Fayetteville, Ark. - Ancient Greeks, Romans, Japanese and Egyptians collected rocks that fell from the sky, using them for trade or to adorn tombs. Derek Sears, a cosmochemist in Fulbright College at the University of Arkansas, offers a comprehensive look at the origin of meteorites and their role in the formation of our solar system in "The Origin of Chondrules and Chondrites," released by Cambridge University Press in December.

In the early 19^th century, Greek names were attached to these objects, renaming the minerals in many meteorites "chondrules" and the type of rocks that contain them "chondrites." These "droplets of fiery rain from the sun," as 19^th century scientist H.C. Sorby described chondrules, have long been objects of wonder and curiosity.

Sears focuses on chondrites, the largest group of meteorites, and on the many theories that have evolved about their formation throughout the centuries.

"Meteorites are the primordial material left over from the formation of the solar system. They are the stardust from which the sun and planets formed," said Sears. "We need to know more about the geology of their source, asteroids, and how the meteorites falling on Earth relate to those asteroids."

Sears meticulously traces the theories that researchers have proposed, from the belief meteorites came from the Moon to their being ejected from volcanoes on Earth. In the process, he provides a comprehensive bibliography of the latest research and a valuable reference guide for students and researchers in planetary science, geology and astronomy.

Meteorites contain in them the story of how the solar system evolved. Chondrites, the most common ones, are as old as the solar system, 4.6 billion years. The youngest are the 20 or so meteorites from Mars, a mere 1.5 billion years old.

In the late 18^th century, it was widely believed that meteorites were produced by lightning. Early chemical analysis showed that meteorites were related to each other, regardless of where they fell, but it wasn't until the 1950s that researchers were able to sort meteorites into two groups: those with large amounts of iron and metallic iron and those with low amounts of the metals.

"Many new methods for examining materials cut their teeth on meteorite studies, beginning with the fledgling techniques of wet chemistry in 1802, which were largely perfected by 1834, followed by optical microscopy of geological thin sections in the 1860s to the methods of instrumental analysis of the mid-20^th century," said Sears.

Mass spectroscopy made possible an extremely important new discipline in chondrite studies: researchers could now determine their chronology, or the time at which events occurred. The types of events are as varied as the chemistry and physics of the many available isotopes found in meteorites.

Space missions hold the best hope for further scientific breakthroughs. The Apollo missions of the 1960s and 1970s brought back samples from six sites on the Moon, samples that completely overturned previous notions of the Moon's origin and history.

"Just as lunar samples revolutionized our understanding of the Moon, returned samples from space missions will revolutionize our understanding of chondrites," writes Sears. "The asteroids remained just points of light until they were visited by spacecraft. The first images of asteroids were provided by the Galileo spacecraft on its way through the asteroid belt to Jupiter."

Scientists now realize that impacts are one of the major forces in the evolution of the solar system. Not only does the Moon's surface show evidence for intense early bombardment by massive objects, but the Moon itself may be the product of a massive impact on Earth.

When the solar system formed, impacts of all sizes occurred, some enormous, Sears explained. "Rocks were crashing into each other, fragmenting, dispersing, and making droplets, or chondrules, all of which were reassembling to make other asteroids and - in the right locations - planets. Studying these rocks has prompted all sorts of ideas in the minds of astronomers and geologists, ideas that will drive, in part, the exploration of the solar system by space missions," he said.

Sears has studied meteorites since 1971. In recognition of his work, on April 4, 2001, he learned that an asteroid had been named "4473 Sears" in his honor.

"These are unique and ancient space rocks, as old as the solar system, containing material from other stars. They offer us a connection to deep space," said Sears.