The Titius-Bode lair predicted that there should be a planet between Mars and Jupiter about 2.8 astronomical units from the Sun. In 1800 six astronomers, with Johann Schroter as president and Baron von Zach as secretary, worked out a scheme jointly to search for this missing planet. However, before their plans could be put fully into effect, another astronomer, G.Piazzi, discovered a planet on 1 January 1801 while he was compiling a star catalogue. The starlike object moved from night to night and Piazzi took it for a tail-less comet; however, he made enough observations for an orbit to be worked out. This showed that the object was not a comet, but a planet 2.77 astronomical units from the Sun, which is very close to the predicted 2.8. Piazzi named the planet CERES, after the patron goddess of Sicily where he was director of the Palermo observatory.

Ceres was found to be a very small body, about 400km in radius and hardly seemed worthy of the name planet. The search for a missing planet continued and in March 1802 H.Olbers discovered another object, now called PALLAS, with a size and orbit very similar to those of Ceres. Olbers suggested that the two bodies may have formed when a larger object met with some disaster. This idea suggested the existence of other similar objects and Karl Harding discovered JUNO in 1804 and Olbers found VESTA in 1807. These two resembled Ceres and Pallas and the four became generally known as MINOR PLANETS or ASTEROIDS. The next asteroids to be found were Astraea in 1845 and Hebe in 1847, both discovered by K.L.Hencke. J.R.Hind found another two hi. 1847 and not a year has passed since then without at least one discovery. Vesta is the brightest asteroid, as seen from the Earth, and can sometimes be seen with the naked eye. Several others are within the range of binoculars and small telescopes.

When an asteroid has been sufficiently observed for a satisfactory orbit to be worked out it is given a number; these numbers there¬fore roughly give the order of discovery. There is also a temporary nomenclature which is used until a permanent number can be assigned. Since 1 January 1925 this has comprised the year of discovery followed by a pair of letters. The discoveries of January 1 -15 are AA, AB, etc., of January 16-31 BA, BB, etc., up to December 16 – 31 when YA, YB, etc. are used (the letter J is not used). At the time of writing almost 2000 asteroids have been given permanent numbers and about another thousand still have only temporary nomenclature. In general these have been seen for a short period only, much less than an orbital period, and then never seen again. Asteroids are only named officially (generally by the discoverer) when they receive a permanent number.

The early asteroids were given dignified names from mythology but in time the supply of these names became exhausted and discoverers have resorted to other sources. Some are quite odd, such as 724 Hapag, the initials of a German navigation line, the Hamburg Amerika Paketfahrt Aktien Gesellschaft, and 694 Ekard. Ekard is Drake spelt backwards and was named by two members of Drake University in the USA.

Systematic searches have been made for asteroids on two occa¬sions: the McDonald survey from 1950 to 1952 and the Palomar-Leiden survey in 1960. From these surveys the number of asteroids brighter than photographic magnitude 21.2 at mean opposition has been found to be near 500 000. It has been estimated that the total mass of all these asteroids is only 2.4 X 1021kg, which is 0.0004 of the Earth’s mass. The largest asteroid, 1 Ceres, accounts for nearly half of this mass.

Nearly all asteroids have orbits with a mean distance from the Sun between 2.17 and 3.3 astronomical units which .places them between the orbits of Mars and Jupiter. The lower limit appears to be due to the influence of Mars. Asteroids in smaller orbits would pass sufficiently close to Mars for their orbits to be significantly perturbed and the region closer to the Sun than 2.17 astronomical units has been almost entirely swept clean.

