The recording starts with the patter of a summer squall. Later, a drifting tone like that of a not-quite-tuned-in radio station rises and for a while drowns out the patter. These are the sounds encountered by NASA’s Cassini spacecraft as it dove through the gap between Saturn and its innermost ring on April 26, the first of 22 such encounters before it will plunge into Saturn’s atmosphere in September. What Cassini did not detect were many of the collisions of dust particles hitting the spacecraft as it passed through the plane of the rings. You can hear a couple of clicks,” said William S. Kurth, a research scientist at the University of Iowa who is the principal investigator for Cassini’s radio and plasma science instrument. The few dust hits that were recorded sounded like the small pops caused by dust on a LP record, he said. What he had expected was something more like the din of driving through Iowa in a hailstorm,” Dr. Kurth said. Since Cassini had not passed through this region before, scientists and engineers did not know for certain what it would encounter. Cassini would be traveling at more than 70,000 miles per hour as it passed within 2,000 miles of the cloud tops, and a chance hit with a sand grain could be trouble. The analysis indicated that the chances of such a collision were slim, but still risky enough that mission managers did not send Cassini here until the mission’s final months.