After orbiting Saturn for 13 years, Cassini has become part of the planet

The Cassini spacecraft, after spending 13 nearly flawless years revealing a complex, ringed gas giant along with its extensive array of enigmatic moons—and finding two worlds capable of supporting life—died on Friday morning. It expired after setting its engines on full thrust and flying directly into the maw of Saturn. It was 19 years old.

The spacecraft made its final significant maneuver on Monday, flying near enough to Saturn's largest moon Titan to nudge Cassini into a collision course with the planet. Cassini took its final image on Thursday and on Friday morning began accelerating to more than 140,000km/hour. There was no return.

Cassini had to die this way. Its fuel reserves were nearly gone. Scientists didn't want to take a chance that, however improbably, some life from Earth had survived on board the spacecraft for 20 years and might one day contaminate one of the planet's moons, such as Titan or Enceladus.

Early Friday, as Cassini began flying deeper into Saturn's atmosphere, molecules from the planet struck the spacecraft. Slowly, it began to heat up. The spacecraft's attitude control thrusters, fighting to keep Cassini's antenna pointed back toward large arrays on Earth amid the turbulence, lost their struggle. At about 1,500km above the planet's cloud tops, Cassini began to tumble, falling, falling... falling. NASA scientists at the Jet Propulsion Laboratory lost contact with the spacecraft at 7:55am ET. It was burning and soon would break apart, becoming part of the gas giant it has so closely studied for so long.

Key discoveries

NASA's Cassini orbiter, along with the Huygens lander built to descend to the surface of Titan, launched on October 15, 1997, aboard a Titan IVB/Centaur rocket and reached an orbit around Saturn on July 1, 2004. That marked the beginning of the end of a more than two-decade journey, beginning in the 1980s, to get the probe into space and onto Saturn.

Voyager 1 had flown by Saturn in 1980, and Voyager 2 followed in 1981. These two iconic probes raised more questions about Saturn than they answered, especially about its moons. Titan, in particular, intrigued scientists with its thick nitrogen atmosphere—the only other world in the Solar System to have a nitrogen-rich atmosphere like Earth's. What might lie there?

"Voyager was a tease," planetary scientist Carolyn Porco, who worked on the imaging team for the Voyager mission, and led it for Cassini, told Ars. "It was like a fleeting moment in time when you could just see, briefly, what was there. We were left with all these questions. And they were important questions."

Questions like: Why is one half of that moon dark, but the other light? Why are they so weirdly shaped? What are all these things in the rings of Saturn? What could it tell us about the formation of the Solar System? And might the atmosphere on Titan allow for some kind of exotic life forms to exist?

During its 13 years at Saturn, Cassini sent back more than just dazzling photos. It found that Titan's nitrogen-rich atmosphere had a surface pressure 1.6 times that of Earth and had large, methane lakes. The presence of nitrogen along with methane, and ultraviolet light reaching the surface, created conditions where rich organic chemistry might flourish.

Similarly, Cassini answered key questions about the upper atmosphere of Saturn—mostly hydrogen but about 7 percent helium—and elucidated the nature of its fine rings. It explored the more than five dozen moons orbiting the planet. And upon closer inspection, Cassini found small moons dancing in gaps between the rings, gaps those tiny moons had created with their own small gravity fields.

And Cassini discovered a huge surprise at one of Saturn's smaller moons, Enceladus. Large, icy geysers were spewing into space. Later, scientists confirmed that a relatively large ocean must exist on the moon, beneath the ice. Cassini's mission was modified to fly through the geysers, and that small world, only about 500km across, is now considered one of the best places in the Solar System to look for extant life.

Survivors

For the scientists who study Saturn and the outer Solar System, Cassini represents a glorious achievement. While they are sad about Cassini's demise, it has been a tremendous run. "It's the finality of this enormous effort that we all personally have put into Cassini," Porco said. It was three decades of her life. "It’s kind of like a death in the family. It will feel that way."

Cassini leaves few survivors out in the Solar System. There is the Juno spacecraft, exploring the Jovian atmosphere for clues about the inner workings of the largest planet in the Solar System. It will fly into Jupiter within a couple of years. On January 1, 2019, the New Horizons spacecraft will fly by the Kuiper Belt object 2014 MU69 before heading ever deeper into space and following the Voyagers out of the Solar System.

And beyond that? NASA is developing an orbiter mission to Jupiter's moon Europa and, potentially, a lander. And while the agency is making progress, neither of these $3 billion to $4 billion missions is guaranteed to fly. Therefore, after Cassini's final transmission this morning, scientists will have lost more than a long-time, faithful friend in the outer Solar System. The worlds out there will have become quite a bit darker.

Listing image by NASA