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Why NASA is crashing the $4 billion Cassini spacecraft into Saturn

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On Friday, September 15, around 6:30 am EDT, NASA will watch its 20-year-old, $4 billion-plus spacecraft to crash into Saturn.

The space agency really has no other choice. Cassini is nearly out of fuel, and has already been stretched years beyond its intended mission duration. What’s more, keeping it going risks potentially contaminating one of Saturn’s moons — like Enceladus, an ice world that has some ingredients for life, or Titan, a dynamic moon where it rains methane — with microbes from Earth.

And so the spacecraft will end its existence by literally going where no human-made object has gone before: into Saturn’s atmosphere.

But up through its very last moments, Cassini will be conducting a scientific investigation. As it descends into Saturn’s atmosphere, several of its instruments will be on, including the mass spectrometer, which can essentially “sniff” the atmosphere and determine the chemical compounds therein.

Cassini has made discoveries that have changed our understanding of Saturn and the cosmos at large. The spacecraft discovered whole new moons around Saturn, lakes of methane on Titan, and jets of water erupting from Enceladus, and has made extremely detailed observations of the planet’s rings, an environment believed to be similar to the rings of debris that formed the entire solar system. But ending is bittersweet: Scientists have dedicated decades of work to the mission and the study of Saturn, and Cassini ends its run with some key Saturn mysteries still unsolved.

Cassini’s final days will be its most dramatic. Here’s what will happen.

The crash will be one last, spectacular moment in Cassini’s “Grand Finale”

Cassini looks at Saturn’s north pole.
NASA/JPL-Caltech/Space Science Institute

For the past several months, Cassini has made 22 orbits in and out of the region between Saturn and its rings, a place where no spacecraft has gone before. There, Cassini made the careful measurements needed to assess the mass of the rings and ultimately determine their age (preliminary analysis suggests they’re younger than expected).

These harrowing inner-ring passes were saved for the very end of the mission, because NASA’s scientists didn’t know if there would be debris in this space that could have destroyed the craft. In fact, Cassini didn’t encounter much dust or debris in this space at all, which in itself is a new discovery about the Saturn system.

On September 11, a final pass-by of the moon Titan put Cassini on a collision course with Saturn (check it out in the animation below). Earl Maize, the project manager of the mission, called this pass “a kiss goodbye.” And it is goodbye. There is no way to stop the spacecraft from crashing now.

The mission’s scientists — many of whom have worked on Cassini since its inception in the 1980s — are overflowing with sentiment for the SUV-size craft.

“Our families have gotten to know each other, in some cases our children have grown up together, and now in the final two weeks we’re sharing the end of this incredible mission,” Linda Spilker, a Cassini project scientist, says. Cassini has spent 13 years orbiting Saturn. It has been in space for 20.

How Cassini will melt and explode in its descent into Saturn

There are some last orders of business for Cassini before it meets its demise. Its cameras are taking some final images of the moons Titan and Enceladus as well as a tiny moonlet in the rings that may soon leave the rings, and some final color images of Saturn itself. “These final images are like taking a final look at your house or apartment just before you move out,” Spilker says. “Memories across the years come flooding back.”

Starting on September 14, Cassini will turn back to Earth, secure a good radio connection, and reconfigure itself to become a makeshift atmospheric probe.

When it moves through empty space, Cassini meets no resistance. So it’s easy to keep flying without deviations. But once it hits an atmosphere, everything changes. As with the wind resistance you feel when sticking a hand out of a moving car’s window, Cassini will begin to wobble it gets deeper into the atmosphere.

There, the craft’s oddly shaped radio equipment and magnetic will catch like sails, throwing Cassini into a tumble. It has few small thrusters designed to counter the tumbling motion, but they weren’t designed to for planetary entry.

At first, those thrusters will fire intermittently to keep Cassini’s transmitter pointed toward Earth. Then “it will get to the point where the thrusters will turn on, 100 percent, trying to fight the atmosphere,” Thomas Burk, a NASA Jet Propulsion Lab engineer who has worked on the mission from the beginning, says. “Of course, it will lose that battle.” In all, Cassini may stay stabilized for just two minutes after it enters the atmosphere.

Once Cassini starts to tumble, it will no longer be able to send data back to Earth. From NASA’s perspective, that’s the end of the mission.

But Cassini will live on in tumultuous silence for at least a few more minutes, traveling at around 20 miles per second. The forces from the tumbling will literally rip its components apart, piece by piece.

