storms that revealed the turbulence lurking within the planet's pastel cloud layers. Cassini also discovered a strangely hexagonal zone of clouds, which appears as a slightly dark region at the top of the planet.

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Ringside seat

This view from NASA’s Cassini spacecraft shows a wave structure in Saturn’s rings known as the Janus 2:1 spiral density wave.

Starting with its initial trajectory, which required passing through Saturn's ring plane before settling into orbit, Cassini was able to reveal a universe of complexity in the patterns and motion of the trillions of icy particles that separately make up the planet's rings. Alternating bands of light and dark found within the thousands of individual "ringlets" that make up the system are the mathematical result of a gravitational tug-of-war between Saturn and its various moons. Cassini also spotted tiny moons orbiting within the ring plane, such as Daphnis, only eight kilometres across, which the spacecraft captured as it skirted along one edge of a ringlet, creating ripples like the wake from a small boat. By any measure, Cassini's detailed observations "have revolutionized our understanding of rings," says Doug Hamilton, a researcher who specialized in ring dynamics at the University of Maryland. "Every aspect of the field has been redefined."

The atmosphere of Titan, one of Saturn's moons, appears similar to that on Earth about 3.5 billions years ago before life appeared. ESA/NASA image

Paradise found

"Titan is an explorer's paradise," says Alexander Hayes, a Cornell University astronomer who has been working with Cassini data for more than 10 years to try to understand the complex processes that have shaped Titan's unique surface. The only moon in the solar system with a substantial atmosphere, Titan is too cold for water except in solid form. However, its surface has been carved by rivers of liquid methane that falls as rain and eventually gathers in lakes and seas. Covered by a thick orange smog that results from sunlight reacting with the methane, Titan was finally revealed by Cassini's infrared camera and radar images. Today, the smooth dark seas define alien coastlines that scientists are keen to explore in the future with amphibious robotic landers. John Moores, a planetary scientist at York University in Toronto, still remembers his time as a graduate student working on the Huygens lander, which Cassini carried with it from Earth and which dropped by parachute onto Titan's surface. After so many years of wondering what was down there, assistant Prof. Moores compares seeing the first glimpse of ice boulders on Titan's surface with seeing the runway materialize when landing at his native St. John's on a foggy day.

Many moons

This false-color view of Hyperion reveals crisp details and differences in color across the moon’s surface that represent differences in the composition of surface materials.

In addition to the massive and complex Titan (which is larger than the planet Mercury), Saturn is circled by dozens of small, icy moons, each with its own unexpected features. Among the strangest is Iapetus, with one side dark and one side bright. Cassini revealed the contrast in extraordinary detail and measured the composition of the darker, leading edge of the moon. Differences in temperature and sublimation of ice are thought to be the cause for Iapetus's two-tone appearance, but the details of the process that produced it are still a matter of debate. Equally strange is the irregularly shaped moon Hyperion, whose low density and porous interior has given it a honeycomb look after billions of years of battering by meteorites.

An ocean of mystery

While coming up on Saturn’s small icy moon, Enceladus, Cassini observed jets of material shooting out from a region near the moon’s south pole.

For many scientists who worked with Cassini through more than three decades including planning, development, launch and execution, the mission's most important discovery was also its most surprising. While coming up on Saturn's small icy moon, Enceladus, Cassini observed jets of material shooting out from a region near the moon's south pole. After many subsequent close passes by the spacecraft, scientists were able to determine that the jets are caused by water vapour escaping from a salty, subterranean ocean and shooting up through cracks in the moon's heavily lineated surface. The existence of the ocean – a spectacular find – has led to one of the most pressing questions that Cassini has left for the next probe to visit Saturn, mission scientist Jonathan Lunine says: "Is there life in the ocean?"

Looking back

On July 19, 2013, NASA’s Cassini spacecraft slipped into Saturn’s shadow and turned to image the planet, seven of its moons, its inner rings – and, in the background, our home planet, Earth - the tiny white dot at lower right.

While orbiting Saturn, Cassini occasionally was in a position to see the planet blocking the sun, with its rings brilliantly backlit by the sun's light in what amounts to a Saturnian version of a total eclipse. At such times, Cassini has been able to spot Earth as a small dot among countless background stars. It's not clear when any spacecraft will return to Saturn. Because of its distance, missions there cost billions and take decades to fulfill. But on July 19, 2013, Cassini captured this view of its distant home, where its images have inspired surprise, awe and countless questions. Asked what he found most memorable about working on the mission, Dr. Moores pauses and says: "I always enjoy how nature is different from what we can imagine."

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