Science: Sprinkling Stardust

Astronomers used to think that only good-sized meteorites reach the earth intact, while the smaller ones “burn” to vapor on passing through the atmosphere. But Dr. H. E. Landsberg at the U.S. Weather Bureau had another idea. He smeared some microscope slides with glycerin and exposed them on a mountaintop just before a shower of “Giacobinid” meteors* was expected. Before and during the shower, he caught nothing unusual. But for many days after the shower he caught highly magnetic particles unlike anything found in normal dust-catches.

Dr. Landsberg argued that his particles were bits of iron-rich material arriving from space with the meteors, but he could not prove it. For one thing, many of them were sharply wedge-shaped. They did not look as if they had received the terrific heating that meteorites usually get.

This week Harvard’s Dr. Fred L. Whippie explained to a Rochester meeting of the National Academy of Sciences how Landsberg’s “micro-meteorites” may have slipped through the atmosphere without getting hot at all.

The speed of Giacobinid meteors is rather accurately known: 23 kilometers (14.3 miles) per second. Working theoretically, Whipple figured out what would happen to a very small particle hitting the thin top of the atmosphere at this speed. He decided that if the particle were small enough, about 4 microns (.000156 in.) in diameter, the heat generated by its friction with the air would be carried away (by radiation and other effects) without heating the particle. The “critical size” that he calculated theoretically was close to the actual size of Landsberg’s particles. This is strong evidence, said Whipple, that Landsberg’s particles really came from space and passed through the atmosphere unscathed.

Whipple urged his listeners to keep on the lookout for micro-meteorites. Large meteorites that hit the earth as flaming “fireballs” are believed to be part of the solar system. A few of the micro-meteorites floating gently down, said Whipple, may be the only specimens available of “interstellar solids.” Geologists should look for micro-meteorites, Whipple said, in places like the Cretaceous chalk beds. Many astronomers believe that a planet blew up fairly recently; if so, there ought to be strata rich in its micrometeorite fragments to date the explosion.

-Recurrent meteors associated with a comet named for Astronomer Etienne Giacobini.