Asteroids—the midgets of the solar system

Press, 23 July 1986, Page 28

Asteroids—the midgets of the solar system

The solar system includes a very large number of small planets, termed asteroids or minor planets. The largest has a diameter of 955 km, and the smallest is a mere rocky fragment. They move, like the other planets, in orbits around the Sun. A German astronomer, Johann Bode, published in 1772 a number series which roughly coincided with the average distances of the then known planets. Uranus was discovered in 1781 and seemed to fit this scheme of distance from the Sun, although not perfectly. Bode’s Law, as it was termed, showed that there was a gap between Mars and Jupiter in which a planet was missing. Baron von Zach, another German astronomer, organised a group to look for the missing planet. This group dubbed

themselves the “celestial police.” However, before they had any success an Italian astronomer, Giuseppe Piazzi, who did not belong to the group, discovered on January 1, 1801, the asteroid Ceres. The “celestial police” soon found three more asteroids. No more were found until 1845, when a fifth was located. Thereafter discoveries were made frequently so that by 1890 300 had been found. These early searches were made visually by comparing star maps with the actual sky. This was time-consuming, but with the advent of photography discoveries soon showed that there were hundreds of these midgets. It is estimated that 50,000 are within the range of the giant telescope at Mount Palomar. Something like 5000 have been discovered, of which about half have precisely determined orbits. The remainder have been observed once and lost because of the lack of further observations. The orbits of asteroids divide them into three main groups. The great majority belong to what is known as the Main Belt with periods of between about two and six years. They move in slightly elliptical orbits between Mars and Jupiter. The Apollo and Amor Group have highly inclined elliptical orbits. At aphelion they lie within the main belt, but at perihelion they are within the inner planets. Some of them can therefore come very close to the Earth. Typical examples are Eros and Icarus. There is little doubt that asteroids from this group have hit the Earth in the past. There is always the chance that one may do so in the future. The third group, the Trojan asteroids, move in the same orbit as the giant plant Jupiter. Only a few are known but it is believed several hundred exist oscillating around two points that form equilateral triangles with Jupiter and the Sun. Those that are known tend to be larger than the average, dark and elongated in

shape. It is thought that they are the result of collisions between large asteroids. Asteroids are unevenly spaced mainly because of the powerful gravitational pull of Jupiter. Gaps occur at precise fractions of Jupiter’s orbital period. Any body within these gaps would be repeatedly subjected to the gravitational pull of Jupiter and swept into a different orbit. Apart from the division into three groups asteroids have been divided into 10 families, some of which are compact and others well dispersed. The compact families are believed to have been formed as the result of collisions between two larger asteroids. The others have probably suffered collisions at widely different epochs. It is obvious that collisions have played a large part in producing the horde of fragments we see today. At one stage it was thought minor planets resulted from the breakup of a normal planet. The general view today is that at the time the Solar System formed a cloud was located between Mars and Jupiter, which condensed into a number of separate bodies of varying sizes. The number of these original asteroids is unknown, but could be as many as 30 or less. Collisions between these created more separate bodies and as their number increased more and more fragments were produced by more frequent collisions. The larger asteroids, such as Ceres, Pallas, Vesta and possibly a few more may be members of the original asteroids that formed out of the cloud. They tend to have retained a spherical shape. On the other hand asteroids of the Apollo and Amor group may be the remains of short-period comets. Their highly eccentric and inclined orbits suggest such an origin. Most minor planets are dark bodies, and therefore, poor reflectors of sunlight. Many show marked variations in brightness, implying that they are irregular in

shape, which is what one would expect if they had suffered collisions. Some have shapes that resemble those of the two moons of Mars, which are probably captured asteroids. Others have peculiar shapes which have been variously described as a tumbling brick; a rotating spindle or a squashed potato. Their composition appears to be very similar to those of meteorites, being either carbonaceous, stony or stony-iron. The stony types have diameters ranging from 100 km to 200 km, whereas the carbonaceous type range in size from large to small. They are also mainly found in the outer parts of the asteroid belt. Asteroids can provide information on the history of the solar system. However, observing them is not popular with astronomers since this largely consists of obtaining accurate positions so that orbits can be accurately determined. A husband and wife team, Alan Gilmore and Pam Kilmartin, at the Mount John Univer-

sity Observatory, have established an international reputation for their work in this field, in the course of which they have also discovered a few asteroids. The display of bright naked eye planets continues in the evening skies of August. Venus becomes even brighter in the western evening sky as it climbs higher. By August 31 it will be setting about 10 p.m. Mars, in Sagittarius, commences to fade but is still a very bright object, visible all night. At 4 a.m. on August 17 it will be half a degree south of the Moon, but we shall not see it occulted. Jupiter will be visible almost all night. It is in Aquarius, and at 11 p.m. on August 21 will be just over Ideg. north of the Moon. Saturn, in Scorpios, will also be in a good position for evening viewing, setting this month after midnight. Antares, the bright red star in Scorpios, will be less than Ideg. south of the Moon at 11 p.m. on August 14.