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The Sciences

Cassini’s Closing Act

One of NASA’s most iconic missions comes to an end.

Lapetus
(Credit: NASA/JPL-CalTech)

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Cassini is running out of gas.

So before mission managers lose control, they’ve steered the spacecraft on a no-return course into Saturn’s atmosphere, where it’s scheduled to burn up Sept. 15 at about 3:45 a.m. PDT. The move is precautionary. A dead spacecraft carrying stowaway microbes could contaminate icy Enceladus, a moon Cassini showed us has a salty ocean and the potential for life. Instead, friction from the high-speed atmospheric entry will destroy Cassini.

NASA launched Cassini from Cape Canaveral on Oct. 15, 1997, and it slid into Saturn’s orbit nearly seven years later. The robotic spacecraft proved so robust that NASA extended the mission twice.

Its twin 1-megapixel cameras, built in the early ’90s, snapped thousands of pictures. Along the way, Cassini discovered seven moons and parachuted the Huygens probe down to Titan, the only known moon with a thick atmosphere. But perhaps its most startling discovery was detecting organic compounds in the saltwater ocean sloshing under Enceladus’ icy shell.

Now the last leg of the mission, which NASA calls the Grand Finale, flings the craft over Saturn’s mysterious polar hexagon and into a series of daring dives between planet and rings. On that final day in September, the team will gather to say goodbye at the Jet Propulsion Laboratory in Pasadena, Calif. Among them will be Cassini’s science team leader, Linda Spilker. She’s been with the team for almost 30 years — or the equivalent of just one Saturn orbit, she says.

“When the signal stops, there will be a moment of silence, then applause and tears,” Spilker predicts. “We will miss her.”

Hexagon Puzzle

DSC-CR0917 04 saturn
Cassini brought new detail to Saturn’s mysterious hexagon-shaped polar jet stream, shown here from 2 million miles away. Researchers created similar shapes in computer models, concluding the unusual cloud patterns are caused by atmospheric disruptions. (Credit: NASA/JPL-Caltech)

Cassini

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The orbiter and attached Huygens lander eased into Saturn’s orbit in 2004. (Credit: NASA/JPL-Caltech)

Northern Exposure

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This false color image, taken in near-infrared light, shows the raging eye of the 1,200-mile-wide storm at Saturn’s north pole. Red shows lower clouds; the green ones are higher. (Credit: NASA/JPL-Caltech)

Tiger Stripes

DSC-CR0917 07 enceladus
Under the icy surface of Saturn’s moon Enceladus, a liquid ocean launches water plumes through the cracks. Cassini also sniffed out organic compounds and potentially life-giving hydrothermal vents. (Credit: NASA/JPL-Caltech/Michael Benson/Kinetikon Pictures)

Gravity Wake

DSC-CR0917 08 daphnis in saturns rings
Only 5 miles across, Saturn’s ring-embedded moon Daphnis kicks up waves of particles in the A ring. (Credit: NASA/JPL-Caltech)

Grand Finale Portrait

DSC-CR0917 09 saturn backlit
Here the ringed planet shows a side never visible from Earth. Cassini took 96 backlit photos for this mosaic on April 13, 2017. Because the sun shines through the rings, the thinnest parts glow brightest and the thicker rings are dark. (Credit: NASA/JPL-Caltech/Jan Regan)

Cracking The Walnut Moon

DSC-CR0917 10 Iapetus
Cassini close-ups found a 900-mile mountain range, up to 12 miles high, encircling Iapetus. Some researchers say the moon’s rotation caused the ridge; others blame a collapsing debris disk, suggesting a ringed moon once circled the ringed planet. (Credit: NASA/JPL-Caltech)

Methane Lakeshore

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As the Huygens probe parachuted to Titan’s surface, it revealed the shorelines of a liquid methane sea, plus geologically active mountains and channels. (Credit: ESA/NASA//University of Arizona)

Many Moons

DSC-CR0917 12 saturns moons
Saturn had 18 known moons when Cassini launched. The latest total is 62 — seven found by Cassini. This view shows (from left) Pandora and Prometheus — moons that shepherd the F ring — and Epimetheus. Each is less than 70 miles across. (Credit: NASA/JPL-Caltech)

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