11/6/98
But how many of these objects actually lurk in the darkness of space?
Mark Showalter's research may help answer that question. A research associate at Stanford's Space, Telecommunications and Radioscience Laboratory (STARLab) who works out of the NASA Ames Research Center, Showalter reports, in the Nov. 6 issue of the journal Science, the first direct and unambiguous detection of basketball-sized meteoroids striking one of Saturn's rings.
Using images taken by the two Voyager spacecraft that flew by the ringed planet in 1980 and 1981, Showalter identified objects that he calls "burst events" on the F Ring, a faint and narrow ring that orbits about 3000 kilometers beyond the outer edge of Saturn's main ring system.
By tracking these bursts long enough to determine that they have a lifetime of about two weeks, he determined that they are clumps of dust kicked out of the ring when it is hit by small meteoroids ranging from two to 40 centimeters in size.
The F Ring has the best conditions for allowing observation of these dust clouds, Showalter says. It is dim enough so that the clumps show up and thin enough so that they do not rapidly collapse back into the ring. That allowed him to follow the evolution of three of these clumps in some detail. Their spreading rates were consistent with that expected if the ring was hit by an object traveling at high velocity, he reports.
Showalter estimates that the F Ring experiences about 20 such impacts per year. This, in turn, allowed him to produce the first crude indication of the abundance of centimeter-sized objects in the outer solar system. When the Cassini spacecraft reaches Saturn in 2004, it will be equipped to do a more thorough job of observing these impacts and so should provide a much more precise estimate, he says.
There are only two other sources of data about meteoroids in the solar system's outer reaches. The Pioneer 10 and 11 spacecraft, which flew by Saturn in the late 1970s, were equipped with a simple detector that provided an estimate of the population of micrometer-sized particles in the vicinity of Jupiter and Saturn. Observations with Earth-based telescopes also have discovered eight asteriods in the 10- to 100-kilometer size range that cross Saturn's orbit. They have been named the Centaurs.
Such research should give scientists a better handle on the number of meteoroids of various sizes that exist in the inner solar system. Objects of this size strike the Earth fairly frequently. They are large enough so that they don't completely burn up in the atmosphere, and the burnt remnant lands on Earth as a meteorite. But they are too small to be picked up in satellite imagery, so the frequency of these fiery collisions is not well known, Showalter says.
"People who model these things estimate that the frequency of meteoroids should be about the same throughout the solar system," he says. And, within the rather large uncertainties involved, his observations are consistent with that assumption.
More importantly, improved knowledge of the abundance of centimeter-sized meteoroids in the solar system will provide better estimates of the numbers of kilometer-sized objects, which are the real concern.
"It is all part of a continuous process," Showalter says. "We know there are so many ping-pong-ball-sized objects for every basketball-sized object and so many basketball-sized objects for every house-sized one. So improved estimates of meteoroids of any given size improve our estimates of the number of meteoroids of all sizes."
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