Spaceflight Insider

Gap between Saturn and innermost ring surprisingly free of dust

Cassini's Grand Finale dive through Saturn's rings

NASA’s Cassini spacecraft is shown diving through the gap between Saturn and its rings in this artist’s depiction. Image & Caption Credit: NASA/JPL-Caltech

Analysis of data returned by NASA’s Cassini spacecraft from its first Grand Finale dive between Saturn and its rings has surprised scientists by revealing the region to be nearly dust free.

The finding is good news for engineers working on the spacecraft because it means the chance of it being damaged by impacting dust particles is low.

However, it is puzzling mission scientists, who expected the region to have more dust based on the spacecraft’s Radio and Plasma Wave Science (RPWS) instrument’s detection of numerous ring particles having impacted Cassini during its earlier crossing of the ring plane outside Saturn’s main rings in December 2016.

During that flight, sensors on RPWS and on Cassini’s magnetometer counted hundreds of ring particles hitting the probe every second.

Data taken by RPWS is converted to an audio format, in which the impact of dust particles on the spacecraft’s antenna, used to shield its science instruments, produces distinctive popping and cracking sounds.

In contrast, the spacecraft’s passage through an environment of charged particles produces waves of whistles and squeaks.

When the RPWS team listened to the audio from the dive, they heard whistles and squeaks but almost no pops and cracks.

Cassini's sounds and spectrograms of its flyby through Saturn's rings

The sounds and spectrograms in these two images represent data collected by the Radio and Plasma Wave Science (RPWS) – an instrument on NASA’s Cassini spacecraft – as it crossed the plane of Saturn’s rings on two separate orbits. Image Credit: NASA / JPL-Caltech / University of Iowa

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

“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.”

Just prior to Cassini’s April 26 first of 22 dives between Saturn and its rings, mission engineers positioned its saucer-shaped 13-foot-wide (four-meter-wide) antenna as a shield, pointing it in the direction of incoming ring particles to protect Cassini’s science instruments.

Sensors on RPWS and on the magnetometer were able to observe outside the antenna’s shielding to record data during the flight.

Cassini is the first spacecraft to ever fly into the 1,200-mile-wide (2,000-kilometer-wide) gap between the innermost edges of the rings and the giant planet, so mission scientists and engineers chose to exercise caution during the first dive.

The absence of dust in the gap region means contingency plans to alter observations to protect the spacecraft from impacting dust particles will not need to be used, except during four dives that will take Cassini to the innermost edges of Saturn’s rings.

Earl Maize, NASA’s Jet Propulsion Laboratory (JPL) and Cassini Project Manager, said: “The region between the rings and Saturn is ‘the big empty’, apparently.

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

The few particles RPWS did detect were tiny, with widths no bigger than one micron.

Cassini’s second dive on Tuesday, May 2, takes it very close to the first dive location. In anticipation of the move, its cameras have conducted close-up observations of the innermost rings.

This time, the spacecraft will roll or rotate faster than it has ever done to calibrate its magnetometer for high-intensity magnetic field observations.

Using its Imaging Science Subsystems (ISS), Cassini will observe Saturn’s rings at very high phase angles to study faint fragments in the main rings as well as monitor structures within the D ring.

These observations are possible because the Sun is hidden behind Saturn during this orbit.

The probe’s Composite Infrared Spectrometer (CIS) will view Saturn’s moon Rhea in infrared wavelengths in an effort to identify its structure and composition.

While flying within 1,820 miles (2,930 kilometers) of Saturn’s one-bar atmosphere cloud tops and 2,980 miles (4,780 kilometers) of the D ring’s inner edge, Cassini will be unable to contact Earth.

Contact is scheduled to resume at 10:13 a.m. EDT (14:13 GMT) on Wednesday, May 3, followed by downlink of the data from the second dive.

Cassini's Grand Finale orbits through Saturn's rings

Artist’s concept of the final orbits of the Cassini spacecraft at Saturn. Image Credit: NASA / Jet Propulsion Lab



Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.

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