After 13 years in the Saturn system, NASA's Cassini spacecraft had become an icon.
The probe's data and imagery reshaped scientists' understanding of the ringed planet and its 60-plus moons, and brought Saturn's beauty and mystery to the masses all over the world.
All of that storied work came to an end on Sept. 15, 2017, when Cassini's handlers sent the craft hurtling into Saturn in an intentional death dive. This plunge wrung the most possible science from the mission while keeping any potentially habitable environments safe from contamination, NASA officials said. [The Biggest Spaceflight Stories of 2017]
"Not only did we do science here at the very end, but we protected the science to be done in the future," Thomas Zurbuchen, head of NASA's Science Mission Directorate in Washington, D.C., said the day after Cassini's plunge.
Sealing its fate
Cassini launched in October 1997 and took nearly seven years to reach Saturn. The spacecraft performed science during the long journey, capturing information about Venus, Earth and Jupiter as it passed each on speed-boosting flybys.
But Saturn was the main event, and Cassini exceeded expectations: The mission transformed our understanding of the ringed planet system.
"I never imagined the discoveries we'd make," Cassini team member Larry Esposito, of the University of Colorado, told Space.com. "Nature was way more creative than I imagined." [Cassini's 13 Greatest Discoveries During Its 13 Years at Saturn]
For example, Cassini spotted weird waves and strange propeller structures in Saturn's rings. And before the probe's mission, scientists had thought that the rings formed with the gas giant, about 4.5 billion years ago. Observations by NASA's Voyager mission raised a few questions about this assumption in the late 1970s and early '80s, but most scientists continued to believe the rings were ancient.
Cassini's data, however, suggest that the rings are only a few hundred million years old, Paul Estrada, a planetary researcher at the SETI (Search for Extraterrestrial Intelligence) Institute in California, told Space.com. (Estrada presented these results at the American Geophysical Union annual winter meeting in New Orleans earlier this month.)
And then there are the moons. Cassini discovered seas of liquid hydrocarbons on Saturn's biggest satellite, Titan. Before this find, Earth had been the only known world with bodies of stable liquid on its surface. The spacecraft also spotted geysers of water ice blasting from the south pole of the ice-covered moon Enceladus. This water is coming from a buried, potentially habitable ocean of liquid water, mission team members later determined. [Photos: Enceladus, Saturn's Geyser-Blasting Moon]
Cassini's observations of Titan and Enceladus "began changing the way we viewed the habitable or potentially habitable moons of the outer solar system," Jim Green, the head of NASA's Planetary Science Division, said after the probe's death dive.
These observations also shaped Cassini's ultimate fate: To make sure Cassini never contaminated Titan or Enceladus with Earth microbes, mission planners decided to hurl Cassini into Saturn, where it would burn up in the thick atmosphere. (Cassini was running out of fuel by September, so the end was near regardless.)
"Because of planetary protection and our desire to go back to Enceladus, to go back to Titan, to go back to the Saturn system, we must protect those bodies for future exploration," Green said.
A Grand Finale
On April 22, 2017, Cassini began the first step of its mission's "Grand Finale." A flyby of Titan reshaped the spacecraft's orbit, allowing Cassini to spend the next five months swooping between Saturn and its innermost rings.
These daring dives took Cassini through the tenuous uppermost layers of Saturn's atmosphere and along the edge of the inner rings — environments the probe hadn't yet studied up close.
On Sept. 8, Cassini made its final dive through the gap between Saturn and its rings. It made its last flyby of Titan on Sept. 11 and captured its farewell image of the Saturn system on Sept. 14.
"For 13 years, we've been running a marathon of discovery, and we're on the last lap," Cassini project scientist Linda Spilker, of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, said a day later. "We're here today to cheer as Cassini finishes that race."
Early in the morning on Sept. 15, Cassini crossed the finish line, barreling into Saturn's atmosphere. An analysis of the spacecraft's final moments suggested that the mostly aluminum spacecraft melted away quickly, said Cassini program manager Earl Maize, also of JPL.
The piece that lasted the longest may have been the plutonium power supply, which was wrapped in iridium to survive high temperatures in case of a problem during launch or an unexpected return to Earth. Still, even that gear wouldn't have survived the burning temperatures created by friction with Saturn's atmosphere for long, Maize said. Shortly after sending home its final signal, Cassini was no more.
Fittingly, Cassini gathered data to the very end — and performed better in its death throes than mission team members had expected.
"We got almost 30 seconds longer than we predicted," Cassini spacecraft operations manager Julie Webster, also of JPL, said of Cassini's data stream.
"We believe we got every last bit of data," Maize said.
Cassini may be gone, but the probe's impact will be felt for a long time to come. For starters, scientists will be sifting through its data sets for years.
"There's so much left, so much incredible science left," Spilker said.
"There's lots of work for everyone," Maize agreed. "The science data, to the extent that it can be funded by research grants, will continue for decades."
And Cassini has blazed a trail that other probes may soon follow. For example, NASA is currently considering launching a quadcopter to Titan that would fly through the moon's hazy skies. Cassini's observations will help select potential landing sites if that mission flies.
The agency is also supporting the development of a possible mission that would look for signs of life in the plume of material generated by Enceladus' geysers.
"I think Cassini was quite a gift to humanity," Spilker said the day after the spacecraft's death dive. "Goodbye, Cassini, and thanks for the ringside seat at Saturn."