Dr. Clayne Yeates, scientist at NASA's Jet Propulsion Laboratory, has apparently confirmed one of the decade's most controversial astronomical theories -- that millions of small water-bearing comets strike the Earth's atmosphere every year.
However, Yeates said, analysis is continuing in order to rule out other possible explanations for his observations.
The theory was first proposed by Dr. Louis Frank, physicist at the University of Iowa, in 1986. His theory, based on observations of water vapor high up in the atmosphere, stated the cometesimals may have been bombarding Earth since it was formed and may have brought enough water to fill the Earth's oceans.
Frank's evidence was gathered on ultraviolet images made by the Dynamics Explorer satellite from 1981 to 1986. Frank said the frequency of the spots matched the rate at which bright meteors are known to enter the atmosphere.
He concluded the spots were small comets being torn apart by gravity about 1000 miles above Earth, and vaporized by sunlight into water vapor.
Frank's theory and his inferential data were immediately controversial and stimulated considerable research within the scientific community.
Yeates proposed to find out if the cometesimals were directly observable, but he needed telescope with sufficient sensitivity to detect the small comets -- the 36- inch Spacewatch Telescope at Kitt Peak, Ariz.
Yeates, deputy project scientist and science manager of the Galileo Project at JPL, said his calculations showed it would be difficult, but possible, to confirm the presence of the cometesimals by using an observation scheme designed precisely for these objects.
Frank had said the bodies have only about two percent reflectivity of light, about as dark as charcoal, and enter the atmosphere at nine to 10 kilometers per second, or about 22,000 miles per hour. They vary in size but are on average about 10 meters in diameter.
Yeates said the detection rates of the small, dark, fast-moving objects would be impossibly low using standard photographic plates, but the Kitt Peak telescope uses the far more sensitive charged coupled detector, CCD.
Yeates conducted observations during three-month- long period from November of last year through February this year. Streaks on the images have characteristics and frequency of occurance which agrees with predictions based on Frank's theory.
Yeates said he was deeply indebted to Dr. Tom Gehrels, planetary astronomer at the Lunar and Planetary Laboratory at the University of Arizona, who permitted use of the telescope while he was abroad on other business.
Among other scientists who also explored the theory were Thomas Donahue and colleagues at the Space Physics Research Laboratory at the University of Michigan.
They used little-known set of data produced by the Voyager 2 spacecraft as it began its journey to the outer planets in 1977.
The Voyager's ultraviolet spectrometer gathered evidence of hydrogen atoms released from water vapor -- known as Lyman alpha emissions. Donahue said he thought the data would show the emissions were no greater than background levels in the interstellar medium near the Earth.
But the data showed the emissions were much higher than the expected background levels and that large number of small comets were losing their ice as they passed through the inner solar system.
Still, the Michigan team arrived at much lower number of cometesimals striking the atmosphere than the number hypothesized by Frank or calculated by Yeates in his supporting study. The number calculated by Yeates is one- billion times the Mighigan team's number.
Yeates said the analysis is still preliminary and final results would be published in about month.
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