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Saturn and rings
The hexagon-shaped cloud system at Saturn’s north polar region looms in the foreground with the planet’s rings stretching across the background, in an image captured by the Cassini spacecraft’s camera on April 26. Click on the image for a larger version. (NASA / JPL-Caltech / Kevin M. Gill Image / CC-BY-2.0)

The researchers behind NASA’s Cassini mission to Saturn are relieved, and a bit mystified, to discover that the narrow gap between the giant planet and its rings is virtually devoid of stray particles.

The discovery comes from the bus-sized Cassini spacecraft’s first dive through the gap on April 26, which marked the beginning of the end for the 20-year mission.

“The region between the rings and Saturn is ‘The Big Empty,’ apparently,” Cassini project manager Earl Maize of NASA’s Jet Propulsion Laboratory said in a news release.

The finding came as a relief, because Cassini’s engineers had worried that the probe might be pummeled by unseen dust particles associated with Saturn’s visible rings. If such a disk of dust were dense enough, it could have done damage to Cassini’s hardware.

To guard against that potential damage, Cassini was commanded to turn its main antenna into the expected stream to act as a shield during last week’s first dive.

Cassini’s Radio and Plasma Wave Science instrument had detected hundreds of particle hits per second last December, when the spacecraft passed through the plane of the rings just beyond their outer edge. Surprisingly, only a few pings were registered during last week’s pass just inside the rings.

“It was a bit disorienting — we weren’t hearing what we expected to hear,” said University of Iowa astronomer William Kurth, the leader for the instrument science team. “I’ve listened to our data from the first dive several times, and I can probably count on my hands the number of dust particle impacts I hear.”

Now that mission managers know that there’s a “Big Empty” inside the rings, they’ve decided it won’t be necessary to take extra protective measures for most of the 20 passes that remain on the schedule. During four of the dives that are still ahead, Cassini will pass through the innermost edge of Saturn’s rings, and the shielding maneuver will be used just for those passes.

“Cassini will stay the course, while the scientists work on the mystery of why the dust level is much lower than expected,” Maize said.

Cassini made its second dive through the ring gap today, and the data gathered during that dive is due to be transmitted back to Earth on Wednesday. The plutonium-powered spacecraft, which was launched in 1997 and reached Saturnian orbit in 2004, is due to be sent crashing into Saturn’s cloud tops to mark the end of the $3.27 billion mission in September.

Here are a few more close-ups of Saturn and its surroundings, gleaned via Twitter:

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