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

The spacecraft begins its “grand finale” before crashing into the gas giant later this year.

An artist’s rendition of what Cassini’s crash into Saturn will look like.
Brian Resnick is Vox’s science and health editor, and is the co-creator of Unexplainable, Vox's podcast about unanswered questions in science. Previously, Brian was a reporter at Vox and at National Journal.

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.

NASA/JPL/Space Science Institute

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”

This artist’s concept shows an over-the-shoulder view of Cassini making one of its grand finale dives over Saturn.

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.