Heavenly Rocks: Asteroids Discovered and Meteorites Explained

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Heavenly Rocks: Asteroids Discovered and Meteorites Explained

Overview

Giuseppe Piazzi discovered the first asteroid, Ceres, on New Year's Day, 1801, then lost it when it traveled behind the Sun. Luckily, a new and valuable mathematical method, least squares, allowed Ceres to be rediscovered. This success established the reputation of Carl Friedrich Gauss, one of history's greatest mathematicians. Just two years later in 1803, the view of the Solar System was further expanded by a study that forced science to accept the idea that space debris came to Earth. French physicist Jean-Baptiste Biot investigated a rock that fell from the heavens, and his report convinced skeptics that meteorites came from space. By the century's end, meteorites were accepted as extraterrestrial in origin, and the first steps were taken toward proving that craters were on Earth as well as on the Moon.

Background

Giuseppe Piazzi (1746-1826) was an eminent astronomer who directed two prominent Italian observatories in Naples and Palermo. He was not looking for an asteroid (more properly, minor planet) in 1801. Rather, he was doing a systematic study of stars, work that ultimately resulted in his cataloging of the positions of 7,646 stars. His knowledge of an upcoming search for a new planet led him to correctly understand what it was he had accidentally discovered during his search, Ceres (named for the patron goddess of his native Sicily), the largest asteroid. The search, and his conclusion, were both prompted by a numerical coincidence that is still unexplained.

In 1766 J. D. Titius discovered a curious regularity to the placement of the order of the planets. He constructed his series of numbers in this way: Take 0 and add 3, then double the number for each succeeding number in the series so that you get 0, 3, 6, 12, 24, etc. Finally, take each of these numbers, add 4, and then divide by 10. The third number in the series is one. The third planet from the Sun, Earth, by definition, is one astronomical unit (AU) away. The first number in the series, 0.40, is a good match for Mercury (0.39 AU). In fact, the rest of six known planets—Venus, Mars, Jupiter and Saturn —all match up fairly well if you ignore the curious gap at 2.8 AU. Johan Bode (1747-1826), a German astronomer, popularized this formula (now known as Bode's law) in 1772. It might have faded into obscurity, but William Herschel (1738-1822) made a discovery that changed the scientific world.

In 1781 the scientific world was complacent. Isaac Newton's physics was king, and, for many scientists, it explained the universe. In the view of much of the scientific community, everything that was important had already been discovered. Their jobs involved merely working out the details. Then William Herschel looked at Bode's law and wondered: Was there another planet to discover? The numbers told him where to point his telescope. He didn't find anything at 2.8 AU, but he did find something by looking further out: Neptune, a new planet, and, at 19.2 AU, a better match to its spot in the series of Bode's law than Saturn was.

Herschel's discovery excited the whole scientific community; it quite literally opened up new worlds. Astronomy was no less energized. In 1800 German astronomers under Baron von Zach were organizing to conduct a thorough search for a planet near 2.8 AU. Before they could commence their work, however, Piazzi had announced his discovery.

While looking for the planet, Piazzi spotted what he thought might be an uncharted star because it was a simple point of light. (Asteroids, meaning "star-like," got their name from Herschel because of this appearance.) To Piazzi's surprise, he found that it had moved with his next observation. It wasn't a star, and it wasn't a comet, either. Comets typically appear fuzzy because of their escaping gases. This image was sharp. At the same time, it didn't have the disc shape of a planet. Whatever it was, Piazzi knew it was right where the Germans intended to look, at 2.8 AU. Piazzi reported his findings to the scientific community knowing they would find the object's position to be significant. Unfortunately, he was only able to make a few observations before the astronomical body was lost behind the Sun.

In 1801 no means existed for predicting where Ceres would reemerge from behind the Sun, and it seemed likely that Piazzi would never see it again. Fortunately, the problem caught the interest of one of the most brilliant mathematicians ever. Carl Friedrich Gauss (1777-1855) was still in his early twenties when, from just three recorded observations, he made a prediction of where astronomers would find Ceres. Late in 1801 Baron von Zach and Heinrich Olbers (1758-1840) relocated the minor planet right where Gauss said it would be.

