September 1, 2020
A thorough guide to rocks that fall from space. Meteorites don't exist in space, explains British geologist Gregory in his first book; that term is what scientists call the object once it hits the ground. In space, it's a meteoroid. Streaking across the sky at night, it's a meteor or shooting star. Readers who assume the author is describing an exotic phenomenon will quickly learn their error. Roughly 40,000 tons of extraterrestrial material fall to Earth every year. Most is "cosmic dust" that rains down unseen, but about 60,000 "finds" have been discovered and moved to museums and private collections, and a small army of scientists and entrepreneurs is scouring the planet for more. Most originate from the innumerable small bodies in the asteroid belt between Mars and Jupiter, the result of collisions. The fragments drift through space for between 100,000 and 30 million years before arriving on Earth. Sometimes a piece is large enough to produce a spectacular show and considerable damage. A fragment perhaps 20 km. in diameter that struck 66 million years ago decimated entire ecosystems and wiped out the dinosaurs. Most consist of stone or a mixture of stone and iron; a few are almost pure iron. So far, none have contained materials unknown to science, and they are sources of priceless knowledge. Many asteroid stones remain unchanged since whirling clouds of dust formed the solar system 4.6 billion years ago--and it was from these stones that scientists determined the age of the Earth. As the molten Earth cooled, iron and other heavy metals mostly sunk to its core out of reach, but iron meteorites provide a sample. Along with long descriptions of hard science, Gregory also explores the inevitable pop-science questions. Stories of humans killed by a falling meteor are unconvincing. Evidence for animals killed is weak, but meteors have definitely struck houses and bruised their occupants. A solid education that is so detailed it will appeal mostly to amateur astronomers and geologists.
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