It’s a story of awe, love, and sacrifice, and its chief actor was a 22-foot-tall computer. On September 15th, NASA’s Cassini probe deliberately dove towards Saturn, burning up in the atmosphere. As the probe hurtled towards the planet with the eyes of the world on it, it transmitted some of the most remarkable photos of its tenure.
After almost 20 years aloft, Cassini was running low on propulsion fuel, and the project’s scientists decided to pilot the probe into Saturn. Technically, Cassini’s mission didn’t have to end; even though it was out of fuel, it could have continued sending back data from its orbit around Saturn. But one of its most remarkable findings had been that there was liquid water with organic components on Saturn’s moon Enceladus.
The longer Cassini orbited without propulsion fuel, the more it risked colliding with the moon, potentially contaminating it with microbes from earth. In essence, Cassini sacrificed itself for NASA’s “Prime Directive,” the agency’s dedication not to contaminate potential life on other planets.
As Eric Maize, Cassini’s project engineer put it, “Cassini’s own discoveries were its demise.”
The probe had been navigating its way through the Saturn system since 2004, sending us back invaluable data about Saturn, its rings, and its moons.
Some of the scientists on the Cassini project have been working on it since 1988. Dr. Spilker, the mission’s project scientist, says, “When I first started working on Cassini…my oldest daughter Jennifer had just started kindergarten. And now, here we are in 2017, she’s married and she has a daughter of her own.”
Cassini’s data was well worth the wait. In its 13 years circling the Saturn system, it made some very important discoveries that “re[wrote] the textbooks of Saturn.”
From observing the plumes of water that shoot up from the icy seas of Enceladus, Cassini helped us determine that the moon has the chemicals most essential in developing Earth-like life — carbon, nitrogen, oxygen, and hydrogen.
Cassini’s data also revealed that the planet’s rings aren’t static; the rocky, icy debris creates and destroys little “moonlets” fairly frequently.
Cassini also showed us the dynamic, lovely surface of Titan. Earlier seen as just a blur, we now know that Jupiter’s largest moon boasts seasons, weather, and persistent lakes and seas.
But all good things must come to an end. And, as Cassini’s fuel ran out, the little probe had one last chance to make history.
In the months before its mission ended, Cassini dove in and out of the area between Saturn and its rings, completing 22 orbits.
Since scientists didn’t know exactly what Cassini would encounter in the ring gaps, these missions were too risky to perform before they knew Cassini was slated to be destroyed.
These difficult missions allowed NASA to collect data on the age and mass of the rings. They also informed us that there was very little space-debris in the inner ring space, a new piece of information about Saturn’s atmosphere.
When these missions ended, Cassini prepared to self-immolate. One of the last photos Cassini ever took was Enceladus setting behind Saturn, the probe seeing the site of its greatest discovery for the last time. Earl Maize, the mission’s project manager, called it a “kiss goodbye.”
On September 15th, Cassini began its descent into Saturn’s upper atmosphere. Right before the little probe prepared to dive towards the planet, it pointed its transmitter at Earth, allowing it to send back a few more minutes of data.
At that point, the probe stopped taking pictures; the atmospheric data it could pick up was far too valuable to waste any of its capacity on photography. The very last picture Cassini took was a blurry shot of Saturn’s “night side.”
Plummeting towards the planet at around 120,000 km/h, Cassini’s components peeled away, jolted apart by atmospheric friction. Brett Pugh, a NASA JPL thermal engineer, says it’s likely that Cassini’s fuel tanks exploded in the heat. If anyone had been on Saturn to see Cassini’s demise, it might have looked like a particularly lovely shooting star.
Cassini kept transmitting to the end; it managed to keep sending back data for half a minute longer than anticipated. But at 9:55 PM on September 15th, its radio signal vanished, and Cassini’s long mission was over.
As Cassini prepared to dive into the gas giant it explored, the general mood was wonder and grief. Even though Cassini was essentially a 4-billion-dollar weather balloon, people had become attached to the probe and mourned its “death” at the same time as they celebrated its accomplishments.
Tributes to the little probe poured in over the days after its death, from a six-year-old hosting a “farewell party” to an adorable comic of Cassini’s demise framed as a triumph.
Even the scientists working on the project weren’t immune to grief over Cassini’s “death”. “It’s like in the death of a loved one—you look back and you think about all the good memories, the times you’ve shared together, went on vacation together, grandchildren,” says Dr. Spilker. “I think of it more like planning perhaps a wake.”
Perhaps the loveliest, most remarkable findings from Cassini’s “death” are what it revealed about human beings.