After 13 years at Saturn, NASA’s Cassini spacecraft is ready for its grand finale

What do you do with a 20-year-old spacecraft that has spent 13 years orbiting Saturn, logged 4.1 billion miles in space and is about to run out of fuel?

You could crash it into one of the dozens of moons that orbit the ringed planet. Or you could let it hang out in a wide orbit around the gas giant where it would stay out of the way.

But the planners of NASA’s Cassini’s mission had a different idea.

On Wednesday, their running-on-empty space probe will dive into the mysterious space between the cloud tops of Saturn and the inner edge of its majestic rings.

Over the next several months, Cassini will fly through this uncharted territory a total of 22 times. Along the way, it will collect new data that could answer important questions about Saturn’s interior, its mysterious storms, the age of its rings and the length of its day.

It’s a noble end to a long trip. Then, after the spacecraft’s final pass through this narrow region, it will hurl itself into Saturn’s atmosphere, where it will vaporize in a matter of minutes.

“I like to say it’s going out in a blaze of glory,” said Linda Spilker of the Jet Propulsion Laboratory, the Cassini project scientist. “It will be trailblazing until the very last second.”

NASA has dubbed this final series of maneuvers the grand finale. But in Spilker’s view, it’s more like the beginning of a new mission.

“Getting this close to the rings and the planet, that’s a once-in-a-lifetime experience for a scientist like me,” she said. “We’ve wanted to do this for a long time.”

In the past, mission planners have been hesitant to explore this part of the Saturn system because it was deemed too risky.

“We’re going to be going 70,000 mph into a 1,200-mile gap — and oh, by the way, we’ll be driving this thing from a billion miles away,” said Earl Maize, Cassini’s project manager at JPL in La Cañada Flintridge.

But it isn’t the navigation that has NASA officials worried.

“Our concern is not accuracy, it’s whether we modeled the system correctly,” Maize said.

At Cassini’s high velocity, even a small piece of dust could take out one of its instruments. If a particle the size of a grain of sand struck it in the wrong spot, it could trip up the entire spacecraft.

Models of the Saturn system suggest that the narrow region between the rings and the planet should be free of dust, but it’s always possible that the models are wrong. (In fact, that’s one thing Cassini’s instruments will test.)

Therefore, on the spacecraft’s first pass through the gap, its large, dish-shaped antenna will go in first to act as a shield for the rest of its instruments.

The odds are in Cassini’s favor.

“Our most conservative, dire models where the engineers awful-ize everything puts us at 97% chance of success,” Maize said.

That may sound pretty safe to you, but NASA really hates risk.

“We would never take a flagship mission on that kind of course on any other time in the mission except when it’s about to end,” Maize added.

In the unlikely event that the models are wrong and Cassini’s path is not sufficiently dust-free, contingency plans are in place. And in the even more unlikely event that the models are really wrong and the spacecraft encounters BB-sized material that causes serious damage, Cassini will still end up vaporized in Saturn’s atmosphere.

Spilker, who leads Cassini’s 300 scientists, said the decision to send the spacecraft on this final mission was a no-brainer.

“We took one look at the possibility of going between the planet and the rings and said this is what we’re going to do,” she said. “No discussion needed.”

The spacecraft arrived at the Saturn system in July 2004, and it has been collecting data on the planet, its moons and its rings ever since. However, there are still fundamental questions about Saturn that have yet to be answered.

For example, scientists believe that deep beneath the planet’s atmosphere lies a rocky core — but whether it is the size of one Earth or two remains unclear.

“Seven hundred and sixty-four Earths would fit inside Saturn,” Spilker said. “So imagine trying to find just one Earth with all this mass on top of it.”

Knowing the mass of the core would help scientists determine the mass of Saturn’s iconic rings. This, in turn, could indicate how old they are.

