On April 26, 2017, NASA’s Cassini orbiter made its first ‘Grand Finale’ dive through the previously unexplored gap between Saturn and its rings. An analysis of data collected by Cassini’s Radio and Plasma Wave Science (RPWS) instrument shows that this dive was nearly particle free. The unexpected finding that the gap is so empty is a new mystery that the mission scientists are eager to understand.
This illustration shows Cassini about to make one of its dives between Saturn and its innermost rings as part of the mission’s Grand Finale. Image credit: NASA / JPL-Caltech.
“The region between the rings and Saturn is ‘the big empty,’ apparently,” said Cassini project manager Dr. Earl Maize, from NASA’s Jet Propulsion Laboratory.
“Cassini will stay the course, while the scientists work on the mystery of why the dust level is much lower than expected.”
Based on images from Cassini, models of the ring particle environment in the 1,200-mile- (2,000 km) wide region between the gas giant and its rings suggested the area would not have large particles that would pose a danger to the spacecraft.
But because no spacecraft had ever passed through the region before, the mission engineers oriented Cassini so that its 13-foot- (4 m) wide, dish-shaped high-gain antenna pointed in the direction of oncoming ring particles, shielding its delicate instruments as a protective measure during its first ‘Grand Finale’ dive.
On April 26, Cassini dove through the ring-planet gap at speeds approaching 75,000 mph (121,000 kph).
Two of Cassini’s instruments, the magnetometer and RPWS, extend beyond the protective antenna dish, and were exposed to the particle environment during the dive.
The mission scientists used these data from RPWS, along with inputs from other components on the spacecraft, to make the decision of whether the high-gain antenna would be needed as a shield.
Based on these inputs the researchers determined this protective measure would not be needed, allowing the team’s preferred mode of science operations to proceed, with Cassini able to point its science instruments in any direction necessary to obtain scientists’ desired observations.
“As tiny, dust-sized particles strike Cassini and three RPWS antennas, the particles are vaporized into tiny clouds of plasma, or electrically excited gas. These tiny explosions make a small electrical signal that RPWS can detect,” the researchers explained.
“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,” they added.
“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.”
“We 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.”
“It was a bit disorienting — we weren’t hearing what we expected to hear,” said RPWS team lead Dr. William Kurth, from the University of Iowa.
“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 analysis suggests Cassini only encountered a few particles as it crossed the gap — none larger than those in smoke (only one micron across).
Cassini will next cross through the ring plane today, May 2, 2017, at 3:38 p.m. EDT (12:38 p.m. PDT) in a region very close to where it passed on the first dive.
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This article is based on a press-release from the National Aeronautics and Space Administration.
