Cassini was the mission of a lifetime for this NASA scientist. Now she must say goodbye.

PASADENA, Calif. — Linda Spilker checks the clock: 12:04 p.m. As the NASA scientist sits in this crowded conference room on the Caltech campus, the aging Saturn orbiter Cassini is flying past the moon Titan for a final time. The maneuver on Monday will give Cassini the gravitational tug needed to sling it straight into Saturn’s atmosphere, where it will vaporize amid roiling clouds of dust and gas.

There’s no turning back now. Spilker’s life’s work is officially doomed.

That is the nature of being a planetary scientist. No mission lasts forever. Every spacecraft eventually runs out of fuel. Spilker knew this when she joined the Cassini team half a lifetime ago. Later, as head scientist, she was part of the group that devised the mission’s “grand finale,” which has sent Cassini on dizzying dives between Saturn and its rings and ends Friday with the fatal plunge.

“I’m trying to be stoic,” Spilker says. The mission could have been prolonged by moving the probe into a safer, more distant orbit. But that isn’t Spilker’s style – or Cassini’s. After 13 years at Saturn, it seemed only fitting to send the spacecraft out “in a blaze of glory,” the scientist says. Use that last bit of fuel to see what no one has seen before. Leave behind one more discovery for scientists to puzzle over after it’s gone.

Spilker stands, and raises a plastic cocktail glass of sparkling apple juice (Caltech doesn’t allow alcohol in school buildings) to a room of fellow scientists who have come to feel like family.

“Titan has given Cassini that last push – a goodbye kiss. Its fate is sealed,” she announces. “A toast to a great spacecraft, a great mission.”

The assembled researchers lift their glasses of juice and chorus their appreciation. A few are close to tears. After Cassini disintegrates, this team will be disbanded, and NASA’s view of Saturn will go dark. For the moment, the space agency has no plans to return to the ringed planet.

But Spilker and a young protege have submitted a proposal for a new mission to the Saturnian system, which would investigate one of Cassini’s most significant finds: jets of water on the moon Enceladus that could contain traces of alien life.

This isn’t a funeral, Spilker constantly reminds her colleagues – and herself. It’s more like a graduation: “Both an end and a beginning.”

She holds onto this idea as the mission’s final minutes tick away. Cassini’s work isn’t over. It’s just turning into something new.

Before Cassini, before Spilker, before NASA, there was Saturn. There has always been Saturn – that gold and glowing gas giant, encircled by shimmering rings of ice and dust. It was the most distant of the planets visible to ancient astronomers, who thought they could divine the secrets of existence from the behavior of lights in the skies. They called Saturn and its fellows “planetes,” or “wanderers,” for the way they roamed the heavens against the steady background of stars.

It wasn’t until the Copernican Revolution of the 17th century that scientists realized the planets are actually bodies orbiting the sun and that Earth is among them, another wanderer. Then Galileo became the first to reveal the planet’s rings with a telescope, Dutch astronomer Christiaan Huygens spotted the largest of its moons, Italian scientist Giovanni Cassini (the spacecraft’s namesake) detected several more satellites. Saturn became a world unto itself – not simply a spot in the sky but a place to explore.

As a child, gazing into the sky with her two-inch refractor telescope, Spilker was captivated. When she graduated from college in 1977, just before NASA’s twin Voyager probes launched on a solar system tour that would send them past Saturn, she sought a job at the Jet Propulsion Laboratory studying the planet’s rings.

The data from Voyager was printed on rolls of paper so long Spilker had to take them into a hallway to study them. She recalls walking among those sheets as a 21-year-old, tracing the rippling pattern of rings a billion miles away and getting the feeling that she was stepping around Saturn itself.

After the flyby, the Voyager probes sailed on to more distant parts of the solar system. And life on Earth marched forward. Spilker got married, got her PhD, had children. Many of her colleagues did the same.

“A whole generation of JPL kids was born in that window,” Spilker says. They would all grow up alongside one another and alongside the new mission Spilker was helping to develop. A flagship voyage that would, for the first time, be devoted entirely to Saturn.

It took nearly a decade to get Cassini approved and built. Budget constraints required the team to scale back the spacecraft and its ambitions: a rotating platform that would make it easier to conduct observations was scrapped; an instrument that can “taste” molecules was downsized.

As the pieces came together in the cavernous “clean room” at JPL, Spilker made time each week to walk by and witness it. Someday soon this school bus-size contraption would be circling another world. But for now, it was almost close enough to touch.

In October 1997, Spilker stood on a lawn at Cape Canaveral, Fla., and watched Cassini streak into the velvet predawn sky. Launch is the most nerve-racking phase of any mission; when you set fire to a tank of rocket fuel beneath a billion-dollar spacecraft, so much can go wrong.

But nothing did. Seven years later, Cassini’s entrance into orbit around Saturn was similarly flawless.

