PASADENA, Calif. — The Cassini spacecraft has investigated Saturn, its distinctive rings and dozens of moons far longer than the experts at NASA’s Jet Propulsion Laboratory had ever hoped.

But the end of the mission is drawing near—as is the end of Cassini itself—so milestones are taking on a tone of bittersweet nostalgia.

On Monday, Cassini zips past the moon Dione for its final time. Cassini's closest approach, within 295 miles of the icy, crater-pocked surface, will occur at 2:33 p.m. EDT.

“We’ve been there four times before,” Todd Barber, the mission’s lead propulsion engineer, said Friday. “But each flyby helps unlock its mysteries.”

Information gathered by the craft’s package of instruments will be added the trove of data collected since launch in 1997. Cassini reached Saturn, a gas giant 900 million miles from Earth, in 2004.

Although the spacecraft remains in fine shape, albeit with a gas gauge approaching “E” and an odometer reading more than 4.3 billion miles, the mission is wrapping up, planned out to the day of Cassini’s death 25 months from now.

Cassini has enjoyed a spectacular life, with research taking place well past its planned four-year primary mission. Accomplishments include:

• The Huygens probe making the first landing on any moon of the outer solar system.

• Discovering that moon, Titan, has Earth-like features of rain, rivers, lakes, and seas.

• Finding icy geysers on another moon, Enceladus.

• Learning that Saturn’s rings are active and dynamic.

Fittingly, Cassini will receive a spectacular sendoff, officially called the “Grand Finale”: The final orbits carving through the 1,240-mile-wide gap between Saturn’s cloudtops and rings, before the craft is deliberately steered into the planet's atmosphere, to burn up, on September 15, 2017.

With Cassini’s propellant tanks almost empty, making course changes impossible, NASA JPL decided this ending was suitable for reasons described as scientifically compelling and, as Barber said, “being good stewards of the Saturn system by cleaning up our trash.”

Because scientists had learned some of Saturn’s moons possess the three building blocks for life—water, organic molecules, and sources of energy and heat—no one wanted to risk exposure from possible microscopic stowaways on Cassini.

“As crazy as it sounds, after 20 years there could be bacterial spores on the spacecraft, just waiting for some liquid water to get going,” Barber said. “That’s called ‘forward planetary protection.’”

But until then, Cassini still has work to do—such as Monday’s buzzing of Dione.

A mass of ice and rock measuring 700 miles in diameter, Dione orbits Saturn every 2.7 days at a distance of 234,000 miles, which is roughly the same distance that the Moon orbits around the Earth.

During the flyby, Cassini's high-resolution cameras will gaze upon the moon’s north pole, while the Composite Infrared Spectrometer will map unusual thermal anomalies. And the mission’s Cosmic Dust Analyzer continues its search for particles emitted from Dione.

“I believe this is a gravity flyby, so we can look at the subtle tugs of the moon on the spacecraft as it flies over, and that gives us hints into the interior of the moon,” Barber said. “The interior is where you learn about geology and what makes that moon tick.”

Cassini has performed this sort of gravity science investigation with only a handful of Saturn's 62 known moons.

Scientists also hope to find evidence Dione has geologic activity, like Enceladus, but at a much lower level.

“Dione has been an enigma, giving hints of active geologic processes, including a transient atmosphere and evidence of ice volcanoes,” Cassini science team member Bonnie Buratti said. “But we’ve never found the smoking gun.

“The fifth flyby of Dione will be our last chance."

Afterward, Cassini will be steered toward other moons. Planned in 2015 are three more flybys of Enceladus—including a plunge deep into the ice-particle jets streaming from the surface.

“As much as we love Dione,” Barber said, “we adore Enceladus.”

In late 2016, Cassini will get a close look at Saturn’s F-rings, an outer band described by Barber as “kinky and braided” where “there’s all kinds of gravitational mischief going on.”

Finally, in April 2017, Cassini maneuvers into position for its 22 inside-the-rings polar orbits. Goals include measuring the mass of the ring particles, using the same gravity science investigation Cassini will perform Monday with Dione, and searching for more clues about what is at Saturn’s center.

But how will Barber feel when the day comes to send Cassini to its destruction in the name of science? After all, he’s been with the mission from the beginning, joining six months before the Oct. 15, 1997 launch from Florida’s Cape Canaveral Air Force Station.

“I imagine through the nostalgia and a few tears, it’ll be a celebration of life,” Barber said.

Barber also noted scientists will be extending the mission’s legacy as they pore over 20 years of data.

“Literally, for decades they’ll still be writing Cassini papers, long after the spacecraft has breathed its last.”

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