Ernst Chladni and Rocks from the Sky
Part of the Cosmic Horizons Curriculum Collection.
If the mass or its fragments survive, they fall to the surface. Many people may see the fireball and hear the explosion, but only rarely are people close enough to see a meteorite hit the ground.
For thousands of years, people in all parts of the world had reported such phenomena, which they often regarded as divine portents. The scientifically-inclined suggested that the rocks were lofted by distant volcanoes or hurricanes, or perhaps congealed from atmospheric vapors under the influence of lightning. No one imagined that their origin was outside the Earth. By the late eighteenth century, scientific opinion, reflecting the spirit of the Enlightenment, questioned the credibility of wonders reported by untrained observers. The whole subject of rocks from the sky was dismissed as common superstition.
Then in 1794, the German physicist Ernst Chladni published a small book asserting that masses of iron and of rock actually do fall from the sky, producing fireballs when heated by friction with the air. He concluded that they must be cosmic objects, perhaps debris ejected from planets by explosions or collisions. Reaction to the book ranged from skepticism to ridicule. How could there be rocks in space? Aside from the stars, planets, moons, comets, and perhaps some vapors arising from their atmospheres, everyone knew that space itself was empty. Aristotle and Newton had said so. And yet Chladni was right. Today he is regarded as the founder of meteoritics.
Ernst Florens Friedrich Chladni (1756–1827) was fascinated by science and music from a young age, but was trained as a lawyer at the insistence of his father, a professor of law in Wittenberg, Germany. Later, Chladni was able to pursue scientific research. He discovered a way to make sound waves visible, by sprinkling powder on a plate of metal or glass and rubbing the edge of the plate with a violin bow. The vibrations of the plate made the powder accumulate in symmetrical patterns, now called Chladni figures. These figures revealed the modes of sound wave vibrations in the solid body.
Chladni’s interest in meteorites was stimulated in 1793 by a conversation with Georg Lichtenberg, professor of physics at the University of Göttingen. Lichtenberg had witnessed a fireball and thought that such phenomena might be due to cosmic bodies entering the Earth’s atmosphere. Chladni began his investigation by searching the literature for eyewitness accounts of fireballs and rocks from the sky. During three weeks in the university library, he compiled what he felt were the most reliable eyewitness reports. The list included twenty-four fireballs and eighteen fallen rocks reported from various countries over many centuries. The similarities of these accounts impressed Chladni, whose legal training had prepared him to evaluate eyewitness testimony. He concluded that the witnesses must have been describing a real physical phenomenon.
Chladni found numerous cases in which fireballs were followed by the fall of rock or iron masses to the ground. For example, a lump of iron weighing seventy-one pounds fell from the sky over Croatia in 1751. It was sent to the Imperial Natural History Cabinet in Vienna, together with sworn testimony by seven witnesses in widely separated towns who described a spectacular fireball in the sky and loud explosions.
By analyzing the descriptions of fireballs, Chladni was able to estimate the speeds of the rocks entering the atmosphere. These speeds were enormous, faster than could be produced by the Earth’s gravity alone, and were only possible for objects of cosmic origin. Another piece of evidence was the scorched appearance of the rocks themselves. They had been heated enough to melt their outer layers.
When Chladni published his book in 1794, many scientists immediately dismissed the work because it relied on eyewitness accounts. However, events in the next few years swung the weight of opinion in Chladni’s favor.
Two months after the book was published, a large cloud of smoke appeared in the sky near Siena, Italy. The cloud, sparking and booming, turned bright red and stones fell to the ground. Some of the stones were recovered and descriptions of the event were published and widely discussed.
A year later a 56-pound rock fell near Wold Cottage, England. Witnesses reported the sound of an explosion from the air. One farmer actually saw the black rock hit the ground only thirty feet away, dousing him in mud.
These and similar events convinced Sir Joseph Banks of the Royal Society, an organization of leading scientists, that an investigation was warranted. He asked Edward Howard, a young chemist, to analyze the chemical composition of the alleged rocks from the sky. Howard read Chladni’s book and other accounts and began acquiring samples of the stones and iron masses. Working with the French mineralogist Jacques-Louis de Bournon, he made the first thorough scientific analysis of meteorites. The two scientists found that the stones had a dark shiny crust and contained tiny “globules” (now called chondrules) unlike anything seen in terrestrial rocks. All the iron masses contained several percent nickel, as did the grains of iron in the fallen stones. Nothing like this had ever been found in iron from the Earth. Here was compelling evidence that the irons and rocks were of extraterrestrial origin. Howard published these results in 1802.
Meanwhile, the first asteroid, Ceres, was discovered in 1801, and many more followed. The existence of these enormous rocks in the solar system suggested a plausible source for the meteorites. Space wasn’t empty after all.
Finally, in 1803, villagers in Normandy witnessed a fireball followed by thunderous reverberations and a spectacular shower of several thousand stones. The French government sent the young physicist Jean-Baptist Biot to investigate. Based on extensive interviews with witnesses, Biot established the trajectory of the fireball. He also mapped the area where the stones had landed: it was an ellipse measuring 10 by 4 kilometers, with the long axis parallel to the fireball’s trajectory. Biot’s report persuaded most scientists that rocks from the sky were both real and extraterrestrial. The science of meteoritics, the first-hand study of samples from other worlds, was finally launched.