In September, the Cassini spacecraft ended 20 years of space exploration with a fatal dive into the atmosphere of Saturn. Mental Floss contributor David W. Brown was there at NASA Jet Propulsion Laboratory (JPL), in Pasadena, California, to give us a behind-the-scenes look at Cassini's final hours—and delve into the history of the spectacularly successful mission, which provided virtually everything we know about Saturn, its moons, and the potential for life in the planetary system. You probably saw the headlines. Here's the inside look—and the big picture.
Hours before Cassini plunged into Saturn on September 15, Dave Doody, a senior engineer at NASA’s Jet Propulsion Laboratory in California, sat at the Ace console in mission control and eyed a computer screen lined with codes. A few were highlighted in orange, but all were indecipherable for the uninitiated. The room was dimly lit in blue and red, with bright projections overhead of Deep Space Network transmission operations and spacecraft status.
Cassini’s final downlink was in process, sent from the Saturn system, some 750 million miles away, to the Goldstone 70-meter antenna an hour and a half northeast of JPL. Engineers were watching closely as telemetry data arrived. “There are 13,000 different measurements. Temperature, pressure, computer state, voltages, and so on,” Doody explained. The desk around him—the closest thing to a cockpit for a robotic mission—was littered with binders, computers, coffee cups, Post-its, pencils, and paperwork. A sign hung on the front of it: NOTICE: DO NOT FEED THE ACE. (Scrawled below: “to the wolves.“) “The programs we are running here are sorting through them all and letting us know with an orange blob if anything is out of spec.”
Returns from the science payload were, as usual, transmitted to the various responsible science instrument teams at universities and research institutions around the world. But the next day, it would all go dark: The team had preprogrammed the spacecraft, which was nearly out of fuel, for a suicide mission into Saturn’s atmosphere. Cassini had less than six hours to live.
As Doody studied the data, another controller reached over and pointed out what might be an anomaly. Doody pulled out his cell phone. Would it have to be called in to the other engineers? The two men discussed it, and teased some deeper obscure truth from the subsequent statistics. Doody pocketed his phone. No cause for alarm today.
I asked Doody what would happen to the spacecraft tomorrow as it absorbed the punishment of a world comprised entirely of heat and pressure. Would those 13,000 measurements freak out one by one, or all at once? I imagined Doody’s face glowing orange with proliferating blobs as Cassini’s systems began to fail.
To my surprise, he explained that it would be a quiet day at the office. The most prominent indicator of Cassini’s impending demise would probably be its overactive thrusters. “We’ll see an excessive thruster alarm because the spacecraft is firing its thrusters to try to keep on point. It’s going to be overcome by the forces of Saturn’s atmosphere, and when it tumbles, we will lose all data.”
He furrowed his brow, thought for a moment, and then proposed a second possible indicator. “If we could stay pointing to Earth and sending data back, we would see temperatures go out of whack, because as we got lower and lower and lower, it would heat up too much,” he said. “But it will definitely lose signal first.”
At the speed of light, it takes 83 minutes for a Saturn-borne signal to arrive at Earth. So by the time JPL stopped receiving a signal from Cassini, the spacecraft would have long been destroyed, a blade of light coursing through the planet’s northern sky.
Almost everything we know about Saturn and its moons comes from the Cassini mission, which launched in 1997 and arrived at Saturn seven years later. To date, it has led to more than 4000 published scientific papers and the discovery of six moons. Among other findings, Cassini allowed us to discover that Saturn's E-ring is created by ice and debris blasted from the plumes of its moon Enceladus. "Propellers" and "peaks" have been discovered or explained in the rings, shedding new light on the formation of planetary bodies and even galaxies. The study of Saturn's magnetosphere is helping scientists tease out the length of a Saturnian day (a stubborn mystery yet to be solved) and the thornier question yet of its composition and internal arrangement.
