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The main-belt asteroid (45) Eugenia with its satellite Petit Prince

Asteroids can be divided into two broad classes: Main-Belt  and Near-Earth objects, depending on their orbital characteristics. Main-Belt asteroids (MBAs) have generally low eccentricity and inclination orbits located in the region between Mars and Jupiter, while Near-Earth Asteroids (NEAs) may move on large eccentricity and inclination orbits allowing them to travel throughout the inner and outer Solar System, thus representing also a potential threat for collisions with the Earth. The two asteroidal populations are dinamically connected: in fact, modern theories on the origin of meteorites imply the delivery of asteroidal fragments from certain regions of the main belt (i.e. close to main motion resonances) on chaotic orbits.

can be considered as a collisionally evolved population of remnants from the original "building blocks" needed for planetary formation (i.e. planetesimals) , which failed to acrrete into a single large planet because of the perturbations by Jupiter. Only a few objects have diameters exceeding 500 km, while the vast majority is represented by km and sub-km sized bodies resulting from intensive fragmentation. The two major characteristics of the distribution of objects in the main belt were first pointed out at the beginning of the last century: the existence of collisional families (i.e. clusters of objects having similar orbital parameters) and of regions devoided of asteroids corresponding to low-order mean motion resonances with Jupiter (i.e. when the ratio between the periods of revolution of the asteroid to that of Jupiter can be expressed as an integer fraction: 1/2, 1/3, 3/4, etc.). In particular, the studies on the origin of these so-called Kirkwood Gaps (named after its discoverer, Daniel Kirkwood) has provided a deep insight on the transport of matter throughout the Solar System. Resonances are in fact often surrounded by regions where chaotic motion can appear: thus an asteroid close to a resonance after millions of years of regular orbital motion can suddenly experience large variations of its eccentricity, lowering its perihelion well inside the inner planetary region. Close encounters with the terrestrial planets subsequently drive the orbital evolution of these objects. 

NEAR EARTH ASTEROIDS are dynamically evolved fragments of main-belt asteroids entering the inner Solar System on chaotic orbits. Thus most NEAs share the orbital paths of meteorites and their final fates - either colliding with the planets, being ejected from the Solar System on hyperbolic orbits or melting into the Sun. The non-zero possibility that a sizeable NEA is on a collision course with our planet has recently prompted a new field of study which addresses the topics of discovering and characterising all potentially hazardous objects (i.e. with diameter larger than 50 m) and studying mitigation strategies (e.g. deflection). To this end the use of both ground-based and space-born facilities is foreseen, making the "accessibility" of a NEA an important parameter for further investigations. Within this framework, the H-plot targeting strategy gives a time-free dynamical evaluation of the accessibility of the NEA population, addressing both, mission designers and astronomers. The formers may find it useful as a quick way of decreasing the number of potential targets according to mission specifications before more accurate (and lenghty) trajectory optimization procedures are attempted. Astronomers can adopt the H-plot as a criterion for observing, among the thousands of known NEAs, those more likely to be selected as targets for direct exploration. In the long run this would trigger a colosed loop: the more accessible asteroids are also those for which more data on their phisical characterization is available, which in turn increases scientific motivation and eases mission design.


The Effects of Commensurabilities on the Eccentricity and Inclination of Nearby Asteroids. E. Perozzi: in proc. 'Asteroids, Comets, Meteors', C.I.Lagerkvist & H.Rickman eds, 149-154, 1983.
A Symmetrical Representation of the Resonant Structure of the Asteroid  Belt.
E. Perozzi: European Astronomical Meeting Abstracts, 1983.
Atlas of Photometric Asteroid Lightcurves.
C.I. Lagerkvist, V. Zappalà, S. Argentini, M.A. Barucci, M.T. Capria, M. Fulchignoni,  L. Guerriero, E. Perozzi, M. Viaggiù: in proc. 'Asteroids, Comets, Meteors II', C.I.Lagerkvist, B.A.Lindblad, H.Lundstedt and H.Rickman eds, 61-62, 1986.
Basic Targeting Strategies for Rendezvous and Flyby Missions to the Near-Earth Asteroids.
E. Perozzi, A. Rossi and G.B. Valsecchi: Planetary and Space Science 49, 3-22, 2001.
Dynamical and Compositional Assessment of Near-Earth Object Mission Targets.
R.P. Binzel, E. Perozzi, A.S. Rivkin, A. Rossi, A.W. Harris, S.J. Bus, G.B. Valsecchi, S.M. Slivan: Meteoritics and Planetary Science (in press) 2004.
On the Attainability of the Near Earth Asteroids.
E. Perozzi, L. Casalino, A. Rossi, G.B. Valsecchi: in proc. 'Planetary Science, Fifth Italian Meeting', (in press) 2004.
Gli Asteroidi.
M.A. Barucci & E. Perozzi: Coelum v.L, 238-246, 1982.
The Discovery of Eros: a peculiar 3-body problem.
E. Perozzi:  Tumbling Stone 20, 2003. full article
Asteroid (9934) CACCIOPPOLI: What's in a Name?
E. Perozzi: in proc. 'Modern Celestial Mechanics: from theory to applications', A.Celletti, S. Ferraz-Mello and J. Henrard eds, 2003.
Orvinio 31 Agosto 1872: l'unica Meteorite Osservata e Rinvenuta nel Lazio. G. De Angelis, P. Lanzara, A. Maras, E. Perozzi:
nel volume 'Monti Lucretili', G.De Angelis e P.Lanzara eds, 299-332, 1983. more
Pioggia di stelle.
E. Perozzi: il Manifesto, venerdi' 15 agosto 1986.

The Spaceguard Foundation Central Node:
Tumbling Stone: a scientific magazine about NEOs, asteroids and comets, and the hazard of Earth impact.
NEODyS: Near Earth Objects Dynamics Site.



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