It was a tense moment. The little Cassini spacecraft was hurtling through the black void of space at 54,500 miles per hour. After a seven-year journey, the probe was finally approaching its destination – Saturn – in June 2004. But the spacecraft needed to slow down for the planet’s gravity to pull it into its orbit. So project engineers were going to pump the brakes by carrying out a controlled burn. It was a planned procedure, but a risky one.

Igniting the thrusters too long could burn up much of the spacecraft’s precious fuel, cutting the multibillion-dollar mission tragically short. Not slowing it enough would leave it flinging past Saturn, lost forever in space. The plan was to fire the engine for 96 minutes. If it failed to burn for even four of those minutes, the probe would pass ignobly into oblivion.

Dozens of mission scientists and their families converged in an auditorium at Pasadena City College in California to witness the moment. Adults passed the time and soothed nerves by answering a steady stream of questions about the mission from children. As the moment neared to find out if the burn was successful, a hush fell over the crowd. Cassini project scientist Linda Spilker couldn’t relax in her seat. She stood frozen.

Then, at 9:12 in the evening local time, the radio transmission arrived from 934,431,318 miles away, blinking the result on a large auditorium screen.

“We waited and waited, and finally there’s the signal ... boom!” Dr. Spilker says, recalling that warm California night of June 30. “Right on the line [in Saturn’s orbit]. We were cheering, hugging each other.”

Bruce Weaver/Getty Images The Cassini Spacecraft is prepared for its voyage to Saturn in 1997.

Thus began Cassini’s auspicious quest to increase our knowledge of Saturn – an enigmatic planet that scientists have yearned to revisit for decades and which has fascinated humans for centuries. Today, 13 years later, the craft is about to end its mission as one of the most successful planetary probes in the history of space exploration.

The mission has lasted longer than expected. Cassini has sent back more information than anyone anticipated – about the physics of Saturn’s iconic rings; about the planet’s mysterious, swirling atmosphere; about the staggering variety of Saturn’s 53 confirmed moons. Perhaps most important, Cassini discovered salty geysers spewing out of one moon, containing hints that it could support alien life.

A joint project of the National Aeronautics and Space Administration and the European Space Agency (ESA), the $3.3 billion mission to Saturn has also shown how scientists from a diverse group of 17 nations can work together to explore the heavens. And the spacecraft’s work may not be done.

A fiery end is planned for Cassini in late September. But before then, the craft may well send back more surprises before it burns up while plunging deep into Saturn’s atmosphere. Even if it doesn’t, its legacy seems set in the annals of humans’ quest to fathom the cosmos.

The Cassini mission, says Matthew Shindell, a historian of science at the National Air and Space Museum, “has a pretty secure place as one of the highest performing outer solar system missions of our lifetime.”

No one knew precisely what discoveries lay ahead when Cassini blasted off from Cape Canaveral, Fla., in the cloudy predawn hours of Oct. 15, 1997. Just getting the spacecraft to that point was something of a triumph.

Budget concerns at NASA had put a number of missions in jeopardy in the 1990s, including Cassini. But the ESA was building the Huygens probe, one of the main features of the mission, and European scientists rallied behind the project. “For the US to pull out after the Europeans had spent hundreds of millions of dollars would not have been a good example of cooperation for the future,” says Richard French, team leader for the radio science instrument on Cassini and an astronomy professor at Wellesley College in Wellesley, Mass.

Scientists connected with the project had already put in more than a decade of work and were understandably excited. Cassini is, after all, the most highly instrumented planetary probe ever put into space.

The size of a small school bus, the nuclear-powered spacecraft consists of an orbiter and a lander. The Cassini orbiter, named for Renaissance astronomer Jean-

Dominique Cassini, would explore the Saturn system while circling the planet. It would also give the ESA-built Huygens lander a ride to Titan, Saturn’s largest moon, which was discovered by Dutch astronomer Christiaan Huygens in 1655.

The goal of the Cassini-Huygens mission was simple: to wring every secret possible from Saturn, its rings, and its moons. Humanity had gotten a glimpse of the planet before, in the early 1980s, when two probes, Voyager 1 and Voyager 2, sped past Saturn on their way farther out into the solar system and beyond. But this mission was different. Cassini-Huygens wasn’t an itinerant window peeper. It would be there to stay.

It offered the promise of unlocking secrets about a planet that has been the subject of curiosity since the ancient Romans tracked it in the night sky and named it Saturnus after their god of agriculture.

But first the craft had to overcome one small challenge: travel a billion miles.

NASA/JPL-CALTECH/SPACE SCIENCE INSTITUTE TITAN, ONE OF SATURN’S 53 CONFIRMED MOONS, HOVERS IN FRONT OF THE PLANET IN THIS IMAGE CAPTURED BY CASSINI.

Spacefaring often borrows terms from aeronautics, but Cassini more closely resembles a skydiver than a plane. It falls more than it flies, as gravity, rather than lift, determines its path.

