Cassini left Earth in 1997, starting a long, lonely mission to meet Saturn. It arrived in 2004, becoming the first spacecraft to orbit the ringed planet. And now, after carefully courting Saturn over the past 13 years, Cassini has finally taken the plunge.

It became one with the gas giant on Sept. 15, as NASA deliberately crashed Cassini into the planet it was born to study. The probe had used up nearly all the rocket propellant it carried to Saturn, and NASA wanted to remove it from orbit "to ensure Saturn's moons will remain pristine for future exploration." If Cassini had become space junk, it could have crashed onto Enceladus or Titan, potentially contaminating the surfaces of moons where scientists hope to search for signs of alien life.

Cassini's swan song concludes "one of the most successful space expeditions in NASA's history," as MNN's Michael d'Estries wrote earlier this year. For a look at what made the mission such a success, check out this list of Cassini's amazing discoveries.

NASA kicked off Cassini's final assignment in April 2017. The probe was placed on an impact course that would unfold over five months, featuring a series of 22 orbits — each of which passed through the roughly 1,200-mile-wide gap between Saturn and its rings, a unique region never explored by spacecraft. "Called the Grand Finale, this final phase of the mission has brought unparalleled observations of the planet and its rings from closer than ever before," NASA explains in a mission summary.

That includes detailed maps of Saturn's gravity and magnetic fields, for example, which could reveal secrets about the planet's interior and its rotation speed. The Grand Finale phase will also "vastly improve our knowledge" about Saturn's rings, according to NASA, and yield "amazing, ultra-close images" of the planet.

Cassini will end its 20-year mission to Saturn with a dramatic death dive into the ringed planet on September 15th. Cassini ended its 20-year mission to Saturn with a dramatic dive into the ringed planet on Sept. 15. (Illustration: NASA)

The spacecraft made one final pass of Titan on Sept. 11, dubbed a "goodbye kiss" by mission engineers. "This final encounter is something of a bittersweet goodbye," project manager Earl Maize said in a statement, "but as it has done throughout the mission, Titan's gravity is once again sending Cassini where we need it to go."

Finally, at 7:55 a.m. ET (11:55 UTC) on Sept. 15, Cassini bid us farewell.

"It's a bittersweet, but fond, farewell to a mission that leaves behind an incredible wealth of discoveries that have changed our view of Saturn and our solar system, and will continue to shape future missions and research," says Michael Watkins, director of NASA's Jet Propulsion Laboratory, in a news release.

On top of sacrificing itself to preserve natural conditions on Saturn's moons, the spacecraft also squeezed some last-minute science into its final assignment. "Telemetry received during the plunge indicates that, as expected, Cassini entered Saturn's atmosphere with its thrusters firing to maintain stability," NASA explains, "as it sent back a unique final set of science observations."

But don't mourn for Cassini. "While it's always sad when a mission comes to an end," the agency adds, "Cassini's final plunge is a truly spectacular end for one of the most scientifically rich voyages yet undertaken in our solar system." And thanks to that legacy, we'll still be learning from Cassini even long after the probe is gone.

"To its very end, Cassini is a mission of thrilling exploration," according to statement from NASA. "And although the spacecraft may be gone after the finale, its enormous collection of data about Saturn — the giant planet itself, its magnetosphere, rings and moons — will continue to yield new discoveries for decades."

As for the probe itself, one NASA scientist notes that it's still up there in some form, living out eternity with its planetary soulmate. "Things never will be quite the same for those of us on the Cassini team now that the spacecraft is no longer flying," says Linda Spilker, Cassini project scientist at JPL. "But we take comfort knowing that every time we look up at Saturn in the night sky, part of Cassini will be there, too."

Editor's note: This article has been updated since it was originally published.

Russell McLendon ( @russmclendon ) writes about humans and other wildlife.