Near Earth Environment
Planet Earth provides an interface to the interplanetary environment; its atmosphere forms a protective shield against direct impacts and erosion and is a medium in which to observe the approach of meteoroids and even to capture intact smaller meteoroids.
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I. INTRODUCTION
The Earth and its atmosphere provide a target for interplanetary particulates which allows us to study incoming objects directly, if they reach the surface; or indirectly, if they interact with the atmosphere or orbiting spacecraft. Although this sample is not necessarily representative of the total population of interplanetary material, it is of fundamental importance for understanding the past and present influx of material to the Earth and currently provides the only source of interplanetary material which can be studied in the laboratory. The relationship between these samples and their original sources depends on the physical and dynamical processes which have characterised their transport to the Earth. We observe only material from sources for which such mechanisms exist. The identification of a source of particular samples may be based on composition or on evidence of past processes, e.g. SNC meteorites associated with Martian origin (e.g. Laul et al. 1986; Ott and Beggeman 1985) or antarctic E. Grün et al. (eds.), Interplanetary Dust © Springer-Verlag Berlin Heidelberg 2001
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meteorites of lunar origin (e.g. Dennison et al. 1987). We sometimes have dynamical evidence such as the collection of meteorite samples from observed fireballs for which orbits may be derived. More revealingly it can be based on a combination of both, such as the association of basaltic achondrite meteorites with the asteroid 4 Vesta. Small asteroids (i.e. large impact ejecta), of similar spectral type to Vesta (Binzel and Xu 1993) have been identified in orbits bridging the location of Vesta and the 3:1 Jovian resonance (Wisdom 1983, 1985; Yoshikawa 1990) which provides a transport mechanism to Earth. Details of the large particle influx to the Earth and associated meteor phenomena are dealt with by Ceplecha et al. (1998) and the properties of airborne microparticles which are decelerated without significant ablation are described by Jessberger et al. (this volume). We are concerned here with the near Earth particulate environment, its interaction with the Earth's atmosphere, in situ detection of sub-mm particles and associated modelling tools for deriving their properties and sources. Orbiting spacecraft (and the Earth itself) can only sample objects with orbits which cross or closely approach the Earth's; the Earth's own gravitational influence has a modest focusing effect dependent on the relative velocity. Although the sources of these particulates may themselves be in stable orbits (planets, main belt asteroids) or distant orbits (comets), small particulates are subject to forces which significantly alter or dominate their dynamics, namely: - perturbations due to close encounters or resonances with more massive bodies; - non-gravitational forces due to the non-isotropic sublimation of volatiles from cometary nuclei or fragments; - radiation pressure which exerts a radial force dependent on particle size and composition which can exceed solar gravity for sub-p,m particles ((3meteoroids)
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