R.I.P. Cassini

PASADENA, Calif. — Cassini went down fighting.

After 20 years in space and 13 years orbiting Saturn, the veteran spacecraft spent its last 90 seconds or so firing its thrusters as hard as it could to keep sending Saturnian secrets back to Earth for as long as possible.

The spacecraft entered Saturn’s atmosphere at about 3:31 a.m. PDT on September 15 and immediately began running through all of its stabilizing procedures to try to keep itself upright. The signal that Cassini had reached its destination arrived at Earth at 4:54 a.m., and cut out about a minute later as the spacecraft lost its battle with Saturn’s atmosphere.

“The signal from the spacecraft is gone, and within the next 45 seconds, so will be the spacecraft,” Cassini project manager Earl Maize announced from the mission control center at NASA’s Jet Propulsion Lab. “I hope you’re all as deeply proud of this amazing accomplishment. Congratulations to you all. This has been an incredible mission, an incredible spacecraft, and you’re all an incredible team. I’m going to call this the end of mission. Project manager, off the net.”

With that, the mission control team erupted in applause, hugs and some tears.

It’s the end of an era. But the spacecraft’s last moments at Saturn will answer questions that couldn’t have been addressed any other way.

Going out in a blaze of glory seems fitting. Since its launch in 1997, the probe traveled a total of 7.9 billion kilometers. It gathered more than 635 gigabytes of science data and took more than 450,000 images. It completed 294 orbits of Saturn, discovered six named moons and made 162 close, deliberate flybys of the ringed planet’s largest and most interesting moons.

The last flyby sealed Cassini’s fate. On September 11, at 12:04 p.m., Cassini passed by Saturn’s largest moon Titan one last time (SN Online: 9/11/17). The moon’s gravity nudged Cassini on an irretrievable trajectory into the giant planet’s atmosphere.

Also blame the moons — particularly lake-dappled Titan and watery Enceladus — for why Cassini went out in such dramatic fashion. The mission team decided to sacrifice the spacecraft when it ran out of fuel, rather than risk a collision with one of those potentially habitable moons and contaminating it with any still-lingering earthly microbes.

“Because of planetary protection and our desire to go back to Enceladus, go back to Titan, go back to the Saturn system, we must protect those bodies for future exploration,” Jim Green, director of NASA’s planetary science division, said at a news conference on September 13.

Even in its months-long death spiral, Cassini collected unprecedented observations. Starting in April, the spacecraft made 22 dives through the unexplored region between Saturn and its rings. Measurements of the gravity and composition in that zone will help solve outstanding mysteries. How long is Saturn’s day? How much material is in the rings? When and how did the rings form?

To answer that last question in particular, “you have to fly between the planet and the rings,” says planetary scientist Matthew Hedman of the University of Idaho in Moscow, who uses Cassini data to study the rings. “That’s risky. We had to wait until the end of the mission to take that risk.”

On September 13 and 14, Cassini took a last look around the Saturn system’s greatest hits, taking a color mosaic image of Saturn and the rings, a movie sequence of Enceladus setting behind Saturn, Titan and tiny moonlets in the rings that pull the icy ring particles around themselves to form features called propellers.

Inside the mission control center on the afternoon of September 14, a hushed operations team waited for Cassini to come online for the last time to start sending the last pictures back (SNOnline: 9/15/17).  Then flight engineer Michael Staab at JPL suddenly broke the silence.  “Yeah!” he yelled, pumping both arms in the air. Cassini’s last signal had just come in.

“That tells us that the spacecraft is nice and healthy, she’s doing just fine. She’s doing exactly what she’s supposed to do, like she’s done for 13 years,” Staab said. “We’re just gonna track her now, all the way in.”

In the wee hours of September 15, the spacecraft reconfigured itself to shift from a recording device to a transmitting probe. As of that moment, its last and only job was to stream everything it could sense directly back to Earth in real time. Turning so that its ion neutral mass spectrometer was facing directly towards Saturn, Cassini could taste the atmosphere for the first time and investigate a phenomenon called “ring rain,” in which water and ice from the rings splash into the atmosphere. This idea was introduced in the early 1980s, but Cassini has already shown that it’s more complicated than previously thought.

“We’re trying to find out exactly what is coming from the rings and what is due to the atmosphere,” Hunter Waite, Cassini team lead for the mass spectrometer instrument and an atmospheric scientist at the Southwest Research Institute in San Antonio, said at the Sept. 13 news conference. “That final plunge will allow us to do that.”  

That plunge happened at about 3:31 a.m., when Cassini entered the atmosphere about 10 degrees north of the equator, falling at around 34 kilometers per second. It took data constantly, directly measuring the temperature, magnetic field, plasma density and composition of the upper layers of Saturn’s atmosphere for the first time ever.

When it hit the atmosphere, Cassini started firing its thrusters to keep its antenna pointed at Earth despite the forces of the atmosphere trying to knock it askew. But about a minute later, the atmosphere won, when Cassini was about 1,400 kilometers above the cloud tops.

What happened next, scientists can only imagine. Models suggest this fiery demise: The spacecraft attempted to stabilize itself, but to no avail. It started to tumble faster and faster. Atmospheric friction broke the spacecraft apart, bit by bit — first its thermal blankets burned off, then aluminum components started to melt. The spacecraft probably fell another 1,000 kilometers as it disintegrated like a meteor, Maize said.

Saturn’s atmosphere crushed and melted the bits and pieces, until they completely dissociated and became part of the very planet the spacecraft had been sent to observe. 

When all was said and done, the spacecraft lasted about 30 seconds longer than expected. That may help ensure the team got enough data to figure out Saturn’s rotation period, science team member Michele Dougherty of Imperial College London  said at a post-mission news conference September 15. “I’m hoping we can do it, I’m not going to promise. Ask me in three months’ time.”

There are no planned future missions to Saturn, although some Cassini alumni are already working on proposals.

Outer solar system astronomers are now setting their sights on Jupiter and its icy, possibly life-friendly moons. The European Space Agency’s Jupiter Icy Moons Explorer (JUICE) and NASA’s Europa Clipper both hope to launch around 2022. Those missions may pave the way for a lander on Europa (SN Online: 2/18/17), which could directly look for life in that moon’s subsurface seas.

Planetary scientist Kevin Hand at JPL, who is leading the science definition team for the proposed Europa lander, feels a debt to Cassini.

“When you’re at the earliest frontiers of exploration, it’s hard to feel sad,” he said. “The wake we’re experiencing right now for Cassini, it’s not so much an end but the early steps that pave the way for the next stage of exploration.”

Even Maize is more proud than mourning.

“This is exactly the way we always planned it. It’s sad that we’re losing this incredible discovery machine,” he said in the moments leading up to Cassini’s disintegration. “But the real sense here is just, all right, we got it!”

Editor's note: This story was updated at 11:05 a.m. EDT on September 15 to incorporate updated information from a news conference.