Arriving at Saturn on June 30, the Cassini spacecraft sped through a 24,730-kilometer gap in the giant planet’s rings. Three hours later, it crossed back in the other direction (SN: 7/10/04, p. 22: Available to subscribers at Titanic Images, Groovy Shots: Cassini arrives at Saturn). These pinpoint crossings helped shape the craft’s orbit about Saturn.
The ring gap that Cassini traversed isn’t entirely empty. It contains bits of dust, which Cassini plowed into at a speed of roughly 20 km per second. The smoke-size dust particles did no harm to the craft, which encountered about 100,000 hits in less than 5 minutes during each of the ring crossings, Cassini investigator Donald A. Gurnett of the University of Iowa in Iowa City announced July 9.
He and his colleagues could determine this number because whenever a dust particle strikes Cassini, the impact produces a puff of ionized gas. An instrument counted as many as 680 of these puffs in a single second during the crossings. Gurnett’s team converted these hits into audible sounds, which “resemble hail hitting a tin roof,” he says. The sounds are posted online (http://science.nasa.gov/headlines/y2004/images/