The distribution of asteriods with distance from the Sun, and this shows clearly that there are very few asteroids at certain distances: these are the so-called KIEKWOOD GAPS, named after their discoverer (1866). At these gaps the orbital periods of the asteroids and Jupiter are in a simple ratio (i.e. they are in RESONANCE or are COMMENSURABLE). For example, at the 5:2 resonance five circuits of an asteroid’s orbit take the same time as two of Jupiter. The gaps at the 2:1,3:1 and 5:2 resonances are particularly prominent but there are several others. The 2:1 resonance in fact marks the outer limit of the main asteroid belt at 3.3 astronomical units from the Sun. An asteroid that was hi one of the Kirkwood gaps would repeatedly pass Jupiter at the same small number of points in their orbits and the perturbations that it received would normally move it quickly into a different orbit. This does not always happen, however, and it is possible for an asteroid to become locked into an orbit at one of these distances under rather special circumstances. The most important of these asteroids are the TROJAN ASTEROIDS which have the same mean distance from the Sun and the same orbital period as Jupiter (the 1:1 resonance). There are two groups of Trojans, the ACHILLES GROUP whose members move an average of 60° ahead of Jupiter, and the PATROCLUS GROUP 60° behind . 588 Achilles was the first such asteroid found (1906) and subsequent discoveries were given names of other combatants in the war between Greece and Troy and the whole group came to be known as the Trojans. Special surveys have shown that there are about TOO asteroids brighter than mean opposition magnitudo 120.9 in the preceding Achilles group but only about half that number in the following Patroclus group. The outer satellites of Jupiter are probably captured Trojan asteroids.

Apart from the main belt, there are a number of asteroids that come much closer to the Sun. Those that cross the Earth’s orbit are called Apollo asteroids after the first to be discovered to do so. Apollo was first seen on 24 April 1932 by Karl Reinmuth and was followed only until 15 May of that year when it became too close to the Sun in the sky to be seen, although on that day it was only 11 000 000km from the Earth. The orbit was determined from the observations and showed that at perihelion Apollo was just 0.65 astronomical units from the Sun and inside the orbit of Venus. From its brightness Apollo is estimated to be only about one kilometer across. Nineteen asteriods are known in the APOLLO GROUP and some details are given in table 12.7. Their diameters are between about one kilometer for Adonis, Hermes and 1976AA and 6km for 1972XA, except for minute P-L 6344 and 1976UA which may be only 200m across. The Apollo asteriod 1566 Icarus, 1976UA and 1976AA are particularly notable as they have the smallest known orbits.

The closest observed approach of an asteroid was Hermes in 1937 when it passed only 780000km from barely double the distance of the Moon. Hermes could me closer and pass between the Moon and the Earth!

One very special object is 944 HIDALGO, the asteroid with the largest known orbit. Its mean distance from the Sun is 5.8 astronomical units but because of the high eccentricity (0.66) it moves as far out as 9.7 astronomical units from the Sun and as close as 2 0 astronomical units. It is also the only asteroid ever known to approach within one astronomical unit of Jupiter, having passed within 0.38 astronomical units in 1673.

The bulk of the asteroids, those in the main belt between Mars and Jupiter, have orbital eccentricities smaller than the Apollo group but larger than the major planets. The mean eccentricity of the objects in the Palomar-Leiden survey is 0.147 with a range from 0 to 0.385. The average inclination of asteroid orbits is about 4° near the inner edge of the main belt rising to 11° near the outer edge. The larger asteroids have, on average, larger inclinations so that they are less well confined to the ecliptic than the smaller ones.

The study of the structure of asteroids requires a knowledge of their masses and radii. Only three masses are known and one of these is very uncertain. These three were obtained by observing the gravitational perturbations of one asteroid on the orbit of another. These perturbations are normally too small to measure but they have been large enough for useful measurements to be made in two cases: a series of close encounters of 4 Vesta with 197 Arete and for the mutual interaction of 1 Ceres and 2 Pallas. From the observations astronomers have been able to calculate the following masses

1 Ceres (11-.7 ± 0.6) x l020kg
2 Pallas (2.6 ± 0.8) x lO20kg
4 Vesta (2.4 ± 0.2) x 1020kg

The uncertainty in the mass is particularly large for 2 Pallas. Some other masses, based on the radius and an assumed value of the density.

Only the first four asteroids have diameters sufficiently large to have been measured directly, but two other, indirect, methods are available for obtaining diameters. An asteroid of a particular visual magnitude may be small, but highly reflective, or large and very dark. However the asteroid heats up more in the second case than in the first, both because it is larger and because it absorbs more of the incident light, and it therefore emits more infrared radiation. By comparing the amount of this radiation with the reflected visual radiation, an astronomer can determine the asteroid’s albedo and then, from its visual magnitude, its diameter. This technique was first applied to asteroids hi 1970 by David Alien and Dennis Matson and several astronomers have now used it to obtain the diameters of some tens of asteroids.

