main-belt asteroid (45) Eugenia with its satellite Petit Prince
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.
ASTEROIDS 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
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
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.
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
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.
of Commensurabilities on the Eccentricity and Inclination of Nearby
E. Perozzi: in proc.
'Asteroids, Comets, Meteors', C.I.Lagerkvist & H.Rickman eds,
Symmetrical Representation of the Resonant Structure of the
Asteroid Belt. E. Perozzi: 7.th European Astronomical Meeting
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.
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.
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.
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.
Barucci & E. Perozzi: Coelum v.L,
Discovery of Eros: a peculiar 3-body problem. E.
Stone 20, 2003. full
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.
31 Agosto 1872: l'unica Meteorite Osservata e Rinvenuta nel Lazio.
De Angelis, P. Lanzara, A. Maras, E. Perozzi: nel
volume 'Monti Lucretili', G.De Angelis e P.Lanzara eds, 299-332, 1983.
Pioggia di stelle. E.
Perozzi: il Manifesto,
venerdi' 15 agosto 1986.
Spaceguard Foundation Central Node: http://spaceguard.ias.rm.cnr.it/