“That’s Cassini,” Spilker says, with affection and pride. “She’s very hard-working, very diligent. And curious. Extremely curious. In that way, she’s an extension of what we are.”

Once at Saturn, the discoveries commenced at a rapid clip. The Huygens craft, provided by the European Space Agency, touched down on Titan – humanity’s first landing in the outer solar system. Cassini revealed the composition of Saturn’s rings and photographed the vast hexagonal storm at the planet’s north pole. Each new batch of images kept Spilker at JPL late into the night; they were transporting. Looking at them, Spilker says, was as close as any human will ever get to being the first explorer at this new world.

Perhaps most astonishing of all was an image of the moon Enceladus backlit by the sun. For the first time, scientists saw that jets of water and ice were spewing out of cracks in the moon’s frozen surface. Later surveys revealed that Enceladus harbors a vast subsurface ocean and important ingredients for life.

Could alien organisms be swimming on that far-flung world? It looked increasingly possible.

The problem was, Cassini wasn’t built to be a life-finding mission. When it launched 20 years ago, such a goal seemed unimaginable. The molecular “taster” that the team downsized to save money wasn’t powerful enough to test for the long carbon chains that could be considered biological.

And with each passing year, Cassini was using up its fuel. If it stayed in orbit too long, NASA engineers would lose the ability to control the spacecraft. A passing moon or gravitational quirk might knock it off course and send it crashing into Enceladus, where it could contaminate the pristine – and perhaps inhabited? – landscape.

So the “grand finale” was set in motion. Cassini began its dives through the rings in April, each precipitous plunge bringing the craft closer to Saturn’s storm clouds.

Meanwhile, Cassini’s human handlers started preparing themselves for the end.

This week is the final meeting of Spilker’s Project Science Group at which there’s still new data to discuss. But it’s also a chance for the team’s few hundred members to collectively mourn. There are group stargazing sessions, group photos, group hugs. One of the engineers hands out purple handkerchiefs embroidered with the details of Cassini’s mission. “You may need this,” she tells Spilker.

People keep coming up to Spilker to shake her hand. “Congratulations,” they say, their voices thick with emotion. “Thank you.”

The days are so hectic Spilker barely has time for her own feelings. Only at night does she stop to think. Thoughts like, “This is really happening.” And, “It’s been so long.” And, “Maybe I am getting old.”

She thinks about Friday. Her daughters are traveling to Pasadena for those final moments – one of them will bring a daughter of her own.

“They’ve been with, in a certain sense, with the Cassini mission their whole lives,” she says, “the launch, Saturn orbital insertion, and now the end of Cassini.”

But this is not really the end. With fellow Cassini scientist Morgan Cable, Spilker has developed a proposal to return to Enceladus and seek signs of life.

“I’ve come full circle now,” Spilker says. “Working on another new mission.”

She and Cable will find out in December whether they get to move forward with their proposal. But even in the best-case scenario, it’s unlikely Spilker will see the idea to fruition. At 62, she’s contemplating retirement. If and when the Enceladus mission gets the go-ahead, Spilker will hand control to her younger counterpart.

Cable is 35, the same age as Spilker when the Cassini mission was officially approved.

Looking down into the same clean room where Cassini was built, Cable can picture the pieces of an Enceladus probe coming together. She can envision the spacecraft sailing through the moon’s plumes, tasting for organic molecules, detecting something her predecessors only dreamed of.

Like generations of astronomers before her, Cable seeks from Saturn the answers to humankind’s biggest and oldest questions: Why are we here? Are we alone?

“Deep down, I think I always hoped that life exists out there somewhere, and I really hope that we find it in our lifetime,” she says. “It’s just a matter of continuing to look, being persistent. Following the clues that missions like Cassini leave for us.”

She, too, will be watching the mission’s final moments Friday. Though she has worked on Cassini for only three years to Spilker’s nearly 30, Cable shares her mentor’s affection for the plucky space robot.

“As a scientist, I always try to be empirical,” she says. “But you get attached to the things you work on.”

And then her eyes fill with tears. “Crap.” She wipes her face and lets out a watery chuckle. “Sorry. This is going to happen a lot this week.”

She can’t help it. That is the nature of being human.

The days until Cassini’s demise turn into hours. On Thursday afternoon, the spacecraft will take its final images. Soon after, it will turn its antenna toward Earth, sending a steady pulse of radio waves about everything it senses as it plunges toward its demise.

“I see that signal like Cassini’s heartbeat,” Spilker says.

Just past 3:30 California time on Friday morning, the spacecraft will cross the threshold into Saturn’s atmosphere and burn up like a meteorite.

But because Saturn is so distant, Cassini’s final heartbeat won’t reach Earth until 83 minutes after it’s gone. When Spilker and her colleagues hear the last of their pioneering probe, it will be a whisper from a ghost: one final piece of insight from an alien planet, beckoning to whoever comes next.