Crossing “The Big Empty”

The image above (as always thanks to NASA/JPL-Caltech/Space Science Institute) was taken on 29 April 2017 and has very minimal processing.

Later today (at 19:38 UTC) the Cassini spacecraft will make its second crossing in the gap between the rings of Saturn and the planet itself.

During the first crossing the spacecraft was oriented so the four-meter wide antenna was leading the way through the gap so it could act as a shield protecting instruments on the spacecraft. Not so long ago only few could imagine flying through that 2,000 km wide gap and nobody knew what they would find.  Will this dive be the same?  We will know tomorrow.

NASA — Cassini’s Radio and Plasma Wave Science (RPWS) instrument was one of two science instruments with sensors that poke out from the protective shield of the antenna (the other being Cassini’s magnetometer). RPWS detected the hits of hundreds of ring particles per second when it crossed the ring plane just outside of Saturn’s main rings, but only detected a few pings on April 26.

When RPWS data are converted to an audio format, dust particles hitting the instrument’s antennas sound like pops and cracks, covering up the usual whistles and squeaks of waves in the charged particle environment that the instrument is designed to detect. The RPWS team expected to hear a lot of pops and cracks on crossing the ring plane inside the gap, but instead, the whistles and squeaks came through surprisingly clearly on April 26.

“The region between the rings and Saturn is ‘the big empty,’ apparently,” said Cassini Project Manager Earl Maize of NASA’s Jet Propulsion Laboratory in Pasadena, California. “Cassini will stay the course, while the scientists work on the mystery of why the dust level is much lower than expected.”

“It was a bit disorienting — we weren’t hearing what we expected to hear,” said William Kurth, RPWS team lead at the University of Iowa, Iowa City. “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.”

The team’s analysis suggests Cassini only encountered a few particles as it crossed the gap — none larger than those in smoke (about 1 micron across).

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