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 the gap between Saturn and its innermost ring on April 26, the first of 22 such encounters before it will plunge into atmosphere in September. What Cassini did not detect were many of the collisions of dust particles hitting the spacecraft it passed through the plane of the ringsen the charged particles oscillate in unison.