Asteroids

1. Early discoveries

The first asteroid was discovered on January 1, 1801, by the astronomer Giuseppe Piazzi at Palermo, Italy. At first Piazzi thought 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. Because of illness, Piazzi was able to observe the object only until February 11. Although the discovery was reported in the press, Piazzi only shared details of his observations with a few astronomers and did not publish a complete set of his observations until months later. With the mathematics then available, 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, so some astronomers did not believe in the discovery at all. There matters might have stood had it not been for the fact that that object was located at the heliocentric distance predicted by Bode’s law of planetary distances, proposed in 1766 by the German astronomer Johann D. Titius and popularized by his compatriot Johann E. Bode, who used the scheme to advance the notion of a “missing” planet between Mars and Jupiter. The discovery of the planet Uranus in 1781 by the British astronomer William Herschel at a distance that closely fit the distance predicted by Bode’s law was taken as strong evidence of its correctness. Some astronomers were so convinced that they agreed during an astronomical conference in 1800 to undertake a systematic search. Ironically, Piazzi was not a party to that attempt to locate the missing planet. Nonetheless, Bode and others, on the basis of the preliminary orbit, believed that Piazzi had found and then lost it. That led German mathematician Carl Friedrich Gauss to develop in 1801 a method for computing the orbit of minor planets from only a few observations, a technique that has not been significantly improved since. The orbital elements computed by Gauss showed that, indeed, the object moved in a planetlike orbit between the orbits of Mars and Jupiter. Using Gauss’s predictions, German Hungarian astronomer Franz von Zach (ironically, the one who had proposed making a systematic search for the “missing” planet) rediscovered Piazzi’s object on December 7, 1801. (It was also rediscovered independently by German astronomer Wilhelm Olbers on January 2, 1802.) Piazzi named that 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 (in contrast to comets, which are named for their discoverers). The discovery of three more faint objects in similar orbits over the next six years—Pallas, Juno, and Vesta—complicated that 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 that 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. The name asteroid (Greek for “starlike”) had been suggested to Herschel by classicist Charles Burney, Jr., via his father, music historian Charles Burney, Sr., who was a close friend of Herschel’s. Herschel proposed the term in 1802 at a meeting of the Royal Society. However, it was not accepted until the mid-19th century, when it became clear that Ceres and the other asteroids were not planets. 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 (see below Distribution and Kirkwood gaps). 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, and that number grew to 108,066 by the end of the 20th century and more than 1,000,000 in the third decade of the 21st century. The explosive growth was a spin-off of a survey designed to find 90 percent of asteroids with diameters greater than one kilometre that can cross Earth’s orbit and thus have the potential to collide with the planet (see below Near-Earth asteroids).

2. Later advances

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 those elements had been formed by explosions of larger parent asteroids, and he called such groups of asteroids “families.” In the mid-20th century, astronomers began to consider the idea that, during the formation of the solar system, Jupiter was responsible for interrupting the accretion of a planet from a swarm of planetesimals located about 2.8 astronomical units (AU) from the Sun; for elaboration of this idea, see below Origin and evolution of the asteroids. (One astronomical unit is the average distance from Earth to the Sun—about 150 million km [93 million miles].) About the same time, 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 those asteroids. At first that 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. 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, those objects went from being merely “minor” planets to becoming small worlds in their own right. The discussion below follows that progression in knowledge, focusing first on asteroids as orbiting bodies and then on their physical nature.

3. Spacecraft exploration

The first asteroid studied during a flyby was Gaspra, which was observed in October 1991 by the Galileo spacecraft en route to Jupiter. Galileo’s images, taken from a distance of about 5,000 km (3,100 miles), established that Gaspra, an S-class asteroid, is an irregular body with dimensions of 19 × 12 × 11 km (12 × 7.5 × 6.8 miles). Nearly two years later, in August 1993, Galileo flew by (243) Ida, another S-class asteroid. Ida was found to be somewhat crescent-shaped when viewed from the poles, with overall dimensions of about 56 × 15 km (35 × 9 miles), and to have a mean density of about 2.6 grams per cubic cm. After Galileo had passed Ida, examination of the images it took revealed a tiny object in orbit about the asteroid. Indirect evidence from as early as the 1970s had suggested the existence of natural satellites of asteroids, but Galileo provided the first confirmed instance of one. The moon was given the name Dactyl, from the Dactyli, a group of beings in Greek mythology who lived on Mount Ida in Crete. In 1999 astronomers using an Earth-based telescope equipped with adaptive optics discovered that the asteroid (45) Eugenia likewise has a moon. Once the orbit of an asteroid’s moon has been established, it can be used to derive the density of the parent asteroid without knowing its mass. When that was done for Eugenia, its density turned out to be only 1.2 grams per cubic cm. That implies that Eugenia has large voids in its interior, because the materials of which it is composed have densities greater than 2.5. The first mission to rendezvous with an asteroid was the Near Earth Asteroid Rendezvous (NEAR) spacecraft (later renamed NEAR Shoemaker), launched in 1996. The spacecraft entered orbit around (433) Eros, an S-class Amor asteroid, on February 14, 2000, where it spent a year collecting images and other data before touching down on Eros’s surface. Prior to that, spacecraft on the way to their primary targets, or as part of their overall mission, made close flybys of several asteroids. Although the time spent close enough to those asteroids to resolve them was a fraction of the asteroids’ rotation periods, it was sufficient to image the portion of the surface illuminated at the time of the flyby and, in some cases, to obtain mass estimates.