“If the rings are more massive than we expect, then perhaps they are as old as Saturn itself, because they had enough mass to survive micro-meteor bombardment and erosion,” Spilker said. “On the other hand, if they are less massive, perhaps they are very young, forming as little as 100 million years ago.”

Spilker said data collected during the grand finale could help answer other lingering mysteries as well — including the makeup of Saturn’s atmosphere, how deep beneath the surface its winds blow and how fast the planet’s interior is spinning.

“All we see on Saturn is its atmosphere, with different bands rotating in different directions and different speeds,” Spilker said. “We don’t have any way of knowing at what speed the interior is rotating.”

New data could give researchers a better understanding of the strange hexagon-shaped jet stream at the planet’s north pole.

And, of course, Cassini will beam back the highest-resolution pictures yet of the planet and its rings.

While the Cassini team is eagerly anticipating a new batch of science, some members are also starting to feel the first pangs of sorrow that their time at Saturn will soon come to an end.

“We, humankind, through this mission, have been at Saturn for 13 years,” Maize said. “Unfortunately, there’s not going to be a substitute for that for a long time.”

He paused, and added: “It’s been the ride of a lifetime.”

An artist's illustration of NASA's Cassini spacecraft crossing Saturn's ring plane. A maneuver this close to Saturn's surface was considered too risky earlier in Cassini's mission. (NASA/Jet Propulsion Laboratory)

The Cassini spacecraft’s dive in between Saturn’s rings, explained

The Cassini spacecraft is going where no ship has gone before: On Wednesday, it begins a series of dives into the space between Saturn and its magnificent rings. The maneuver — a series of 22 orbits that will bring Cassini increasingly closer to Saturn’s surface before crashing into it — is called the spacecraft’s “grand finale.” And to mark this final journey, Cassini is being honored with a Google Doodle.

Over its last 13 years in orbit, Cassini has had an amazing run studying Saturn and its moons. Here’s what the spacecraft has taught us so far — and why its final mission may be its most spectacular yet.

In its last days, Cassini keeps generating fascinating insights
Cassini — named after the 17th-century astronomer Giovanni Cassini — launched from Cape Canaveral in October 1997 in collaboration with the European Space Agency. When it launched, 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.

From there, it took Cassini and the Huygens lander (destined to touch down on the moon Titan) seven years to reach Saturn. Once it arrived, it started to make impressive discoveries.

On Titan, Cassini and Huygens revealed surprisingly Earthlike geographic features and great lakes of liquid natural gas on the moon’s surface that outweigh all the oil and gas reserves on Earth. Cassini found evidence of an underground ocean on the moon Enceladus. It learned how new moons could form out of Saturn’s rings. And it has taken detailed, beautiful photographic surveys of the planet’s rings and surface features.

Nearing the end of its life, Cassini is still producing scientific discoveries at a fast clip.

Earlier in April, NASA announced that the spacecraft had found the most compelling evidence yet that the ocean underneath Enceladus could contain life.

Previously, the Cassini spacecraft has observed jets of water containing organic chemicals streaming from Enceladus. This latest finding adds a key ingredient for life to the mix: hydrogen. The presence of hydrogen in the jets makes NASA scientists suspect there are geothermal geysers on Enceladus’s ocean floor. 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 for life.

Now Cassini is beginning a series of harrowing orbits that bring it into the space between Saturn and its rings — a region no spacecraft has been before. When Cassini is in the inner rings, it will finally be able to take the measurements that will aid in calculations to determine the mass of the rings.

Why NASA is diving into the space between Saturn and its rings
On Wednesday, Cassini begins a maneuver that is unprecedented in the history of spaceflight: It’s adjusting its trajectory to bring it inside the 1,500-mile-wide gap between Saturn and its rings for 22 orbits.

This is what that dive will look like from Cassini’s perspective.

In these illustrations, the blue lines represent each of the 22 orbits getting closer and closer to the atmosphere of the giant planet. The red line represents the final orbit, which will end with Cassini crashing into Saturn’s atmosphere.

