It was not yet dawn on the U.S. West Coast when the Cassini spacecraft sent its final message to Earth and began its suicide plunge into Saturn. At NASA's Jet Propulsion Laboratory in Pasadena, California, scientists and engineers crowded into a packed mission control room, while others watched the signal unfold down the road on the campus of the California Institute of Technology. At just after 4:55 a.m. local time, September 15, 2017, the tiny orbiter ended its 20 year mission.
"I liken it to an undefeated boxer, or a baseball player that retires at the end of the season," said Brent Buffington, an aerospace engineer at JPL who helped plot Cassini’s path over the past six and a half years. "They went out on their terms."
Still, Cassini managed to wring out the last drop of science possible as it met its end in Saturn's dense clouds. Even as it hurtled toward oblivion, it was also investigating the planet's atmosphere for the first time. This was characteristic of the orbiter, which has been uncovering troves of incredible insights about Saturn and its moons ever since it arrived at the ringed planet in 2004. The mission's lifetime was extended not once but twice to give the craft more time to probe Saturn’s mysteries.
Cassini didn't stop at Saturn, either: The spacecraft pierced the thick smog of Saturn's largest moon, Titan, to discover lakes of methane and ethane, the only liquid known to exist on a planet other than Earth. It unveiled strange landforms, from dunes to labyrinths to possible ice volcanoes. Cassini also captured incredible images of geysers spouting from the southern pole of the icy moon Enceladus, and unmasked a liquid ocean hidden beneath the moon's icy crust.
These and other observations have helped make the case that our solar system is filled with ocean worlds—and that life may be able to evolve and even thrive far from the sun.
Ultimately, it was NASA's concern for Enceladus and Titan that mandated Cassini's death. Both worlds are ripe for life to evolve on their own, and scientists hope to hunt for possible signs on future missions. One very real worry is the possibility of contaminating these kinds of worlds with our microbes (to the point that we have an entire Planetary Protection Office devoted to preventing that from happening).
"The last thing we want to do is pollute these pristine bodies with the Earth microbes that could be on our spacecraft," Buffington said. So he and the navigational team sat down to figure out how to maximize how much science they could get out of Cassini, while keeping these potentially habitable worlds clear of contamination.
The navigational team pursued several potential orbits for Cassini once its fuel tank was on empty, Buffington said. They could park the spacecraft in permanent orbit around Saturn, sending back information about the system for years to come. They could smashed it into the rings to see how they would react, a collision that could also provide insights. They could crash it into one of the many Saturn moons. Or it could leave the system entirely, traveling to another giant planet or the strange asteroids of the outer solar system.
Each possibility was presented to the science team, who looked for the best way to make the most of the spacecraft's final days. The selection process was, Buffington says, "Darwinism at its finest.”
Smashing into the rings was swiftly ruled out. Trying to prove that none of the resulting pieces would end up falling onto—and potentially contaminating—Titan or Enceladus was all but impossible. Exploring another world was also rejected, given how many outstanding questions about Saturn remained.
And while an eternal orbit around Saturn sounded good, there was one big problem: Titan, one of the worlds they hoped to preserve, had the potential to cause chaos, and could one day send Cassini spiraling into one of the habitable moons.
So the team decided to put Titan's power to good work. Buffington, who left the mission in 2012 but returned to JPL to witness Cassini’s grand finale, said that one of the major breakthroughs was the realization that the massive moon could be used as a workhorse. That is, engineers could take advantage of the fact that, when a small body passes by a larger moving body, the small body’s path is altered in a way that scientists can calculate and predict.
"A single Titan gravity assist could be used to jump the entire main ring system," allowing the spacecraft to skirt the danger zone and travel between the planet and its rings, he said.
After the navigation team mapped out Cassini's final orbits a full half a decade before its demise, they sent the plans to the Cassini flight controllers. Every 10 weeks, they sent out a packet of navigational commandsto the spacecraft. They didn't chart the course, but they're the ones who make sure Cassini received it.
"They hand us the reference trajectory and then we fly it," said David Doody, head of JPL's Realtime Flight Operations department. Doody and his team of seven "Aces" (which is the official name for engineers who talk to the spacecraft in real time) input the small maneuvers that put the spacecraft where it needs to be. But while they helped nudge Cassini onto the correct path, it was Titan and its vast gra that did the heavy lifting.
"Titan is our big engine," Doody said. If Cassini were traveling down the freeway, he continued, the Aces would be responsible for keeping it in the correct lane. But the massive moon exerts the most control. "Titan is our offramp," he said.
In April, death by Saturn became inevitable. That was when the gravitational effects of a Titan flyby led to the last shift in a series of changes that pointed Cassini straight at Saturn, with no possibility of escape. Even if the mission planners had somehow changed their minds, the tiny boosters made for small shifts wouldn't be powerful enough to get the satellite off the crash course Titan had set it on.
At 3:53 a.m on September 13, Cassini mission operations engineer Michael Staab uploaded the last tweak to the spacecraft. Staab was at the console two weeks before the Grand Finale to send the last packet Cassini would ever receive, the final nudge from the thrusters that would put it on a precise path toward its demise. Although the spacecraft's course was already set, this last series of commands sealed its fate.
Did he feel any regret?
"I'm a heartless engineer," he laughed, sitting at the Ace console hours before the spacecraft met its fate. Unlike many of the scientists, who refer to Cassini as 'she,' Staab reminded us that Cassini is a robot, doing what it's designed to do.
For Doody, it was not his first time sounding the death knell on a beloved satellite. In 1994, he sent the final command to NASA's Magellan spacecraft that told it to duck into the clouds of Venus. But while Magellan required a single, specific command to meet its demise, Cassini's final road required a series of incremental changes that took half a decade to reach. "This time, it's so elegant," Doody said.
As Cassini hurled into Saturn's atmosphere, Doody stood in mission control at the Jet Propulsion Laboratory in California. The telescopes of the Deep Space Network telescopes in Australia, Spain and California connect mission control with satellites plunging through the depths of space. A plaque in the floor next to the Cassini Ace console identifies mission control as "the center of the universe."
After working on the mission from the very beginning, Doody says, the conclusion feels both exhilarating and final. "This is the end of a 20-year commitment," he said. "It's been blood, sweat and tears the whole time, and now that it's over, it's like jumping off a cliff."
Staab was standing in mission control as well, working 27 hours straight and serving as the backup Ace for the Grand Finale. "I'm sad to see it gone," he said. "But I'm very proud of what we accomplished."
Buffington was also at JPL, though not inside mission control. Like Staab, he says he didn't become overly emotional about the spacecraft, instead saving his admiration for the scientists and engineers who made this mission possible.
"If there's any emotion involved, it's just thanking the people for the amazing job they did engineering and building the spacecraft before I was even old enough to write my name," he said.
Cassini met its fiery fate with the help of flight controllers, engineers and Titan, but its legacy will continue in the years to come. The information it provided about the Saturn system, including its final measurements of the planet's atmosphere, will spur more than a decade of research.
"Cassini is inspiring all of us, young and old, to keep looking up and wondering what's out there," Buffington said.