Then it will melt, and explode. “The outer surface materials might start to char at first; then you’d see some breaking apart; then when you get down to the metal, once it gets hot enough, it will glow,” says Brett Pugh, a NASA JPL thermal engineer. There’s no oxygen on Saturn, so there’s no fire. But “the propellant tanks will explode eventually as the temperatures get high enough,” he says.

That might produce a flash, like the streak of light that explodes from a meteor above Earth. (NASA will point some telescopes at Saturn to watch, but they’re unlikely to see anything.) After the tanks explode, there will be nothing left. “It would be a tremendous view, if anyone could witness it,” Pugh says.

As NASA spokesperson Preston Dyches has said: “We’re going out in a blaze of glory.”

An artist’s depiction of Cassini breaking apart as it descends into Saturn’s atmosphere.

Whatever’s left will continue to sink deeper and deeper into Saturn’s atmosphere, “where intense heat and pressure will cause all of its materials to melt and completely dissociate, eventually becoming completely diluted in the planet’s interior,” NASA explains. The trace bits of metal, composite materials, and even some of the plutonium that powers Cassini will become undetectable in the planet, which is 764 times the size of Earth.

Cassini has completely transformed our understanding of Saturn

Cassini — named after the 17th-century astronomer Giovanni Cassini — launched from Cape Canaveral in October 1997 in a NASA collaboration with the European Space Agency. At that time, we were still a few months away from Bill Clinton’s damning “I did not have sexual relations with that woman” remark. Harry Potter had not yet been published in the United States.

This view from NASA’s Cassini spacecraft shows a wave structure in Saturn’s rings.
NASA/JPL-Caltech/Space Science Institute

From there, it took Cassini, and the Huygens probe (destined to touch down on the moon Titan), seven years to reach Saturn. Once it arrived, it started to make discoveries that utterly changed our understanding of the planet and its system.

On Titan, Cassini and Huygens revealed surprisingly earthlike geographic features and huge lakes of liquid natural gas on the moon’s surface that outweigh all the oil and gas reserves on Earth. There are great clouds on Titan that rain down liquid methane. There are rivers of it on the surface.

Cassini found evidence of an underground ocean on the moon Enceladus, an incredible discovery. It learned those oceans may contain hydrothermal vents and the right ingredients to support life. Like the geothermal vents deep within Earth’s oceans, these could be home to microbes that use the chemical energy of hydrogen and carbon dioxide to produce methane and energy. And Cassini regularly sees great plumes of water vapor and gases erupt from the surface of Enceladus.

The ice world Enceladus.
NASA/JPL-Caltech/Space Science Institute

“Enceladus may have all the ingredients for life — as we know it — to currently exist,” Curt Niebur, a Cassini program scientist, told reporters at a recent press conference. The discovery of the ocean “changed our idea that ocean worlds — like Earth and Europa — are rare in the universe,” he said.

This movie sequence of images is from the last dedicated observation of the Enceladus plume by NASA’s Cassini spacecraft (taken over the course of 14 hours).
NASA/JPL-Caltech/Space Science Institute

Cassini learned how new moons could form out of Saturn’s rings. And it has taken detailed photographic surveys of the planet’s rings and surface features — like the beautiful hexagonal cloud patterns bigger than Earth that exist at the North Pole.

Saturn’s hexagonal, hypnotic North Pole.


Cassini’s last look at Saturn will be our last look at the planet for at least a generation

When the spacecraft sends the last image of Saturn back to Earth on Friday, it will be the last time our human eyes will get to see Saturn up close for many years. There are no upcoming missions planned for the Saturn system. A new one could take a decade to plan and launch, and then it takes about seven years to reach the planet.

NASA’s next big planetary science effort is the Europa Clipper, which will launch in the 2020s. Its goal is to investigate Jupiter’s ice moon.

So for all we know, this is the last chance NASA has to make direct measurements of Saturn, its atmosphere, and its vexing and beautiful rings for a very long time.

And that’s why Cassini’s finale is so special: It’s not just a spectacle. It’s a scientific operation that honors the precious limited time we have to explore other worlds. And that pain is compounded by the fact that Cassini leaves Saturn with mysteries left unsolved.

Scientists still don’t know exactly how quickly Saturn’s core rotates, which would determine the length of a Saturn day. This is a huge, basic question about the planetary system, and it will remain unanswered, for now. And scientists still don’t have an exact figure on the mass of Saturn’s rings or their age.

“It’s been part of my life for so long, this spacecraft, it’s going to be a shock to have this happen,” Burk says, anticipating the moment Cassini goes offline. “It’s bittersweet in that regard. But it’s a really exciting ending. When we stop getting data, that will be the moment of truth.”

Goodnight and good luck.

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