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Physical Properties of Near-Earth Asteroids As Principal Impactors Onto the Earth

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Catastrophic Events Conference 3040.pdf PHYSICAL PROPERTIES OF NEAR-EARTH ASTEROIDS AS PRINCIPAL IMPACTORS ONTO THE EARTH. D.F.Lupishko, Astronomical Observatory of Kharkiv National University, Sumskaya str. 35, Kharkiv 61022, Ukraine. [email protected] INTRODUCTION. Besides the main-belt asteroid. 3109 Eger with very high albedo (0.64) asteroids, which rotate around the Sun in stable orbits corresponds to assemblages of iron-free silicate located between the orbits of Mars and Jupiter, there is minerals, such as enstatite. Five NEAs classified as V- a population of asteroids in unstable eccentric orbits class, have spectra indentical to those of main-belt which can approach and cross the Earth's orbit. This asteroid 4 Vesta, which is known to have a population is called "near-Earth asteroids" (NEAs). differentiated structure. The variety of taxonomic More than 800 NEAs have been discovered to-date. classes among NEAs reflects the diversity of their They are the objects of a special interest from the point surface mineralogy and an overall analogy with the of view not only of fundamental science but of the MBAs. Taking into account their small sizes, one applied science as well (NEAs as the potential sources might infer that they are the products of much larger of raw materials in the nearest to the Earth space, the differentiated bodies which were later injected into the problem of asteroid hazard, etc.). There is no doubt present orbits. that Earth-crossing asteroids are the principal bodies SIZES AND SHAPES. NEAs are much smaller in who strike our planet ocasionally and therefore they sizes in comparison with main-belt asteroids (MBAs), are a real threat to the Earth civilization. the largest of which is 1 Ceres (about 950 km in di- TAXONOMY AND MINERALOGY. ameter). Asteroid 1036 Ganymed (D=38.5 km) re- Taxonomic classification of the discovered near-Earth mains to be the largest among NEAs, two other aster- asteroids shows that among the classified NEAs oids 433 Eros and 3552 Don Quixote are about 20 km practically all taxonomic classes are represented with in diameter, all others are about or less than 10 km and the exception of one or two low-albedo classes which approximately 3/4 of them are less than 3 km. The are located in the outer part of the main belt. The main smallest known NEAs are about 6-9 m across. The question of the NEA taxonomy is a relative abundance population of NEAs can be approximated by a power of the two most numerous C (carbon) and S (silicate) law, which reflects a general exponential increase of classes. About a half of the classified near-Earth the number of asteroids as we go to smaller sizes: objects belong to class S, and the observed number of −b S-types exceeds the number of low-albedo types (C n = k D and others) as much as a factor of three. Using the where: n is a number of asteroids larger than a given results of the selection effects modelling [1] it was diameter D, k is the constant and b is the power law obtained [2] that the relative number of C and other exponent. low-albedo objects among NEA population is According to the availble estimates, Earth-crossing approximately 2.5 times less than in the main asteroid population contains about 2100 objects larger than 1 belt. This could be an important constraint for the km in diameter and about 300000 objects larger than possible sources of NEA replenishment. The simplest 100 m. Among ECAs 1866 Sisyphus is the largest ob- explanation could be that NEAs are coming ject with diameter of about 8 km. The absence of preferentially from the inner regions of asteroid main NEA-objects larger than 40 km in diameter is usually beit, where the relative abundance of C and other interpreted as an indication that near-Earth asteroids lowalbedo asteroids is much lower. are not primordial objects, but they are collisional Most of the NEAs for which mineralogical fragments of larger main-belt asteroids. information exists, represent differentiated The data of ground-based observations and space assemblages. Among them there are objects with missions show that NEAs have irregular and elongated monomineral silicate composition and purely metalic shapes. The NEAs on the average are elongated to the ones. For example, small asteroid 1915 Quetzalcoatle same extent as those of MBAs of corresponding sizes. appears to have little or no olivine, and diogenitic But radar observations showed a striking diversity of meteorites (Mg-pyroxenes) are the best analogs of it. NEAs shapes from nearly spherical (1566 Icarus) to 3199 Nefertity has the same content of pyroxene and very elongated (1620 Geographos, 1865 Cerberus) and its