One of the last pieces of data captured by Cassini was an infrared image of the place into which it took its final plunge. The image, taken 15 hours before the spacecraft's demise, reveals a spot on Saturn's dark side just north of the planet's equator where the spacecraft disintegrated shortly after losing contact with Earth.
Cassini was the first probe to orbit Saturn. Built and operated at JPL, it was launched in 1997 and inserted into orbit in 2004. The spacecraft revealed the structure of Saturn's rings and, by delivering the Huygens probe to the moon Titan, executed the first landing of a spacecraft in the outer solar system. It also exposed two moons — Titan, a land of methane lakes, and Enceladus, which has jets of water streaming from its southern pole — as prime targets in the search for life beyond Earth.
After 13 years in orbit, Cassini leaves researchers with still more mysteries to ponder: They don't know the length of the Saturn day or understand the quirks of its magnetic field. And it will fall to a future mission to discover whether one of Saturn's potentially habitable moons could truly be home to alien life.
“Most of what we have in science textbooks about Saturn comes from Cassini,” JPL Director Mike Watkins said. “The discoveries are so compelling, we have to go back.”
It's precisely because of its successes that Cassini had to die. Once the spacecraft ran out of fuel, NASA would not risk letting it remain aloft, where it might be knocked into Titan or Enceladus. In April, Cassini began 22 close-in orbits that took it between and behind Saturn's rings. Earlier this week, NASA flew Cassini past Titan one last time, taking advantage of the moon's gravitational pull to slingshot the spacecraft toward Saturn.
That “goodbye kiss” set Cassini on its final, fatal course. Just after 3:30 a.m. California time on Friday, Cassini entered Saturn's atmosphere, plummeting at a pace of about 77,000 miles per hour. For a few minutes, the spacecraft's thrusters fought to keep its high-gain antenna pointed toward Earth, so it could continue to send back real-time data from this uncharted territory.
During its last moments, the spacecraft's instruments sampled the molecules in the planet's atmosphere — information that scientists will use to understand the planet's formation and composition. It also collected data that researchers hope will help them solve the mystery of the speed of Saturn's rotation.
NASA was able to maintain a link with the spacecraft 30 seconds longer than the team anticipated.
“Those last few seconds were our first taste of the atmosphere of Saturn,” Watkins said. “Who knows how many PhD theses are in that data?”
Minutes later Cassini vaporized, just a small flash of light streaking across an alien sky. But because Saturn is so distant, Cassini's final signals didn't reach Earth until 83 minutes after the spacecraft was gone.
That last communication was displayed as a green spike of data on a screen above mission control. The spike shrank, then flickered, then flatlined.
“We call loss of signal,” said spacecraft operations manager Julie Webster at 4:55 a.m. local time.
There was utter silence at mission control. And then Maize spoke: “I'm going to call this the end of mission. Project manager, off the net.”
The room burst into applause. Maize immediately stood, strode over to Webster, and gave her a hug.
“I'm almost without words,” Webster said at a news conference an hour later. She has worked on the Cassini mission for more than two decades and is among the few people at NASA who ventured inside the craft as it was being built.
“A perfect spacecraft,” she said, her voice almost cracking with emotion. “It did exactly what it was supposed to do.”
“Even better,” chimed in Project Scientist Linda Spilker.
“Even better,” Webster agreed. “Exactly as it always did.”
In a JPL auditorium minutes after the end, science planner Jo Pitesky gazed up at the video from mission control with a slightly stricken look on her face.
“She's us,” said Pitesky, who has worked on Cassini's operations team since 2001. “We can't go there ourselves, so we build a spacecraft and load it up with instruments, and then we put on our hopes and desires and we send them there.”
Thanks to its scientific successes, stunning images, and the sad circumstances of its demise, Cassini is viewed with deep affection by NASA researchers and space enthusiasts alike. Some of the features in Saturn's rings are named for team members' pets; one of the engineers named his daughter Phoebe for one of Saturn's moons.
Many members of the Cassini team refer to the spacecraft as a “she” and they ascribe “her” human traits: curiosity, intelligence, determination, valor.
“It's like the loss of a friend,” said Spilker, who has worked on the mission since its inception in the late 1980s.
The start of the grand finale in April set off a months-long period of protracted public mourning for the spacecraft. The nonprofit Planetary Society filmed a short operatic tribute to the mission. Fans on Twitter posted silly cartoons and tearful eulogies. Maize told the story of a 6-year-old boy from Florida who sent a letter to JPL inviting staff to his end-of-mission party.
“It's very heartwarming,” he said. “It's not science in the ivory tower. It's for humanity.”
The Cassini Virtual Singers — a group of JPL employees who perform Cassini-themed parodies of popular music — rewrote the lyrics to “Seasons of Love,” a ballad from the musical “Rent.”
“The truths that we learned, and the things that we tried,” they crooned at a meeting of the Project Science Group this week. “The fuel that we burned. And the way that she'll die.”
Trina Ray, a senior science systems engineer for Cassini and founding member of the singing group, handed out handkerchiefs to her colleagues so they could mop up their inevitable tears.
But the mood was all business at mission control early Friday. Conversations about the spacecraft's status were conducted in the same serious tones the flight team has always used. The only difference was a clock displayed above one of the room's main monitors, counting down the minutes until the signal from the spacecraft was lost.
So many current and former Cassini team members have flocked to Pasadena for the end of the mission there wasn't room for them at JPL. Instead, a viewing party was arranged on the campus of nearby Caltech.
In the predawn dimness, hundreds of bleary-eyed scientists gathered to watch the live stream from mission control. Three jumbotrons had been set up on a lawn outside the auditorium; they played a slickly produced NASA video showing some of Cassini's greatest images. The glow of the screens and the soundtrack's dramatic drumbeat made the proceedings even more intense.
Sean Hsu, a researcher at the University of Colorado at Boulder who works on Cassini's Cosmic Dust Analyzer, flew out with his wife and two children to attend. When he explained to 5-year-old Liese why they were waking up so early to celebrate a spacecraft, the little girl started to cry.
Hsu feels mournful, too.
“It has been a tremendous mission to be a part of,” he said. “It has been a lot of new science, a lot of new data, and suddenly there will be no more data.”
With the loss of Cassini, the space around Saturn has gone dark. There are no missions in progress to return to the ringed planet.
But Cassini's revelations at Titan and Enceladus inspired NASA last year to add the moons to its call-out for proposals for the New Frontiers program — a group of medium-size missions that includes the New Horizons flyby of Pluto and the Juno orbiter around Jupiter.
Spilker is co-investigator on a New Frontiers proposal to study Enceladus, a tiny body that harbors a subsurface ocean and boasts jets of water spouting from cracks in its icy surface. She called Cassini's revelations about this moon “one of the most astonishing discoveries for planetary science … that has really changed our thinking about where to look for life.”
Spilker would like to return to Saturn and sample the Enceladus plumes for large organic molecules that could be signs of biological activity. Others have proposed similar missions to test for “biosignatures” in Titan's atmosphere.
If and when a spacecraft is sent back to Saturn, it will arrive at a place ever so slightly touched by humans. Because NASA chose to end Cassini's life by plunging it into the planet, “its bits and pieces are now one with Saturn itself,” Spilker said. “So when I look up at Saturn in the future, I'll know … Cassini is there too.”