Meteors and Meteorites
Meteors, commonly (and inaccurately) called shooting stars, are particles of interplanetary dust that burn up as they enter the Earth's atmosphere from space. Meteorites are larger pieces of interplanetary matter that are too large to burn up entirely and therefore survive the passage through the atmosphere to the surface of the Earth.
Although these objects attracted the attention of early astronomers, their origin was anything but clear. The Greek philosopher Aristotle speculated that they were an atmospheric phenomenon akin to lightning. Many people believed the stones that occasionally "fell from the sky," accompanied by spectacular flames, had a supernatural origin or were thrown into the air by exploding volcanoes. At the close of the 1600s, neither shooting stars nor objects falling from the sky could be explained.
In 1714, the English astronomer Edmond Halley finally shed some light on the origin of meteors. Halley examined the various reports of bright meteors that observers had seen, and he was able to calculate both the altitude and the velocity of each object, coming to the conclusion that each must have come from outside the earth's atmosphere. His calculations, though painstaking, were not widely accepted.
Beginning in the mid-1700s, a rash of spectacular meteorites attracted the attention of a number of scientists. In 1751, a large fireball seen over Germany crashed to earth in Croatia, leaving a seventy-one-pound chunk of iron. Geologist Andreas Stütz conceded that the object may have fallen from the sky but insisted it was caused by some unexplained electrical phenomenon in the atmosphere. When a group of stones fell over France almost forty years later, an intrigued German physicist, G. F. Lichtenberg, assigned his assistant, E. F. F. Chladni (1756-1827), to look into the matter. After a painstaking study of two centuries of records, in 1794 Chladni concluded that the chunks of iron found where fireball sightings occurred were, indeed, of cosmic origin, possibly the remnants of a disintegrated planet. On the afternoon of April 26, 1803, another sensational fireball appeared over France, accompanied by loud explosions. Two to three thousand stones of various sizes then showered the French countryside.
Spurred into action by this remarkable event, the Academy in Paris decided to send in Jean-Baptiste Biot to gather witnesses' reports and study fragments of the object. Biot traced its remains over a wide area, running along an axis from southeast to northwest. His report at last convinced scientists that the stones had, in fact, fallen from the sky, but meteorologists and astronomers still debated their origin for many years.
It took a better understanding of the solar system and improvements in technology before the matter could be clarified. Radioactive dating of meteorite fragments indicates that they are roughly the same age as the solar system, while their composition (often a combination of iron and nickel, not unlike the composition of the earth's core) suggests that they are simply chunks of material left over from the formation of the solar system that occasionally collide with the Earth. Some astronomers believe that they may originate in the asteroid belt.
About the same time that Chladni's hypothesis about meteorites was published, two German scientists, Johann Benzenberg (1777-1848) and Heinrich Brandes (1777-1834), were immersed in a study of shooting stars, the streaks of light that we often see cross the sky on clear nights. Basing their observations on a method similar to Halley's, they concluded that meteors did not originate from within the atmosphere but were far above the clouds and weather phenomenon. In addition, the meteors were apparently moving at speeds of several miles per second, close to planetary velocities. This meant that meteors were objects that approach the Earth from space and that the "flash" of a meteor was a result of its burning up as it passed into the Earth's atmosphere.
In the winter of 1833, astronomers had another chance to enhance their understanding of meteors when a shower of thousands of shooting stars occurred in November. The meteors seemed to radiate from a single point in the sky in the constellation Leo. An American astronomer, Olmstead, pointed out that this meant that the Earth was running into the objects as they were in parallel motion, like a train moving into falling rain. A look back into the records revealed that a meteor shower occurred every year in November. It looked as though the Earth, as it orbited the sun, crossed the path of a cloud of meteors every November 17th.
Astronomers realized that another shower in August, in the constellation Perseus, was also an annual event; apparently another group of meteors crossed the earth's orbit. Italian scientist Giovanni Schiaparelli (1835-1910) used this information to fit the final piece into the puzzle. He carefully calculated the velocity and path of the Perseid meteors--so named because they appear to radiate from a point within the constellation Perseus--and found that they circled the sun in nearly parabolic orbits. Since comets follow parabolic orbits, he simply examined catalogs of cometary orbits, finding that the path of these Perseids corresponded to that of a bright comet that had passed the earth in August 1862, and that the Leonids followed the same orbit as Temple's comet of 1866. In other words, the paths of comets and meteor swarms were identical. Most annual meteor showers can now be traced to the orbit of a comet which intersects the earth's orbit.
Today astronomers know that comets leave behind a trail of dust and ice as they circle the Sun. As the Earth passes through some of these clouds on its journey around the Sun, the particles contained in the cloud burn in the upper atmosphere of the Earth, about sixty miles (100 km) in altitude. As they combust, they briefly leave behind a bright trail of ionized gas--the "flash" of the meteor.
Not all interplanetary objects are mere specks of dust. About 22,000 years ago, a chunk of rock about 100 feet wide blasted a 600-foot-deep hole into the desert of northern Arizona. In 1908, hundreds of square miles of forest in the Tunguska region of Siberia were flattened by a devastating explosion now thought to have been caused by a large meteor. And in the 1980s and 1990s, scientists gradually moved toward acceptance of the fact that an unimaginably huge impact 65 million years ago, near what is now the Yucatan peninsula, had caused the extinction of the dinosaurs. Impacts by such large near-Earth objects, or NEOs, became a frequent topic in the news and even in Hollywood scripts. Early in 1998, a public stir was raised by the announcement that a sizable asteroid would pass within 30,000 miles of Earth in 2028, with an impact not out of the question. Improved data revised the distance to a safe 600,000 miles, but the event served as a reminder that not every "shooting star" is innocuous.
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