A Big Empty between Saturn and its rings

Some data from the RPWS instrument (Image NASA/JPL-Caltech/University of Iowa)
Some data from the RPWS instrument (Image NASA/JPL-Caltech/University of Iowa)

The first results of the analysis of the data collected by the Cassini space probe during its dive into the space between the planet Saturn and its rings on April 26, 2017 provided a remarkable surprise. The researchers discovered that Cassini’s instruments recorded very few particles during the crossing of that space and all of them were tiny, around a micron across. This is a mystery to which the researchers hope to find an explanation.

The space between the planet Saturn and its rings’ inner rim is only 2,000 km (about 1,200 miles) wide. The models of that environment created based on the photos collected by the Cassini space probe suggested that there were no large particles in the area that could damage it but no spacecraft ever went into that space. The researchers expected that there were still particles, though small, and they had reoriented the Cassini’s 4-meter (13-foot) dish-shaped high-gain antenna to use it as a protective shield.

The Radio and Plasma Wave Science (RPWS) and the magnetometer were the two Cassini’s instruments with sensors outside the antenna / shield. RPWS detected the impact of hundreds of particles per second when the space probe crossed the plane of the rings just outside the main ones but detected very few of them when it crossed the space between Saturn and the rings on April 26.

The detected data were converted into audio format so the impacts were heard as cracks and pops. Those relating to the impacts in the space between Saturn and the rings were counted on the fingers of a hand. The analysis suggests that Cassini met on its way very few particles and all around a micron across, like smoke particles.

The image compares some data collected by the RPWS instrument in December 2016 during a Cassini passage through the small Janus-Epimetheus ring and others collected during its  April 26, 2017 dive. In the first case there’s a peak matching the ring area while in the second case there’s nothing comparable.

Given the proximity of the rings, researchers expected Cassini to encounter a much larger amount of particles. For this reason they planned to use the antenna as a shield in all 22 passeages through the inner rim of the rings and also in the space between Saturn and its rings. Such emptiness in that space is a mystery that researchers hope to solve with the next passages.

This mystery is good news because it means that using the antenna as a shield during the next dives into the space between Saturn and the rings will no longer be needed. Without this need, black-outs will not be as long as the one of first dive and there will be no restrictions on the instruments available for the detection. This will allow to collect a lot of different data and take more photos of the area.

Saturn is offering surprises until the end of the Cassini mission and the space probe needed to dive into that area to see that it’s practically empty. Cassini has just completed its second dive and in the coming hours is scheduled to contact the Earth and send new data.

Two NASA videos show with a greater completeness data collected by the RPWS instrument during a Cassini passage through the small Janus-Epimetheus ring and during its April 26, 2017 dive with its audio version.

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