Journeying to the outer solar system against the gravitational pull of the sun is no simple feat, and even Cassini’s three tons of fuel wasn’t enough to chart a direct flight to Saturn. So mission engineers had to rely on a method often used to propel probes, from Pioneer 10 to Galileo to Ulysses, deep into space: a cosmic slingshot.

Cassini’s first connection happened at Venus, a year after launch. Venus careens through space at nearly 80,000 m.p.h., and as Cassini passed by, the planet’s gravity dragged the craft along for the ride and then flung it away at more than 14,000 m.p.h. faster than it had been traveling before. It would repeat this trick, known as a gravity assist, the following year again with Venus, and then Earth, getting catapulted faster and farther from the sun each time.

Even with all this help, however, when Cassini finally settled into an orbit of Saturn in 2004, the spacecraft would have only about 20 percent of its liquid fuel left. The rest had been consumed by all the maneuvers needed to get there and the controlled burn to slow it down. The propellant would turn out to be enough, however, to carry the probe on nearly 300 orbits of the planet over the next dozen years – a mission life more than three times as long as scientists planned.

Yet none of this means the mission was easy. Studying a system as vast and complex as Saturn’s meant that hundreds of scientists around the world had to communicate regularly and compromise in making decisions. Consider just the itinerary alone. In an ideal world, planetary scientists would prefer to have sent Cassini on equatorial orbits of Saturn to get closer to its moons. But ring observers favor higher, tipped orbits. So, to try to satisfy everybody, Cassini went everywhere.

Spacecraft maneuvers had to be planned months in advance. Under its original design, Cassini was be fitted with an elaborate turntable to allow instruments to point where they needed to. But this “scan platform” was ditched to save money. Instead, engineers mounted all the instruments on the side of the spacecraft. So if, say, the imaging team wanted to point a camera in a different direction, the entire spacecraft would have to turn.

“Learning how to be cooperative with other scientists when you have a competition for resources has been really eye-

opening,” says Dr. French. “For me personally, the collaborations have been the most fun part of the mission, where you acknowledge that working together brings out the best science.”

The politics of all these decisions became more routine once Cassini got down to the process of discovery. A prime target of interest right from the start was Titan.

The moon, which is about the size of Mercury, has long been enshrouded in mystery. The Voyager probes couldn’t pierce the thick smoggy veil of Titan’s nitrogen and methane atmosphere decades earlier, but scientists were intrigued by its chemical composition. The atmosphere’s makeup hinted that Titan’s surface might look remarkably like Earth’s.

“There’s nothing like having a place where you know there’s a surface under there” but you can’t see it, says Jonathan Lunine, director of the Cornell Center for Astrophysics and Planetary Science at Cornell University and a scientist on the Cassini mission. “That kind of mystery is exactly what motivates future exploration.”

The Cassini spacecraft was to act as the eyes in the sky, swinging close to Titan for observation and gravity assists, while the Huygens lander would plant its feet on the ground. It would sample the surface chemistry and scour for other details. The results would be radioed back to the mother ship.

But that data almost didn’t make it back to Earth. A design flaw made Cassini’s “ear” too inflexible to receive radio waves from Huygens. Fortunately, a test by ESA communications specialist Boris Smeds uncovered the problem in early 2000, as the craft drifted through the asteroid belt. The flaw came down to one line of computer code, according to Mr. Smeds.

Nevertheless, it took a joint NASA-ESA committee a year to develop a way to solve the problem. Ultimately, an extra orbit around Saturn put Cassini on a path that would better position its ear.

At a cost of some backup fuel and a months-long detour from the painstakingly crafted tour plan, the $300 million Huygens probe finally touched down on Titan on

Jan. 14, 2005. It marked the first landing by a spacecraft in the outer solar system.

Smeds received a pair of champagne bottles for his find, but he says the real reward was salvaging the work of so many: “All these scientists, they work for this for years ... and it would have been very painful if they would have been there and everything was for nothing.”

Not all would have been lost had the Cassini receiver remained deaf to Huygens’s transmissions. Even from its vantage point in Titan’s skies, Cassini’s radar revealed rolling hydrocarbon dunes and the crinkled coastlines of methane lakes. But Huygens highlighted just how much Titan’s hydrological system resembles Earth’s own water cycle.

Indeed, the moon’s organic chemistry is thought to be much like that of Earth before life began. Liquid methane rains down on Titan, filling its lakes and rivers, and carving out gullies as it flows over the surface. Water fills that role on Earth, as it freezes, evaporates, and condenses at much higher temperatures than methane.

But water is more plentiful on Earth than methane is on Titan, which scientists say may make the moon’s climate system a good model for Earth’s climate system in the far-off future. “I like to call Titan the once and future Earth,” Dr. Lunine says.

While Titan was supposed to be the marquee attraction among Saturn’s moons, another planetary satellite ended up sharing equal billing: Enceladus.

Before the mission, Enceladus was just another small moon to planetary scientists. But Cassini exposed it as a complex, dynamic world, perhaps capable of supporting life.

