A series of spectacular images produced by state-of-the-art cameras on NASA's Polar spacecraft have confirmed that Earth is being pelted by thousands of small comets each day. These never-before-seen images establish the physical reality of the house-sized "snowballs" that weigh tens of tons, break up as they approach Earth, and deposit large clouds of water vapor in the upper atmosphere.
This image, for example, which was taken on Dec. 31, 1996 by the Low-Resolution Visible Camera aboard the Polar spacecraft, is composed of three consecutive snapshots of a cometary water cloud taken about six seconds a part.
These Earthbound comets, however, represent no threat to either people on the ground or in orbit as these objects disintegrate far above the Earth's atmosphere. "In fact," says Louis A. Frank, the University of Iowa space physicist who first proposed the existence of these small comets more than a decade ago, "this relatively gentle cosmic rain and its possible simple organic compounds may well have nurtured the development of life on our planet."