When Voyager 1 and 2 flew past Saturn in 1981, the spacecrafts photographed three billowing clouds of dust streaming from one of the planet's outer rings. These puffs went unnoticed among the many other details that fascinated astronomers at the time. But now a scientist is calling these photos the first unambiguous proof of basketball-sized meteoroids in outer space. He estimates that the main rings of Saturn are probably bombarded by a rock of this size every second.
Until now, astronomers knew only the abundance of the very largest and the very smallest meteoroids, says Stanford space scientist Mark Showalter, whose findings appear in the latest issue of Science. Eight meteoroids 20 to 100 kilometers in diameter, called the Centaur objects, have been spotted in the Saturn system by telescope. At the other size extreme, the Pioneer 11 mission to Saturn tallied the dust grains it encountered. But objects ranging in size from peas to basketballs--the size of objects that may erode moon surfaces over millions of years to form rings--are too small to be seen by Earth-based telescopes and too rare to run into a spacecraft.
An ideal detector for these rocks is Saturn's dusty F ring, which lies just outside the main rings visible from Earth. When a basketball-sized chunk of ice or rock plows into the F ring, it is pulverized and the shrapnel spreads out for kilometers. This means that the fleeting clouds are produced by meteoroid impacts, rather than the slower bumping of objects within the ring, says Showalter. "The most telling thing is how fast they appear and how fast they spread." Because the F ring is so faint, the dust clouds are much more visible there than they would be in the main rings.
Now that scientists know what to look for, they should be able to spot many more impacts when the Cassini mission arrives at Saturn for a 4-year stay in 2004. "This opens a window into a size range [of meteoroids] we have almost no information about," says planetary scientist Douglas Hamilton of the University of Maryland, College Park. "Kilometer-sized objects might have killed the dinosaurs," adds space physicist Mihaly Horanyi of the University of Colorado, "but centimeter-sized objects are important bits and pieces to understand how the solar system works," such as piecing together the formation of planetary rings.