How Scientists Engineered Cassini’s Final Demise – Smithsonian

 In Science

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.






Saturn’s northern hemisphere in May 2017, observed by the Cassini mission.

(NASA/JPL)

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.

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