Grand Finale: Cassini Preps For Last Days at Saturn

Cassini, which has been circling Saturn since 2004, will dip inside the planet's rings during its 13th and final year on the job.

Image: Cassini snapped this backlit view of Saturn in July 2013. Next year, Cassini will end its mission with 22 passes through the narrow gap between the planet and its rings. Credit: NASA/JPL-Caltech/SSI Before the Cassini project bids farewell to one of NASA's most successful probes, the spacecraft will make a daring jump across Saturn's rings, setting up 22 passes through the narrow, unexplored gap between the planet and its closest ring.

"It really and truly for Cassini is a brand new mission," said project scientist Linda Spilker, with NASA's Jet Propulsion Laboratory in Pasadena, Calif.

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The Cassini spacecraft left Earth on Oct. 5, 1997, and set sail for Saturn, the second-largest planet in the solar system after Jupiter. Seven years later, after traveling nearly 2.2 billion miles, Cassini put itself into orbit around the ringed beauty.

Data collected by Cassini and radioed back to Earth will keep scientists busy for decades, but the spacecraft's days are numbered: Next September, Cassini will fly itself into Saturn and incinerate.

The suicidal plunge, scheduled for Sept. 15, eliminates the chance that one of Saturn's potentially habitable moons becomes a new home to an uber-hearty colony of hitchhiking Earth microbes, still alive on a crashing Cassini spacecraft after decades in deep space.

Cassini will fly into Saturn using its steering jets, leaving little to chance.

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During the 22 final orbits, and particularly during Cassini's closest approach, scientists want to collect data that will let them figure out how much material is in Saturn's rings. That, in turn, would resolve a long-standing mystery about if the rings are as old as Saturn, or a more modern-day phenomenon.

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They also want to measure the length of a Saturn day, learn more about planet's magnetic field, map its internal structure and take close-up pictures of the planet's powerful aurora. Cassini even could discover a moon-like mass inside Saturn, which could explain some mysterious wave motions detected in the C-ring, Spilker said.

"The rings are actually a very sensitive detector of what's going on inside Saturn," Spilker said last week during a webcast meeting of the National Academy of Sciences Committee on Astrobiology and Planetary Science.

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"We saw a series of waves in the rings and they were damping in the wrong direction. Usually, when there's resonance with a moon, the waves damp out toward the moon. These particular waves were damping in toward Saturn. It looks almost like you have a mass or like tiny moon inside Saturn itself," Spilker said.

The journey in toward Saturn promises suspense. Scientists aren't exactly sure how many particles Cassini might encounter as it flies closer to Saturn. To be on the safe side, Cassini will makes its first few close passes using its big communications antenna as a shield. If too many particles hit the antenna, Cassini can fly that way the rest of the mission, Spilker said.

"In just a little over an hour, you go from north pole to south pole, through the ring gap. You're going incredibly fast -- 75,000 mph. That's why even a particle a millimeter or so in size hitting in the wrong place wouldn't be so good for Cassini," Spilker said.

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"Fortunately, most of the particles we encounter are more like smoke," added Cassini project manager Earl Maize.

"For the Grand Finale, we strive to make the probability of serious impact less than a few percent, although four of the 22 passes in the Grand Finale slightly exceed that threshold. That being said, our models and risk calculations are very conservative and we expect to be absolutely fine," Maize wrote in an email to Discovery News.

Beginning Nov. 30, Cassini's orbit will be shifted so that it passes just beyond the outer edge of the main rings. During these weekly orbits, Cassini will fly within 4,850 miles of the center of the narrow F ring.

In April, a flyby Saturn's large moon Titan will bend Cassini's trajectory so that the spacecraft leaps across all the rings. Cassini's first dive through the unexplored 1,500-mile gap between Saturn and its rings is scheduled for April 27.