Cassini brings back shocking data from Saturn's rings, Nasa announces

There is very little dust in the narrow gap between Saturn and its rings, leading scientists to call it 'the big empty'

Scientists have received puzzling and shocking data back from the first part of Cassini's Grand Finale.

The spacecraft just completed its first flight through the narrow gap between Saturn and its rings. And it came back with surprising information: that the gap itself is free of almost anything at all, according to the data it gathered as dove through.

The news is a delight for Cassini's engineers, since it will be much easier and safer to manoeuvre the craft as it makes the rest of its dives through the region. But it is a puzzle to Nasa's scientists, who had expected there to be far more filling the gap between the rings and the planet.

"The region between the rings and Saturn is 'the big empty,' apparently," said Cassini project manager Earl Maize of NASA's Jet Propulsion Laboratory in Pasadena, California. "Cassini will stay the course, while the scientists work on the mystery of why the dust level is much lower than expected."

If the spacecraft had seen more dust, as expected, it might have meant that engineers would have had to use the saucer-shaped antenna as a shield during its dives, as it did the first time through. That would have meant that Cassini would have been restricted in what observations it could make and how much information it could send back as it did so.

But now the craft will be able to make the 21 more dives without such worry, and find out more about the mysterious gap around Saturn.

Scientists had used images from Cassini and models of Saturn to project that there wouldn't be any large particles in the gap that would pose a danger to the craft. But since nothing had ever flown through the gap before, engineers used the antenna to point in the direction of oncoming particles, helping to shield the delicate instruments.

That meant that the craft couldn't phone home during its dive, meaning that engineers were left in the dark about whether it had done so safely and according to plan. And it meant that some sensors were covered up and couldn't do the science work that they normally would.

However, Cassini's Radio and Plasma Wave Science (RPWS) instrument was poking out the side of the shield and so was able to detect the hits of particles as the craft continued its dive. RPWS picked up a lot of pings as it moved through the area just outside the rings – but picked up very few when it made the first of its Grand Finale dives on the inside of the rings.

"It was a bit disorienting -- we weren't hearing what we expected to hear," said William Kurth, RPWS team lead at the University of Iowa, Iowa City. "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."

Cassini will be heading back through the rings on its next dive this week, in an area very close to where it went last time around.

The dives mark the beginning of the Grand Finale that will conclude when Cassini destroys itself by flying into the atmosphere of Saturn, and burning up. That is being done partly to ensure that the craft doesn't land on a habitable planet and accidentally populate it with our own life.