Just two years later in 1803, what was probably another asteroid accounted for another challenge to astronomy. The question in this case wasn't where the rock—a meteorite—was, but where it came from. Reports of rocks falling out of the sky extend throughout recorded history, but the scientific community did not believe these fantastic tales. Even when respected physicist Ernst Chladni (1756-1827), who collected meteorites, suggested they were the debris from an exploded planet, the notion was still considered nonsense. The kindest explanation was that the rocks fell from a short distance, having been thrown up from the Earth by a great force. In 1807 in America eminent Yale chemist Benjamin Silliman (1779-1864) and a colleague reported that they had seen a meteorite fall and found it. Even this strong a testimony met resistance. United States President Thomas Jefferson, an amateur scientist and a committed rationalist, declared that it was easier to believe that two Yankee professors were lying than that rocks fell from the sky.

By then, however, the point had already been won in Europe. In 1803 there were reports of a recent meteorite fall in France. The French intellectual community, fiercely rational, decided to settle the question of the rock's origin once and for all. They sent Jean-Baptiste Biot (1774-1862), one of their best young scientists and a protégé of the powerful and influential astronomer Pierre-Simon Laplace (1749-1827), to listen to the reports and to examine the rock. Biot, going against the flow of the scientific community, declared his finding that the meteorite had indeed fallen from the heavens.

Impact

The discoveries of Piazzi and Biot led to renewed wonder and curiosity about the heavens. Asteroids were different from anything else in the known heavens, and within a few years three more had been discovered, two of them by Heinrich Olbers. Today we know there are thousands of asteroids, mostly in the asteroid belt at about 2.8 AU. This belt became one of the prime points of evidence that caused the scientific community to accept the theory for which Laplace is perhaps most famous, the nebular hypothesis for the origin of the Solar System. By this hypothesis, a rotating cloud of matter cooled and contracted to form the Sun and its planets. The belt represents matter from this earlier stage, and now it is generally believed that the asteroids did not come together because of the disruptive gravitational forces of its huge neighbor, Jupiter.

Asteroid hunting captured the public imagination during the nineteenth century—300 were found before the advent of photographic astronomy in 1891. One of the oddest "finds" was by Frederic Petit. In 1846 he claimed to have discovered Earth's second moon, a captured asteroid. The details of the report were somewhat absurd, with a regular close approach to Earth by the asteroid of just 7 mi (11.4 km). Petit was tenacious. Fifteen years later he was using his second moon to explain anomalies in Lunar motion. Most scientists ignored him, but Jules Verne did not. In Verne's 1865 novel, From the Earth to the Moon, Petit's asteroid makes a frighteningly close approach to the heroes and becomes a reference point that allows them to determine their position.

Mathematical prediction seemed to take a giant step forward with the second success of Bode's law, but this triumph was short lived. Astronomers hunted near 38.8 AU for the next planet in the series in vain. In 1846, after precisely calculating the orbit of Uranus and looking at how the actual orbit deviated, Urbain Leverrier (1811-1877) found Neptune at 30.06 AU. This planet was too close to the Sun and too large to explain away, and it broke the series. To date there is no explanation for the success (or failure) of Bode's law, and it is largely considered to be merely a numerical coincidence.

Perhaps the largest impact of the fresh look at the heavens at the beginning of the nineteenth century was the world's discovery of a genius, Carl Friedrich Gauss. His creation of the method of linear least square alone has had a profound effect on all observational science. Not only has it provided a convenient and reliable way of reducing error in natural sciences, such as chemistry and physics, but it has become one of the standards for assessing the legitimacy of observations in the social sciences, such as psychology and anthropology.

The work on Ceres also helped Gauss to continue his career after his sponsor died in 1807. His contribution to astronomy had enhanced his reputation and helped win him a position as professor of astronomy and director of the observatory at the University of Göttingen even though his nation was at war. He did not disappoint his sponsors. He went on to show how probability could be represented as a bell-shaped curve and to make contributions to geometry, the understanding of fluids, gravitation, and magnetism.

Biot's report on meteorites meant that it was possible, without traveling to other planets or collecting moon rocks, for scientists to chemically analyze extraterrestrial material and get a better understanding of the Solar System. Meteorites also brought into question the origin of craters. Craters on the Moon (and on Earth) had always been assumed to be of volcanic origin. In 1873 Richard Proctor was the first to suggest that the craters on the Moon came from bombardment of rocks, an idea that quickly gained broad acceptance. The first study of an Earth crater, Barringer Meteor Crater in Arizona, began just before the end of the nineteenth century. Oddly enough, the original work indicated that the crater did not originate with a meteorite. However, early in the twentieth century this position was reversed.

Biot's report changed more than the scientific community's view of meteorites. It helped to open scientists up to the reports of non-scientists, which, among other things, has led to the discovery of new medicines and new animal species.

PETER J. ANDREWS