Less than six months after the spacecraft arrived at the Saturn system, it sent a probe called Huygens to the surface of Titan, the largest of Saturn's moons. Titan is the only known moon in the solar system to possess a dense atmosphere, which has, since the invention of the telescope, denied astronomers a view of its surface. Huygens would solve that problem, and once Titan's veil was pierced, what it discovered threatened to upend all of NASA's plans for outer planets exploration.
That's because Huygens revealed unambiguous evidence of liquid activity on Titan's surface. Even mighty Europa, long the darling of the outer planets community for its subsurface ocean, for a brief time seemed staid and uninteresting. Something flowed on Titan! Cassini would only build on this discovery, detecting massive, stable lakes and seas on Saturn's largest moon, making it the only non-Earth body in the solar system to possess such liquid bodies. It's also the only known moon with a weather system.
Titan's lakes are not filled with water, but liquid methane. If the surface harbors life—and it is certainly thought to be habitable—it will be methane-based, and thus alien in the truest sense of the word for us carbon-based lifeforms on Earth. As if its weather system and seas weren't amazing enough, beneath its surface, Titan likely possesses a liquid water ocean.
Cassini, meanwhile, shocked scientists with its discovery of massive plumes of water blasting from another Saturnian moon, Enceladus, which originate from a global, subsurface saltwater ocean that possesses organic compounds and possibly hydrothermal vents on its seafloor. If you're keeping score, those are all the ingredients necessary for life. Enceladus even helps scientists along with a nice interface between space and the ocean by way of a boiling, slushy surface at the plumes' points of origin.
"We thought this tiny moon would be frozen solid and inactive," said Linda Spilker, the project scientist of the Cassini mission. "And what a surprise to find not only geysers of water vapor and water particles coming out, but to find organics, a salty global ocean underneath the icy crust, and even the possibility of hydrothermal vents: the conditions that could be right for life."
Cassini, in other words, expanded the habitable zone of our star. Once it was thought to extend only to Earth, and then to Mars, then to the Jovian system (by way of Europa). Now the zone extends as far as Saturn—10 times the distance of the Earth to the Sun. Planetary scientists already have plans to revisit both Titan and Enceladus, and as early as next year, NASA might select a mission for development and eventual launch. One proposal is to send a submarine to explore Titan's methane ocean.
Cassini is the culmination of American space science and engineering, the heir of the Voyagers, Magellan, Galileo, and Mars Observer. It cost $3.9 billion to build and represents one of the best investments the U.S. government has ever made.
"We’ve had 13 years at Saturn, but 20 years of an incredible spacecraft that was designed by people that had 30 years of experience when they designed it," said Julie Webster, the Cassini spacecraft operations team manager at JPL. "And they built a perfect spacecraft." She was part of the mission when Cassini was only schematics, parts, and dreams. During assembly, she even sat inside it. She was the engineer responsible for the health and safety of the spacecraft throughout the mission, and monitored its health from the spaceflight operations center.
The spacecraft performed almost impossibly well for 20 years, lacking even the slightest hint of a close-call, a mechanical flaw, or a design limitation to be overcome mid-mission. Only toward the end, when engineers and scientists set Cassini on a course of daring dives between the rings and Saturn itself, was there a moment of worry. Would some unknown space debris smash the spacecraft? But Cassini emerged unscathed from the region—which proved remarkably hospitable to a visiting vessel—and spent 13 years in the Saturnian system zipping around the planet as though it were the latest model designed for ring dives.
But Cassini did find worlds habitable by alien life forms, and if the spacecraft, left derelict, were to somehow impact one of those worlds, there could be terrible repercussions. If humans were to contaminate Enceladus with microbes from Earth, and then with some later spacecraft find life on the moon, we wouldn't know whether the life we found was life we brought there. Or worse yet: Imagine if we were to find life that emerged independently on Enceladus only to kill it with life from Earth. (The much publicized planetary protection officer position at NASA is not so much focused on protecting Earth from aliens as it is focused on protecting aliens from Earth.)
