Before dawn on Friday, scientists will gather at Pasadena’s Jet Propulsion Laboratory to witness the death of an old friend: Cassini, a spacecraft that has awed and inspired generations as it spun around Saturn.
Devoted to the end, Cassini will send them a strong and steady pulse of radio waves, like a healthy heartbeat on a monitor, onto a large auditorium screen.
Then, at 4:55 a.m. Pacific time, it will abruptly flatline. And there will be only silence.
“It will be a difficult moment – not only for me, but for other scientists who have spent their professional life with it,” said San Jose State electrical engineering Professor Essam Marouf, a member of the Cassini Radio Science Team for more than two decades.
“Everything comes to an end, one way or another. I am at peace with the fact that the mission has to end – and it has to end for good reasons,” he said. “But I feel very emotional.”
The spacecraft – the size of a 30-passenger school bus – has provided scientists with discoveries beyond their wildest imagination, illustrating the very best of NASA’s scientific and technological prowess.
Now, nearly 20 years after its launch and 13 years in Saturn’s orbit, Cassini has reached the point of no return. It’s running out of fuel, as NASA always knew it would.
NASA scientists decided that rather than having Cassini die in a crash – risking contamination of Saturn’s moons by Earth’s microbes – it will burst into flames.
Burning and breaking apart, the billion-dollar spacecraft will first lose its antennas, then perish as it is pulled toward the ringed planet, killed by the very conditions it studied.
Since ancient times, we have pondered the mystery of Saturn as we gazed up into the vast canopy of the night sky. The planet was named after a Roman god of harvest, portrayed as a strong old man with a beard and scythe, because its wanderings followed the passing of time and seasons.
Cassini – named after Giovanni Domenico Cassini, a 17th century Italian astronomer who discovered four of Saturn’s moons – changed all that.
For the first time, we saw churning gas storms at Saturn’s poles. We discovered that one of its 60 frigid moons, Enceladus, has many of the right ingredients for life. We saw that another moon, Titan, has seas, rivers and clouds filled with methane gas.
We took photos of its long-mysterious rings, hundreds of thousands of kilometers wide but only 10 meters thick – “like a piece of paper the size of Golden Gate Park,” said Jeff Cuzzi, research scientist at NASA’s Ames Research Center in Silicon Valley.
“Cassini has shown us that the Saturn system is very dynamic – evolving before our eyes, with things colliding and smashing into each other,” said Cuzzi. Now 71, he was 31 when he helped design Titan’s first entry probe.
“Features change, objects fly in from outer space and hit the rings,” he said. “We see storms coming and going. On Titan, we see clouds form and dissipate. We see lakes evaporate and get deeper again. That’s what Cassini has brought to us.”
San Jose State’s Marouf first started studying Saturn when he was only 24 and a Ph.D. student at Stanford. His work focused on Voyager 1, the second spacecraft to fly by Saturn, after Pioneer 11. Then he was offered a role on the Cassini mission.
While Voyager just flew by the planet, Cassini was far more ambitious.
The Cassini tour was a more in-depth mission – “designed to execute different orbits of different natures to try to understand Saturn, its atmosphere, its rings and its environment, as well as major satellites like Titan, Enceladus and others,” Marouf said. “It had the luxury of finding things and then going again to learn more.”
Now 73 years old, he recently took his two grandchildren to visit the Pasadena lab.
Stanford’s Howard Zebker was 33 years old when he joined the mission at JPL. Now 63, he is a professor of geophysics and electrical engineering and uses radar to understand the surface of Titan – a complex, challenging and cold place where temperatures average 300 degrees below zero Fahrenheit.
“We knew very little about Titan in those days – it was covered by a dense and opaque atmosphere,” he said. “But our radar can penetrate clouds just as well on Titan as on Earth, and we used this data to make images of the surface of Titan.
“We’ve seen all kinds of great things: a landscape of mountains, rivers, lakes and sand dunes. There is a lot of liquid methane that behaves the same way water does, in a cycle like ours – evaporating, clouding over land and raining.”
Zebker has already paid his respects to the intrepid spacecraft. Earlier this week, as Cassini made its final approach toward Titan, his team interrupted an 8 a.m. meeting to pour champagne and offer a toast. They toasted again a few hours later, when Cassini made its final command to radar instrumentation.
“That was sad,” he said.
Even after its death, Cassini will continue to contribute, the scientists said. It could take 10 to 20 years to analyze all the data that it has sent.
“We all feel a great satisfaction with a job very well done,” Cuzzi said.
“But in a way, we’re just getting started,” he said. “We have decades worth of data to analyze, model and interpret – and, ultimately, we hope to understand. “