In this space, Cassini will be able to take new measurements to better determine the total mass of Saturn’s rings. NASA already knows the mass of Saturn plus its rings. Getting closer to the planet will allow Cassini to take its mass without factoring in the rings. That information will help scientists better understand how the rings formed (which in turn can help them understand how all the planets formed from rings of material around the sun).

The orbits will also produce the closest-ever observations of Saturn’s clouds — yielding incredible images.

It will be a thrilling journey, but also a perilous one. NASA has saved the ring-grazing orbits for Cassini’s finale in part because they are dangerous. The orbits will bring Cassini close to debris and rocks that could take it offline.

“We’re going out in a blaze of glory”
Come September 15, Cassini will crash into Saturn, having spent all of its fuel. But the death dive isn’t just for fireworks. If the spacecraft doesn’t plunge into Saturn’s atmosphere, it runs the risk of potentially contaminating one of the planet’s moons with debris and microbes from Earth.

And there’s no turning back: “The spacecraft is now on a ballistic path,” Earl Maize, a Cassini project manager, said in a press statement, meaning that the spacecraft’s path is shaped mostly by gravity, not by thrusters. “Even if we were to forgo future small course adjustments using thrusters, we would still enter Saturn’s atmosphere on Sept. 15 no matter what.”

Cassini’s dramatic finale is also a last chance to squeeze some more insights out of the 20-year-old spacecraft. As it descends into Saturn’s atmosphere, “several of the instruments will be on,” including the mass spectrometer, Preston Dyches, a NASA spokesperson, says. This instrument essentially can “sniff” the atmosphere and determine the chemical compounds it’s composed of.

On April 12, days before it made its final flyby of Titan, Cassini captured this incredible image of Earth shining through Saturn’s rings, as if to remind us of how far it’s come since beginning its journey. From Saturn, we’re just a tiny bright speck in the darkness.

When Cassini finally goes offline in September, it will die doing what it’s been doing all along: exploring.

Cassini Spacecraft Starts Weaving Between Saturn And Its Rings

If all goes to plan, NASA's Cassini spacecraft will beam new images of Saturn and its rings to Earth early Thursday, sharing data collected Wednesday from its first dive through the gap between the planet and its striped belt of ice and rock particles.

Today's dive also marks the start of the final phase in the craft's 13-year visit to Saturn. Days ago, it used the gravity of Saturn's moon Titan to bend its path toward its eventual destruction on the planet.

Cassini descended below the ring plane around 5 a.m. ET Wednesday, but the antenna it would normally use to send images is instead being used to deflect potentially harmful objects away from its instruments. As it performed the move, the craft's Twitter feed announced, "Shields Up!"

"Because that gap is a region no spacecraft has ever explored, Cassini will use its dish-shaped high-gain antenna (13 feet or 4 meters across) as a protective shield while passing through the ring plane," NASA says. "No particles larger than smoke particles are expected, but the precautionary measure is being taken on the first dive."

For today's maneuver, the craft crossed the ring plane as it moved from north to south across Saturn. The gap between Saturn and its rings is about 1,500 miles wide.

Cassini is expected to get back in touch with the Deep Space Network antenna in Goldstone, Calif., by around 3 a.m. ET Thursday; the first images should be available shortly afterward, NASA says.

The move is the first in what NASA is calling Cassini's Grand Finale, as it weaves its way between Saturn and its rings in a series of 22 dives that will culminate in what the agency describes as "a science-rich plunge into Saturn's atmosphere on Sept. 15."

Cassini was launched in 1997; its mission is slated to end one month before the 20-year mark.

Among the photos sent back by Cassini is a unique view of Earth as seen from Saturn, in what was hailed as a new perspective of our "pale blue dot" in space.

When that image was released in 2013, the Two-Way wrote, "Earth and all its trillions of creatures are seen as a speck of light — lower right — in the vastness of space. What's more, Cassini captures Earth while also capturing Saturn's rings."

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