There were a few other options. It could have been sent to Uranus, though would have taken decades to arrive and would have been able to perform only limited science. Or it could have been sent to Neptune, but it would have taken twice as long and yielded an equally paltry science return. Neither ending really befitted Cassini, the greatest science laboratory ever built by human hands.
Earl Maize, the Cassini project manager at JPL, called it "a superb machine in an amazing place doing everything we could possibly do to reveal the mysteries and secrets of our solar system."
There was no more dignified ending to the spacecraft, then, than to send it into the world that it gave to humanity, doing the sort of frontier science that it had done for so long. The spacecraft's brief time in Saturn's atmosphere during its descent was spent directly sampling its hydrogen-to-helium ratio, which will help eventually determine the planet's age and enlarge our knowledge of the history of the solar system. Such sampling has never been done before. In addition, its efforts will help scientists work out the effects of a phenomenon called "ring rain," in which particles from the rings fall into the atmosphere.
Cassini's last weeks were characterized by many "lasts": the last flyby of Titan, the last return of Titan data, the last picture of Enceladus setting on Saturn, the last image of the rings, the last view of Saturn itself. It will be a very long time before we get new photos from the system, and every shot counts. Cassini's data will last forever, and will fuel discoveries for decades if not centuries. Consider only the camera data in comparison to previous missions: The spacecraft Galileo returned around 750 images in its entire dataset. Cassini returned that many images every orbit.
Cassini was the farthest spacecraft to orbit another world, and Huygens was the first probe to land somewhere beyond the asteroid belt. (Unlike Cassini, it was designed with planetary surface protection in mind, which is why it could land safely on Titan.) The Cassini-Huygens mission has somehow touched every member of the field of planetary science, even if only as a point of pride. What does a planetary scientist do? Study the formation, history, and geologic, chemical, and possibly biologic activities of bodies in space, and determine what science is necessary to better understand these mysterious worlds and the solar system in which they reside. They work with engineers to design spacecraft and scientific instruments to embark on missions of exploration, and to use the data returned by these spacecraft to refine an increasingly complex mosaic of celestial knowledge. Those things everybody now knows about Saturn—its weather, seasons, weird hexagonal poles, and mysterious moons—we didn't know until we had Cassini, a spaceborne laboratory with a full suite of instruments gathering data. That beautiful photo of Saturn on your computer desktop came from Cassini.
Sitting astride the San Gabriel mountains, by day JPL looks like a college campus (and formally it's part of Caltech). But at 3:31 a.m. on Friday, September 15, in a darkness free of stray light that might pollute the evening sky, it seemed like one part military facility, one part scene of some unspeakable crime. Security guards were posted across the facility keeping VIPs—scientists from prestigious institutions, members of Congress, and family members of the Cassini team—from wandering where they didn't belong. A Starbucks kiosk on campus was open late into the night for the events; everyone needed the caffeine. Scientists sat obscured by darkness at tables on the campus mall. TV camera vans lined the curbs.
Things were quiet in the control room. Cassini was still fine, its signal still going strong. Two hours earlier, engineers in the Mission Operations Center at JPL had switched the Cassini spacecraft to a "bent-pipe" mode. Science from Saturn would pass through the spacecraft and be sent directly to Earth, as though, as the name implies, it were a bent pipe. In other words, no longer would data collected by its instruments be stored to a hard drive on Cassini for later transmission to Earth, as had been done for just under 20 years. Instead, Cassini rotated until its ion and neutral mass spectrometer (INMS) was pointed at Saturn, and its high-gain antenna pointed at Earth. Every byte of collected data was now sent straight back to Earth for analysis. There was no longer any need for a hard drive because for Cassini, there soon would no longer be any "later."