There were clues that some sort of activity might be happening on the surface of Enceladus before Cassini arrived. Enceladus sits in Saturn’s E ring, the planet’s second outermost ring, which is particularly tenuous and likely needs a continuous source of dust, rock, and other material to exist. Perhaps some activity on the surface of Enceladus was providing that material, some researchers thought, but others said that was unlikely for such a small, presumably dead moon.

Scientists had also noticed in Voyager images that Enceladus’s bright surface bore too few impact craters to be completely inactive, but it wasn’t until Cassini took a closer look that an explanation emerged.

“I guess there was more or less the bloody glove at the scene,” Lunine says. “But it was Cassini that discovered the perpetrator. And the perpetrator was this giant plume fed by these many jets of material coming out of the south polar region.”

The Cassini mission was initially planned to spend four years at Saturn, looking at Enceladus just a handful of times. But having made such tantalizing discoveries on the moon, and with the desire to witness seasonal changes on Saturn, NASA approved two extensions to the mission: the two-year Equinox Mission and the seven-year Solstice Mission.

“Enceladus really reshaped Cassini’s mission,” Spilker says. “Once we discovered the geysers, then it became important to go back again and again and again.”

What began as an ordinary survey of a seemingly boring moon turned into a new front in the hunt for alien life. Cassini has zipped close to Enceladus 23 times in all, with seven trips passing directly through the ice crystals spewing out into space.

The spacecraft identified a subsurface ocean as the source of the geysers. Instruments showed the ocean contains salts, carbon-bearing molecules, silica grains, nitrogen-bearing compounds, and molecular hydrogen – all conditions conducive to life.

“It was just one ‘wow’ after another,” Spilker says.

NASA/JPL-CALTECH Cassini found evidence that Saturn's moon Enceladus, shown here, has a subsurface ocean with elements conducive to supporting life.

For all the excitement over Titan and Enceladus, many scientists never strayed far from the mission’s central star, Saturn, and its eye-catching rings, which have been a source of intrigue ever since Galileo first noticed them in 1610 and mistook them for a trio of celestial bodies.

New data from Cassini sets the thickness of the rings at about 30 feet – vanishingly thin compared with their 175,000-mile circular span. “People say that’s as thin as a razor blade, but it’s much, much thinner,” says Jeff Cuzzi, a ring specialist at NASA’s Ames Research Center and a scientist on the Cassini mission.

Cassini also found evidence that the particles, chunks, and boulders of ice that make up the rings are engaged in a sort of galactic dance. When Voyager swooped in for a look at the planet, its images suggested that the ring particles’ random bumping gave rise to a relatively smooth disk. But Cassini’s measurements indicate something more intricate is going on.

Gravity orchestrates the rings’ structure and motion. Saturn’s gravity keeps particles in its orbit, with the inner rings turning faster than the outer ones. As the particles circle the planet, they jostle each other in a way that makes the rings behave like a fluid.

And just as boats make waves in the ocean on Earth, the movements of Saturn’s moons trigger waves in the sheet of rings. As a result, the rings seem to dance with vibrations, ripples, and horizontal compressions that spiral tightly inside the rings.

“Our solar system that we think of as so solid and reliable,” Dr. Cuzzi says, “is actually full of dynamical surprises.”

And those surprises bring lessons with wide-reaching applications. The spiral compressions going on inside the rings have supported theories explaining the pinwheel arms of galaxies, while some of the other dynamics mirror the formation of planets early in the solar system.

Despite the trove of information already pouring in from Cassini, many questions remain to be answered. How old are the rings? How much do they weigh? What gives them their tawny color? Saturn’s rings appear redder than a typical ice cube, suggesting the presence of some mystery ingredient.

Of the multitude of riddles remaining about Saturn, a few may yet be resolved as Cassini plummets to its fiery demise. The spacecraft is running out of fuel, and mission planners didn’t want to let it keep flying right down to its last kilogram of propellant. That would risk having it crash on Titan or Enceladus and contaminate a potentially life-bearing world.

Instead, they set it on a course to plunge toward the planet. In recent months, Cassini has been going through a series of daring dives as part of what NASA calls the “Grand Finale.” It has been lunging through the wide gap between Saturn and its rings and, more recently, down into the planet’s upper atmosphere – areas never before explored.

On its final dive, Cassini will drop sideways toward Saturn’s voluminous cloud cover. Thrusters will stabilize the spacecraft as it encounters the atmosphere and keep its antenna locked on Earth so scientists can glean data down to the last second. Seven instruments will remain switched on, including an atmospheric probe.

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Then, on Sept. 15, the heat of plummeting through Saturn’s thick atmosphere will become too much: It will consume the craft in a final moment of incendiary glory at 4:56 a.m. Pacific Standard Time. Spilker will be among those listening when the radio waves carrying Cassini’s final messages reach Earth, from halfway across the solar system. But, for many scientists, the end of the mission is far from the end of Cassini’s ability to convey knowledge.

“Now we have this mountain of data that we need to start working our way through,” says Cuzzi. “Cassini’s going to go on for decades, and I look forward to seeing that.”