CHAPTER ONE The Minor Bodies of the Solar System It is widely believed that about 4600 million years ago our solar system suffered a chain of disruptive events associated with condensation, intense heating, melting and eruptions, and that such events never occurred again with the same intensity. Throughout the remainder of cosmic history, the primary planetary and meteoric bodies are assumed to have cooled more or less homogeneously. Mutual collisions have increased their number and reduced their average size. On the other hand, through gravitational pull of the planets, a large number of the smaller bodies have already been eliminated in crater-producing events, while a few have been captured as satellites of the larger planets, Jupiter, Saturn, Uranus, Neptune and Mars. It is now well known that the larger bodies of our solar system, Earth, Mars and Moon, are scarred to a considerable degree by craters which on Earth are called astroblemes (Dietz 1963). The fact that the smaller bodies are also severely damaged, or perhaps are themselves fragments, was proved when in November 1971, the U.S. spacecraft Mariner 9 succeeded in relaying the frrst detailed views of any natural satellites in the solar system except the Moon. Photographed from a distance of 5500 km, Mars' two satellites, Phobos and Deimos, were found to be irregular, angular objects. The roughly potato shaped Phobos is 26 km long and 21 km wide and displays at least a dozen impact craters. The biggest depression, about 6 km across, probably indicates where a large fragment broke off during an asteroidal collision. The morphology of these tiny satellites not only suggests that they are very old but also Figure lA. The Lost City Fireball. A photograph taken at the Hominy, Oklahoma, camera station, operated by the Smithsonian that they possess considerable structural strength. Astrophysical Observatory, showing the spectacular meteor descend­ On January 3, 1970, at 2014 local time, a fireball ing the eastern sky on January 3, 1970. It remained visible for nine brighter then the full moon descended over Oklahoma and seconds as may be deduced from the series of dashes into which the trail was automatically broken by a chopping shutter. Star trails of caused sonic booms that were heard over a 100 km long Taurus, Orion, and Canis Major cross the background. (Courtesy zone below the line of flight. The fireball was photographed R.E. McCrosky.) by the Prairie Network and the analysis of the trajectory indicated that it had produced some sizable meteorites. A closely associated with the astronomical interpretation of careful search resulted in the recovery of four fragments of the orbits of meteoroids and minor planets of our solar a stone meteorite totaling 17 kg. The successful recovery system. The other is mainly concerned with the physical­ was immediately followed by extensive studies of the chemical study of meteoritic matter itself and with the mineralogy, chemistry, and isotope chemistry, with the solution of the problem of the origin of meteorites and the result that the Lost City meteorite is today perhaps our size of the parent bodies. A start will be made by examining best known from all points of view (McCrosky et a!. 1971; some basic aspects of the solar system. We will then turn to Clarke eta!. 1971b). the physics of the meteorite fall and eventually examine the The entire range of basic problems in meteoritics may fallen meteorite. Finally, in the main part of the handbook, be divided into two main parts. One comprises the study of the individual iron meteorites will be described in alpha­ the circumstances of meteorite falls on the Earth and is betical order. 6 The Minor Bodies of the Solar System This assumption proved to be correct. By 1800, 280 had been discovered and subsequently, after the introduc­ tion of the systematic photography of the skies, the number increased sharply. Today, 1779 are numbered (Ephemerides, Chebotarev, 1971) and accurate orbits have been calculated. There are in addition, however, a large number of smaller asteroids which usually have only been seen and identified once, at the time of their discovery. According to the recent extensive photographic survey conducted at the Palomar and Leiden observatories, the total number of asteroids that become brighter than photographic magnitude 20.4 at mean opposition is about 40,000; see Figure 2. The steady growth in numbers as that magnitude limit is approached makes it very likely that the sequence continues down to bodies as small as meteorites and dust grains. The great majority of the asteroids move in orbits which lie within the range of 2.1 to 3.5 A.U. from the Sun, so that the approximate average