The second indirect method uses measurements of the amount of polarization in the sunlight reflected from an asteroid and the way in which t his varies with direction. This approach has been used by Joseph veverka. Ben Zellner and Edward Bowell to obtain the diameters of a couple of dozen asteroids. They make measurements of the polarization and then use the results of laboratory experiments to deduce the albedo. A measurement of the apparent visual magnitude and a simple calculation then give the diameter. Some of the results from all three methods are compared . Which shows that the values obtained by the photometric and polarimetric methods agree quite well with each other but are obviously systematically larger than the direct measures. It must be remembered, however, that the direct measurements are of angular diameters of, at most, only a few tenths of a second of arc and are therefore subject to large errors.

Most asteroids have an irregular shape or some variability in reflectivity over their surfaces (or both) so that as they rotate the amount of light that they reflect to the Earth varies. It is therefore generally possible to measure the rotation period by observing the light curve and this has been done for about 50 asteroids. The accurately measured periods range from 2.273 hours for 1566 Icarus to 18.813 hours for 532 Herculis, but there appear to be a few objects with much longer periods. An example of the difficulties of interpreting the light curves of asteroids is furnished by 4 Vesta. Astronomers have known for many years that Vesta varies its light by about 0.15 magnitude with a maximum every 5 hours 20 minutes. Two interpretations of this variation are possible. Vesta could be a nearly spherical, spotty body with a rotation period of 5.33 hours, or an elongated body with twice this period. Tom CJehrels made a detailed analysis of photoelectric observations in 1967 which appeared to show conclusively that the shorter period was correct. However in 1971 R.C.Taylor made a new series of observations which clearly showed that successive maxima of Vesta’s light curve alternated in height so that the true period was 10 hours 41 minutes during which two maxima and two minima occur. These observations were made at a time when Vesta’s southern hemisphere was turned towards the Earth. When we view the northern hemisphere the two maxima during each rotation period become indistinguishable because of the different albedo variations in the two hemispheres. The observations can be explained if Vesta is a spheroid with one diameter 15 per cent longer than the other two. There is a flattened region near the south pole which may be a large impact crater like those on Phobos and Deimos.

Now that astronomers are able to measure the radii of asteroids accurately, they can convert measurements of the light reflected at various wavelengths into albedos and deduce something about the composition. Roughly speaking, there are two classes of asteroid. Some are moderately bright reddish objects which reflect 10 to 20 per cent of the incident blue light and 14 to 23 per cent of the red light. The others are dark grey and reflect nearly the same amount of the incident sunlight at all wavelengths; this amount varies between about three and nine per cent. This means that most asteroids are at least as dark as the Moon and some, such as 324 Bamberga (the blackest asteroid known), are much blacker still. In very general terms we can say that the red asteroids are composed largely of silicate-type material and the dark grey asteroids of carbonaceous chondritic type.

Many astronomers have suggested that there is a connection between asteroids and meteorites. Clark Chapman and John Salisbury have made detailed comparisons of the albedos of asteroids and meteorites. They have found only three asteroids which resemble ordinary chondritic meteorites: the Apollo asteroid 1685 Toro and, from the inner edge of the main belt, 8 Flora and 43 Ariadne. A further five, 2 Pallas, 4 Vesta, 16 Psyche, 29 Amphitrite and 192 Nausikaa, match other, less common, types of meteorite. More important than these matches is the fact that there are many asteroids that resemble no known meteorite and meteorites that resemble no studied asteroid. This is consistent with the idea that most stony meteorites are derived from a few atypical asteroids.

Three asteroids, 433 Eros, 1566 Icarus and 1685 Toro have been detected by radar when close to the Earth. Icarus and Toro have been only marginally detected but improved techniques and the favourable circumstances of its close approach to the Earth in 1975 resulted in data of much higher quality for Eros. The observations were made at a wavelength of 3.8cm and show that the surface of Eros must be rough on a scale of centimeters. Since the results from the optical polarimetry used to measure the diameter also tell us that Eros is dusty, the radar results suggest that the dust is too thin to smooth out rock outcrops, edges and depressions on a scale of centimeters

The width of the radar spectrum, together with the rotation period, gave a measurement of the diameter of Eros independent of the results from the polarimetric and photometric methods. All agree that the longest and shortest diameters are about 36 and 15 kilometers.

Comments are closed.