Little dust between Saturn and rings

News from Cassini is surprising, scientists say

LOS ANGELES — What lies in the narrow gap between Saturn and its rings?

According to new data from Cassini, the answer is — not much.

NASA scientists are reporting that the Cassini spacecraft has encountered less dust than expected in this formerly unexplored region.

This is great news for the mission’s engineers, who worried that even a small piece of dust the size of a grain of sand could damage one of the spacecraft’s instruments as it made its first dive through the region last week.

For scientists, however, the nearly complete lack of detectable dust particles came as a head-scratching surprise.

“I was kind of disoriented when we first saw the data,” said William Kurth, a physicist at the University of Iowa and the team leader on Cassini’s radio and plasma wave science instrument. “Typically we can pick out a cross through the ring plane as a very bright feature in our data due to the way the dust impacts affect the instrument. This time we didn’t see that.”

Computer models of the space between Saturn and its rings suggested it would be relatively dust-free, but computer models can be wrong.

And as Kurth pointed out, every other time Cassini has passed through even the faintest parts of the ring plane where Cassini’s cameras showed there would be no dust, his instrument still picked up a dust signal.

“Even though it looks like clear zones between these faint rings there are still particles there,” he said.

Kurth said it is still possible that there is some dust in this region, but it would have to be smaller than what his instrument can pick up. That means the dust particles, if they are there, would be even smaller than the particles in cigarette smoke.

“You might think of it as very large molecules,” he said.

Scientists will know more after Cassini completes its second dive through the gap between Saturn and its rings, which was to occur Tuesday.

This time around the spacecraft’s cosmic dust analyzer will take measurements of the dust environment. It can detect much smaller particles than its radio and plasma wave instrument, Kurth said.

Why wasn’t the cosmic dust analyzer tasked with doing this job the first time around? Well, that’s because mission planners didn’t want it to get destroyed by an unexpectedly large piece of dust.

The radio and plasma wave science instrument sticks out of the side of the spacecraft and there was no way to protect it during Cassini’s first pass through the gap. However, most of the spacecraft’s other instruments, including the cosmic dust analyzer, were shielded by Cassini’s 13foot-wide antenna during that first dive.