of 2.8 is in agreement with the requirement of Bode's rule. See Table 2. The orbital periods vary between 3.3 and 9 years, with an average of 4.5 years. Most eccentricities lie between 0.02 and 0.3, with an average of 0.15. The orbital inclinations range from 0° to 35° with an average of about 10°. They all move in their orbits in the same direction as the major planets, i.e., direct or counterclockwise. The large asteroids, such as Ceres, Pallas and Vesta, are nearly spherical, but many others have irregular, angular shapes suggesting that they are secondary collision frag­ ments. The angular shape is indicated by the large variation in brightness observed as the asteroid rotates and reflects sunlight from different regions on its surface. See Figure 3 I or McCord et al. (1970). From the fluctuations in bright­ Figure lB. Lost City (U.S.N.M. no. 4848). Reconstruction by Roy ness of Eros, for example (table 3), it has been estimated S. Clarke, Jr. of the mass. The 9.8 kg main mass was found on January 9th after analyzing the trail in Figure lA; three other that it measures about 24 x 8 x 8 km in three perpendicular masses were found Ia ter. Lost City is an olivine-bronzite chondrite directions. The masses of even the largest asteroids are too of a common type (H5), containing about 15% (by weight) small to be determined by conventional methods. Rough karnacite, 1.5% taenite, 6% troilite and 0.5% chrornite (Clarke eta!. 1971b; S.I. neg. 1636c.) Scale bar 5 ern. estimates based upon the observed, not too precise dimen­ sions, and on the assumption that the density is comparable Asteroids When comparing the orbits of the planets, there mooo~----------------------------------- 0 appears to be an exceptionally large gap separating Mars 0 o Palomar- Lei den 0 10000 0 and Jupiter. At an early date Kepler suggested that a planet 0 Vl • McDonald 0 might be found in this region of the solar system, and in 0 f:i1 0 1772 a German astronomer, J.E. Bode, publicized what has UJ 0 1- 1000 Vl later become known as Bode's rule (Jaki 1972). According <t • u. to this, the distances, in astronomical units (A.U.), of the 0 • • 0: • UJ m • • successive planets from the Sun are obtained by adding 0.4 ro • ::E • • to each of the following numbers: 0, 0.3, 0.6, 1.2, 2.4, 4.8, z::::> • etc.; see Table I. The discovery of Ceres, in 1801 , appeared 10 • • • to fill the gap in the system, but by 1807 three other • similar bodies (see Table 2) had been discovered with orbits • in the same region. These also happened to be the largest of 10 12 11. 16 18 20 all asteroids and their orbital elements are typical for the MEAN OPPOSITION MAGNITUDE majority of asteroids. Chladni (1819: 412) discussed the Figure 2. Each point represents the cumulative number of bodies in the entire asteroid ring. (Adapted from a diagram by C.J. van Hou­ then known four asteroids as possible sources of meteorites, ten in Astronomy and Astrophysics Supplement, Vol. 2, Springer and he expected more asteroids to be found. Verlag, 1970.) The Minor Bodies of the Solar System 7 3 to that of the Moon, 3.3 g per cm , leads to a figure of Earth-Sun Distance, the astronomical unit. Kepler and 7.6 x 10 17tons for Ceres, or about one percent of the mass Amor belong to a small family of asteroids with Mars­ of the Moon. The total mass of all asteroids is estimated to crossing orbits. Apollo , Hermes, Ikarus and Geographos are be about 3 x 10 18 tons (Putilin 1952). members of the Apollo family of eight asteroids with high Among the relatively few asteroids whose orbital eccentricities and Earth-crossing orbits. The Amor and characteristics are outside the range given above are a Apollo asteroids are very probably former normal asteroids number which have attracted special attention; see Table 3. which were perturbed into their present orbits by Mars. Hidalgo has the largest orbit known, almost touching the The known asteroids are no doubt accompanied by a orbit of Saturn. It was possibly deflected into its present huge number of smaller fragments not large enough to be orbit relatively recently as the result of a collision with observed from Earth. The 80 em telescope on board Skylab another minor planet (Marsden 1970). Eros was the first may, however, have a chance to identify some of the asteroid found to cross inside the orbit of Mars and use was smaller asteroids since the optical resolution is much made of this to calculate a much improved value for the improved outside our atmosphere. As the asteroidal orbits Table 1 - The Planets Planet Bode's Mean Eccen- Inclination Sidereal Diameter Mass Mean and date Rule distance tricity to ecliptic period, km tons density of discovery 1771 from Sun, years g/cm 3 A.U.