A crowd slowly gathered over the next hour. There were hugs everywhere, the way family members who've not spoken for ages embrace at a funeral or a graduation. For the grand finale, the mission team wore purple polo shirts embroidered with Cassini-Huygens. As the minutes elapsed, the shirts proliferated on bodies throughout the JPL. And yet they represented a small fraction of the total Cassini team. It was an enormous mission. Thousands worked on it—so many that JPL could not accommodate those who gathered to celebrate the spacecraft's life. Most of the 1500 scientists who had a hand in Cassini's successes in one way or another gathered instead at the nearby Caltech campus, where a NASA broadcast from mission control was projected on massive outdoor screens.
As the Cassini spacecraft sped toward Saturn at 75,000 miles per hour, it was nearly running on fumes, down to 1 percent fuel, though that measurement had a margin of error of "plus or minus 2 percent," explained Todd Barber, the propulsion lead engineer. According to the plan, once it entered Saturn's atmosphere, it would transmit data about the composition of the atmosphere for about one minute before being silenced forever.
There were no commands left for mission control to send it. So instead, the team watched. "We’re taking in-situ instrument data, sensing the magnetic field, mass spectrometers, feeling out the atmosphere and what its constituents are, and sending it right to Earth," Doody said from the Ace. The team checked the spacecraft's subsystems. In keeping with Cassini's peerless record of reliability over the last two decades, every system reported nominal.
As the spacecraft entered Saturn, the spacecraft's altitude controls grew more active. Coming in a little easterly of the North Pole, it plunged into Saturn's atmosphere at 10-degrees latitude. The atmosphere was thin but swift—about the same density as experienced by the International Space Station at Earth. Cassini’s thrusters began firing furiously in all directions in a desperate bid to maintain stability, just as Doody had anticipated they would. They were not built to fight an atmosphere, and certainly not one with Saturn's torque and drag.
And as predicted, the spacecraft hung in there for about a minute, gathering data and passing it back immediately to the Deep Space Network for eventual analysis.
In the JPL Von Karman Auditorium, scientists, engineers, and the media cheered and bemoaned every spike and wobble of the signal from Cassini. It would take weeks for the science data to yield preliminary results, and there were no more images to come from the spacecraft. (The camera instrument was disabled for the finale; there was no time to transmit images.) All that mattered at that moment was the signal.
Over the course of the mission, Cassini traveled a total of 4.9 billion miles and performed 294 orbits of Saturn. Even as it plunged into Saturn, where it would be vaporized from extreme heat and pressure, its internal electronics ran at room temperature. It tumbled and twirled, breaking its line of sight with Mother Earth, ending communications.
Cassini was gone.
"Project manager, flight director," Webster said over her headset. "We call loss of signal at one one five five four six."
Maize, the project manager, who was seated next to her, then followed protocol: He called the end of mission and signed off the communications network.
There was at first a stunned silence. All around, the teams' eyes were bleary and teary. Their faces showed fatigue and sadness.
But there was also pride and acceptance. The most surprising reaction that day was the applause. It came from every corner, and it came from somewhere deep within the team. Cassini's life—to anthropomorphize it—was a life well-lived, one of meaning and purpose. Why not celebrate it?
In the end, the spacecraft survived 30 seconds longer than was predicted. Every fraction of a second was profoundly valuable from a scientific standpoint. "Who knows how many Ph.D. theses might be in those final seconds of data?" wondered Spilker, the leader of the Cassini mission. She had worked on Cassini for 30 years, or—as she described it—one Saturn orbit.
After Cassini became a shooting star in its northern sky, scientists and engineers from JPL and NASA reflected on the mission and the spacecraft that carried it out. "I've been so busy, I haven't had a chance to deal with the emotional stuff," Barber, who had been Cassini's propulsion lead engineer for 20 years, told me. "Next week is going to be hell."
For Webster, Cassini had been a constant of life for decades. Because celestial mechanics do not recognize U.S. holidays, the spacecraft was part of her Christmases and New Years, Easters and Thanksgivings. "I no longer have a spacecraft that will keep me up at night," she said, her voice slightly trembling. "And in a few days, I think I'm going to really miss that."