composition corresponds to that of stony-iron to bifurcated and contact-binary ones (4179 Toutatis meteorites - pallasites. The object of A-class 1951 Lick and 4769 Castalia). The most elongated asteroid also appears to have a composition indentical to among observed NEAs is 1865 Cerberus (D=1.2 km), olivine achondrites or pallasites. There are two M- the axis ratio a:b of its figure is estimated to be equal objects, one of which, 6178 1986 DA, has radar albedo to 3.2. The opinion that NEAs have more exotic shapes clearly indicating the real metalic composition of this than MABs may belong only to the large MBAs, be- Catastrophic Events Conference 3040.pdf PHYSICAL PROPERTIES OF NEAs: D. F. Lupishko cause we know practically nothing about the shapes of surface, such as bare rock without developed regolith. kilometer-sized main-belt objects. The images of one of the largest NEA 433 Eros AXIS ROTATION. The distributions of the obtained by the NEAR-spacecraft in February 2000 rotation rates of Earth-approaching asteroids in showed a surface covered with large number of craters comparison with that for MBAs of comparative sizes of different sizes. Ground-based radar observations show that both asteroid samples have very similar also showed that even the relatively small NEA 4179 means equal to 4.85±0.30 and 4.50±0.24 rev/day, Toutatis (D=3 km) is cratered at about the same extent respectively, similar dispersions and similar maxima of as MBAs 951 Gaspra and 243 Ida [3]. their distributions. At the same time the mean rotation CONCLUSION. The new results of NEA studies rate of the large MBAs is equal to 2.90±0.12 rev/day. allow us to better understand their nature, origin and Thus, one can conclude that on the average NEAs relation to the comets and meteors. Indeed, small sizes rotate practically in the same manner as the small of NEAs, almost the same variety of taxonomic MBAs and considerably faster than large MBAs. The classes, the same mineralogy and predominance of fastest rotators among NEAs have the rotation periods differentiated assemblages among them, approximately equal to 2-2.5 hrs, but the slowest one (3102 Krok) the same shapes and rotation, optical properties and rotates with the period equal to 147.8 hrs. The main surface structure as compared to those of MBAs, all peculiarity of NEA rotation is that among this these clearly indicate that the main asteroid belt is the population there are objects with very complex and principal source of NEA origin. It means that most of non-principal axis rotation (so-called "tumbling" the NEAs are the fragments of main-belt asteroids asteroids). ejected on their current orbits by processes of OPTICAL PROPERTIES AND SURFACE collisions and chaotic dynamics [4]. This conclusion STRUCTURE. The analysis of photometric, pola- does not contradict the recent results of dynamic rimetric, radiometric and other observing data clearly considerations, according to which the main asteroid demonstrates that the surfaces of NEAs display the belt can supply a few hundred km-sized NEAs per 1 same optical properties as the surfaces of MBAs, and Myr, well enough to sustain the current NEA most likely they have the same microstructure. The population [5]. On the other hand, the identification of whole range of NEA albedos (0.04÷0.60) is basically a few asteroids with extinct or dormant comets does the same as that of MBAs and it corresponds to the not exclude the cometary origin of some of them. same distribution of taxonomic classes and mineralogy Hence, the main problem of NEAs' origin now is the within these two populations. But at the same albedo, relative contributions of both sources. colors and similar surface composition, the strict simi- The discovery rate of NEAs increases greatly over larity of the other photometric and polarimetrical pa- the last years and the organization of new programs for rameters gives evidence of the same surface textures at their discovery and investigation in the USA, Europe, submicron scale. Japan and Australia gives good reasons to hope that a Most of NEAs are covered with regolith of low new era of NEA studies is coming and active ground- thermal inertia. But the conditions of formation, accu- based and space-mission (like Galileo and NEAR) mulation and evolution of regolith on NEAs are differ- investigations will give us all information necessary ent from those on MBAs because of the much smaller for the solution of fundamental and applied problems gravity of the former ones, the higher flux of impactors connected with NEAs. in main belt than in the region of NEA orbits (1-3 or- ders of magnitude), and the difference in intensity of [1] Luu J. and Jewitt D. (1989) Astron. J., 98, Solar wind. As a result, the regolith of NEAs tends to 1905-1911. [2] Lupishko D.F.
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