Cassini Just Dove Into ‘The Big Empty’ Between Saturn and Its Rings

Cassini's first dive through the gap between Saturn and its rings didn't just send back some great views of the planet's upper cloud deck, rings, and storms. It also transmitted some mysterious data that has left mission scientists at NASA's Jet Propulsion Laboratory puzzled.

Scientists expected the spacecraft, which is exploring Saturn and its rings, would encounter dust in the 2000-kilometer-wide gap between the planet and its rings. But it encountered no particles bigger than what you'd find in smoke. They measured the dust density by the creative use of an instrument intended to detect radio and plasma wave signals.

"The region between the rings and Saturn is 'the big empty,' apparently," said Cassini program manager Earl Maize in a statement. "Cassini will stay the course, while the scientists work on the mystery of why the dust level is much lower than expected."

Cassini has been exploring Saturn and its gorgeous planetary rings for the last 13 years. The venerable probe has already captured incredible scenes of the large gas giant, and taken up-close pictures of the shrouded moon of Titan (which was explored by the Huygens lander), as well as the towering geysers of Enceladus. Now it's performing a Grand Finale, 22 orbits that bring it closer than ever to Saturn's surface, ending in a fiery death-dive into the planet.

Cassini's mission was intended to study the unseen aspects of the solar system's most striking planet, including its magnetic field and the radio signals it emits. Using the spacecraft's instruments, they surmised the space between Saturn and its rings were squeaky clean.

For a little refresher, while Saturn's rings look like a smooth dinner plate or vinyl LP, they're actually made up of particles of ice and dust in a range of sizes. Some are as big as mountains, while others can be tiny flecks of ice. Although the gap close to the planet obviously has fewer particles, JPL scientists weren't taking any chances that these particles could take out Cassini, travelling at 124,000 kilometers per hour over the clouds of Saturn.

Read More: We Have Strong Evidence Saturn's Moon Enceladus Can Support Microbial Alien Life

They had good reason to be cautious: previous close interactions with the ring plane had dust show up as audible cracks and pops in the data, as tracked by a device called the Radio and Plasma Wave Science (RPWS) instrument, also used this time around.

To safeguard the spacecraft, they rotated it so that Cassini's main 4-meter-wide communications dish could be used as a protective shield during the crossing of the gap. Luckily, they could still check the particle density, as the RPWS instrument peeks out from behind the dish. Any impact from dust would make some noise.

Here's what they heard last time they pulled a similar maneuver:

And this is what they got this time:

"I was a bit disoriented," said Bill Kurth, lead investigator for Cassini's radio and plasma wave science instrument. "In most of our data, the ring plane stands out as a marker or signpost. It wasn't there for this pass. Hence, we spent a very long night analyzing the data, looking at the data with various tools, and then discussing what we had seen."

Kurth said in a statement he could count on one hand the times the data registered a dust particle hit the RPWS instrument. The team focused on a 20-minute span of when the spacecraft cleared the ring plane, out of an observation period of roughly 10 hours.

The Cassini team might have room for different maneuvers during the Grand Finale, as they don't need to account for a rain of pebbles every time they'll be passing through the ring-plane of the gap.

Even in its final days, the Cassini probe keeps surprising us.

Subscribe to Science Solved It, Motherboard's new show about the greatest mysteries that were solved by science.