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Astron. Astrophys. 327, 388-391 (1997)

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1. Introduction

Near-Earth Asteroids (NEA) represent one of the most peculiar classes of objects in the Solar System. Their orbits can approach or even intersect the terrestrial one. More than 360 NEAs have been discovered to date. The largest object of this population has a diameter of 38 km, two others are about 20 km in size, the remaining have diameters less than 10 km, and about 3/4 of them are smaller than 3 km. According to the more recent estimate (Rabinowitz et al., 1994; Muinonen et al., 1995), NEAs with diameter [FORMULA] 1 km are about 2000, and at present we know the orbits of only about 7% of them.

The population of NEA is extremely heterogeneous in all the aspects of their physical properties. Available data show that shapes, rotation rates and albedos of NEAs are on the average practically the same as those of main-belt objects. However, among NEAs there are objects with unusual shapes (very elongated, dumb-bell like and possibly binary), with very complex non-principal axis rotation (tumbling asteroids) and with peculiar mineralogical compositions. Recent radar observations allow to assume that a substantial part of NEAs could be binary systems. In the NEA population all the taxonomic types have been identified, except the B and the P classes, even though the most numerous classes observed are S and C respectively.

The discovery of the very dark (albedo about 0.03) and reddish D-type Amor asteroid 3552 Don Quixote has been rather unexpected because most of the asteroids belonging to this class are located in the outermost parts of the main belt (Trojan and Hilda groups) and they represent the most primitive objects among asteroids. The variety of taxonomic classes discovered among NEAs indicates that this population is heterogeneous in origin and composition.

The importance to study NEAs is connected to several reasons: a) the impacts by these objects are the principal cause of the craterization of the Earth and the Moon, at least in the last 3.8 Gyr (Wetherill and Shoemaker, 1982). The discovery of the majority of the possible "dangerous" objects and the knowledge of their physical properties are two of the main research lines to be followed in order to solve the problem of "asteroid hazard". b) They could be the sources of chondritic and achondritic meteorites (Wetherill, 1976; Di Martino et al., 1995); c) It is likely that a good number of these objects represent the final evolutionary state of comets, that is a devolatilized nucleus. d) They could be potential sources of metals and other raw materials in the neighbourhood of the Earth space. At present we know two M-type asteroids (3554 Amun and 6178 1986 DA) and the results of radar observations leave no doubts about the metallic nature of them (Tedesco and Gradie, 1987; Ostro et al., 1991).

Anyway, one of the most interesting aspects in the study of NEA is to understand their origin. In fact, their dynamical lifetimes are shorter than the age of the Solar System. Moreover, owing to the constant craterization of the inner Solar System bodies from their formation, it is believed that the population of NEA is practically constant in number. So, it has to be continuously supplied by some sources and/or mechanisms which have been identified in: (i) the dynamical evolution of the fragments coming from catastrophic collisions in the main belt (Greenberg and Nolan, 1989), and in (ii) extinct or dormant comet nuclei (Weissmann et al., 1989; Binzel et al., 1992).

An example of the latest case is comet P/Encke, a low active comet on an Apollo-like orbit. On the other hand, some NEA have been discovered on cometary-like orbits as 2201 Oljato (McFadden et al., 1993, Lazzarin et al., 1996) or dynamically connected with meteor streams, that are believed to be cometary in origin (Asher et al., 1994).

The most striking evidence of this connection is the Apollo object 4015 1979 VA, discovered as an asteroid and then recognized as the non active comet Wilson-Harrington.

Another important aspect of NEAs is that they are very likely the principal sources for meteorites, in particular for ordinary chondrite (OC) meteorites. The OC are considered the remnants of the primitive solar nebula: they have been scarcely thermally processed during the evolutionary stages of the Solar System.

Binzel et al. (1996) have recently found a quite clear relationship between OC and some NEAs. If the idea that part of NEAs could be the parent bodies of OC is confirmed, it would help to understand the origin of part of these objects: they would have been injected into near-Earth orbits from the main-belt reservoir.

In order to try to answer to all these open questions we started a long term spectroscopic survey of NEAs, which preliminary results we present in this paper.

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© European Southern Observatory (ESO) 1997

Online publication: April 8, 1998