Spaceflight Insider

The Big Empty: Cassini finds virtually no particles between Saturn and its rings

Cassini image of Saturn in 2016

An image of Saturn taken by the Cassini spacecraft in 2016. Photo Credit: NASA

Based on data collected on the first of the Cassini spacecraft’s planned 22 “Grand Finale” orbits, the area between the cloud tops of Saturn and the inner-most ring seems to be mostly dust-free. Instead of the heavy distribution of dust particles Cassini had detected when it made its ring grazing orbits in late 2016, the spacecraft instead revealed a “big empty.”

As Cassini approached Saturn on its current trajectory, it focused its camera and science instruments at the atmospheric surface of the gas giant. Not knowing what to expect, and at speeds upward of 70,000 mph (112,650 km/h) relative to Saturn’s cloud tops, the plunge between the planet’s ring gap could have proven fatal to the spacecraft. Had it impacted with large or dense concentrations of fine particles, the mission very well could have ended months ahead of its anticipated Sept. 15, 2017, end of mission.

With this risk in mind, scientists and engineers instructed the craft to turn, positioning the high-gain antenna face first into the breach to act as a makeshift shield from the anticipated barrage of particulates.

The Cassini spacecraft has been in orbit since 2004. Since that time, it has revolutionized our understanding of the ringed planet. Image Credit: James Vaughan / SpaceFlight Insider

Artist’s rendition of the Cassini spacecraft in orbit around Saturn. Image Credit: James Vaughan / SpaceFlight Insider

Much like the “seven minutes of terror” experienced by the Curiosity rover as it made its rapid approach and untried sky crane landing on the Martian surface in 2012, scientists at the Jet Propulsion Laboratory in Pasadena, California, had their own tense moment of waiting. They had 20 hours of terror, having to wait until after midnight April 27 to receive the signal that Cassini had survived the plunge it made nearly a full day earlier.

In a packed auditorium on the campus of JPL, employees waited. As the signal from the Goldstone Radio Telescope relayed the strong signal that had traveled more than 70 light-minutes to reach it, the crowd erupted in cheers and applause. It didn’t take long for the first raw images to be received and shared with the world.

Close-up images of the hexagonal polar storm some two Earth diameters across, details of white ammonia clouds speeding above the rest of the atmosphere, and then the stunning data from the Radio Plasma Wave Science detector, were all downlinked.

Instead of the loud pops scientists expected to hear indicative of particle impacts on the RPWS, the recording was virtually silent. Even at the point where the probe passed directly between the gas giant and the main body of the inner ring, the gap sounded nearly empty in stark contrast to the hundreds of particles per second that hit the detector on its ring grazing orbits in 2016.

When Linda Spilker, a ring scientist who is one of the original Cassini scientists, was asked if she had any suspected reasons for the lack dust particles in the ring gap, she responded, “I have no idea.”

Scientists hope the orbits leading up to the final dive in September will provide some answers to this and many other questions about Saturn. Some of those questions include the mass and age of Saturn’s rings, the internal structure of the planet – whether it has a solid core, and if it does, what is it composed of – as well as the actual length of a Saturnian day.

The day was believed to be known from Voyager 2’s visit in August 1981. The length was discovered to be incorrect after Cassini arrived in 2004. Instead, it was found that the number was wrong, and not only was it wrong, but it has continued to change throughout Cassini’s mission.

The hour long periapsis (closest approach) of each of the weekly Grand Finale orbits, happening through mid-September, should allow for this data to be gathered. In addition to the images taken every 17 seconds as it zooms above Saturn’s surface, the vehicle will simultaneously be gathering information from a suite of a dozen science instruments for a whopping 27 different science investigations. This combination should allow the spacecraft to look deeper inside the body of the second largest, yet least dense planet in the Solar System as well as provide a deeper and far more detailed look at the iconic ringed planet.

Video courtesy of JPL



A native of the Greater Los Angeles area, Ocean McIntyre's writing is focused primarily on science (STEM and STEAM) education and public outreach. McIntyre is a NASA/JPL Solar System Ambassador as well as holding memberships with The Planetary Society, Los Angeles Astronomical Society, and is a founding member of McIntyre is currently studying astrophysics and planetary science with additional interests in astrobiology, cosmology and directed energy propulsion technology. With SpaceFlight Insider seeking to expand the amount of science articles it produces, McIntyre was a welcomed addition to our growing team.

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