Major Milestones In Asteroid Research

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The first asteroid was discovered on Jan. 1, 1801, by the astronomer Giuseppe Piazzi at Palermo, Italy. At first Piazzi thought that he had discovered a comet. However, after the orbital elements of the object had been computed, it became clear that the object moved in a planetlike orbit between the orbits of Mars and Jupiter.

Owing to illness, Piazzi was able to observe the object only until February 11. As no one else was aware of its existence, it was not reobserved before it moved into the daytime sky. The short arc of observations did not allow computation of an orbit of sufficient accuracy to predict where the object would reappear when it moved back into the night sky, and so it was "lost."

There matters might have stood were it not for astronomers searching for a "missing" planet between Mars and Jupiter during an astronomical conference in 1796. (Unfortunately, Piazzi was not a party to this attempt to locate the missing planet.) In 1801, German mathematician Carl Friedrich Gauss developed a method for computing the orbit of an asteroid from only a few observations.

Using Gauss's predictions, the German Hungarian astronomer Franz von Zach rediscovered Piazzi's "lost" object on Jan. 1, 1802. Piazzi named this object Ceres after the ancient Roman grain goddess and patron goddess of Sicily, thereby initiating a tradition that continues to the present day—asteroids are named by their discoverers.

The discovery of three more faint objects (at least when compared with Mars and Jupiter) in similar orbits over the next six years—Pallas, Juno, and Vesta—complicated this elegant solution to the missing-planet problem and gave rise to the surprisingly long-lived though no longer accepted idea that the asteroids were remnants of a planet that had exploded. Following this flurry of activity, the search for the planet appears to have been abandoned until 1830, when Karl L. Hencke renewed it. In 1845 he discovered a fifth asteroid, which he named Astraea.

There were 88 known asteroids by 1866, when the next major discovery was made: Daniel Kirkwood, an American astronomer, noted that there were gaps (now known as Kirkwood gaps) in the distribution of asteroid distances from the Sun. The introduction of photography to the search for new asteroids in 1891, by which time 322 asteroids had been identified, accelerated the discovery rate. The asteroid designated (323) Brucia, detected in 1891, was the first to be discovered by means of photography. By the end of the 19th century, 464 had been found; this grew to more than 100,000 by the end of the 20th century

Bode's Law

Bode's law, also called the Titius-Bode law, is an empirical rule giving the approximate distances of planets from the Sun. It was first announced in 1766 by the German astronomer Johann Daniel Titius but was popularized only from 1772 by his countryman Johann Elert Bode. Once suspected to have some significance regarding the formation of the solar system, Bode's law is now generally regarded as a numerological curiosity with no known justification.

One way to state Bode's law begins with the sequence 0, 3, 6,12, 24,... in which each number after 3 is twice the previous one. To each number is added 4, and each result is divided by 10. Of the first seven answers—0.4,0.7,1.0,1.6,2.8,5.2,10.0—six of them (2.8 being the exception) closely approximate the distances from the Sun, expressed in AU), of the six planets known when Titius devised the rule: Mercury, Venus, Earth, Mars, Jupiter, and Saturn. At about 2.8 AU from the Sun, between Mars and Jupiter, the asteroids were later discovered, beginning with Ceres in 1801. The rule also was found to hold for the seventh planet, Uranus, which lies at about 19 AU, but it failed to predict accurately the distance of the eighth planet, Neptune (30 AU), and that of Pluto (31.5 AU), which was regarded as the ninth planet when it was discovered in 1930.

and to more than four times that number by 2009. This explosive growth was a spin-off of a survey designed to find 90 percent of asteroids with diameters greater than 1 km (0.6 miles) that can cross Earth's orbit and thus have the potential to collide with the planet..

Later Advances in Asteroid Studies

During much of the 19th century, most discoveries concerning asteroids were based on studies of their orbits. The vast majority of knowledge about the physical characteristics of asteroids— for example, their size, shape, rotation period, composition, mass, and density—was learned beginning in the 20th century, in particular since the 1970s. As a result of such studies, these objects went from being merely "minor" planets to becoming small worlds in their own right.

In 1918 the Japanese astronomer Hirayama Kiyotsugu recognized clustering in three of the orbital elements (semimajor axis, eccentricity, and inclination) of various asteroids. He speculated that objects sharing these elements had been formed by explosions of larger parent asteroids, and he called such groups of asteroids "families."

In the mid-20th century, calculations of the lifetimes of asteroids whose orbits passed close to those of the major planets showed that most such asteroids were destined either to collide with a planet or to be ejected from the solar system on timescales of a few hundred thousand to a few million years. Since the age of the solar system is approximately 4.6 billion years, this meant that the asteroids seen today in such orbits must have entered them recently and implied that there was a source for these asteroids. At first this source was thought to be comets that had been captured by the planets and that had lost their volatile material through repeated passages inside the orbit of Mars. It is now known that most such objects come from regions in the main asteroid belt near Kirkwood gaps and other orbital resonances.

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