To space-conscious laymen, the very word meteoroid is heavy with menace. It conjures up visions of rocklike objects streaking the sky as shooting stars, moving at such enormous speeds that a lump as big as a pea could punch a fist-sized hole through any spacecraft. Scientists, who have calculated the probability that a spacecraft and a meteoroid would collide, are less worried than laymen, but even so, they have planned on protecting long-range space vehicles with meteor bumpers. Now it seems that spacecraft will need no such shields. Space is indeed teeming with meteoroids, but most of them are fluffy stuff, harmless as thistledown.
Cosmic Dust. This reassuring news was delivered to the National Aeronautics and Space Administration with the authority of a firsthand report. For seven months, the satellite Explorer XVI orbited earth, inviting meteoroids to hit the instruments that encrusted most of its surface. There were cylinders of thin sheet metal containing helium gas that escaped when they were punctured by a meteoroid. There were instruments that gave an electrical signal when sunlight showed through a puncture hole in plastic film. There were also sensitive microphones that registered 15,000 occasions when something hit them hard enough to make them vibrate.
Only at rare intervals, though, did Explorer XVI collide with anything bigger than a microscopic bit of cosmic dust. There were 44 meteoroids that succeeded in penetrating a sheet of beryllium-copper one-thousandth of an inch thick, which is slightly thicker than household aluminum foil. The most powerful meteoroid encountered knocked a tiny hole in stainless steel three-thousandths of an inch thick. Metal as thick as the wall of a beer can went unpunctured. NASA's tentative conclusion is that the plentiful meteoroids are too small to do harm, and the dangerous ones are too few.
Harvard Astronomer Fred Whipple explained that the low penetrating power of most meteoroids is partly due to their fluffy structure. Even very small meteoroids, Whipple said, are probably loosely bound clumps of much smaller particles. They may be half as dense as water, so when they hit the skin of a spacecraft they spread their effect over a larger area than if they were solid.
Dirty Snowballs. Whipple is the author of the "dirty snowball" theory of comets. He believes comets form, molecule by molecule, out of frozen gases beyond the outermost planets. They pick up bits of dust and start drifting ever so slowly toward the distant sun. When they gather speed as they approach the sun, their surface gets hotter, turning some of the frozen gas to vapor and freeing some of the dust to form the comets' glowing heads and tails. When an old comet disintegrates, it leaves bits of fluff to wander in space.
But for all such intriguing theories, no one really knows much about comets. Even the brightest ones have a central nucleus only a few miles in diameter, too small to be picked out with the biggest telescope. In one of the most imaginative proposals yet, Space Technology Laboratories of Redondo Beach, Calif., plans to send a probe aloft to intercept a comet.