traditionally divided into two distinct populations: Short-Period
and Long-Period Comets, depending whether their period
of revolution is in excess of 200 years. The study of the orbital
evolution of comets and the hypothesis on their origin, has now clearly
shown that this distinction is somehow arbitrary - being driven by
observational needs (i.e. their re-appearance in historical times). The
possibility that comets are original samples of the pristine materials
from which planets were formed (the so-called "planetesimals") re-entering
the Solar System after a long and complex dynamical history, was first
pointed out by Jan Oort at the beginning of our century. He
also hypothised the existence of a distant reservoir of comets - now
known as the Oort Cloud - from which individual objects
are removed by stellar and galactic perturbations. This model could
explain the orbital characteristics of Long-Period and "new" comets
(i.e. those passing for the first time close to the Sun on nearly
parabolic orbits) but not the observed overabundance of Short-Period
comets in low eccentricity and inclination orbits. The discovery of the
first Trans-Neptunian Object (TNO) in 1992 and the many
others found so far, has eventually brought evidence of the existence
of a preferred source region of Short-Period Comets just beyond the
orbit of Neptune..
The Long-Term Evolution Project studies in
detail the past and future orbital evolution of the whole Short-Period
Comet population. The aim of the project, carried out in close
collaboration between the Planetology Branch of the Space
Astrophysics Institute of CNR and the Department of
Interplanetary Matter of the Astronomical Institute of the Slovak
Academy of Sciences, is to create a statistically significant
sample of cometary orbital evolutions. The
orbit of each comet is integrated numerically using an high-precision
method in the full N-body problem, including the perturbation of all
planets for 800 years roughly centered at the present epoch.
Among the many
peculiar events found, such as close encounters and resonance trapping,
it has been possible to trace back the breakup of a cometary
nucleus, due to the striking similarity of the orbital
evolution of comets Van Biesbroeck and Neujimin 3 prior
to an encounter with Jupiter in 1850.
Subsequent developments focus on the characteristics of the so-called Halley-type
comets, identifying the occurrence of frequent librations
around high order resonances with Jupiter. A study devoted to comet
P/Halley allows to put a lower limit (about 11.000 years) to
the presence of the comet in its present orbit. LTEP
orbital evolutions (132 as to January 1985), were published as an Atlas
by Adam-Higer ltd in 1985 and subsequently updated on-line.
MISSIONS TO COMETS
The complex physical
and dynamical evolution undergone by Short-Period comets (witnessed by
the images of Halley's dark-crusted nucleus sent by the Giotto
spacecraft in 1986 or the spectacular break-up of comet Shoemaker-Levy
9 around Jupiter in 1997) has put into question their consistency as
representative "primitive bodies" of the Solar System.
Therefore Long-Period and "new" comets
can be considered as better targets for direct exploration.
Unfortunately their appearances are rare and unpredictable, thus making
the planning of a conventional space mission (which needs
the target being known well in advance) a rather difficult task. The
recent advances in small satellite and launchers technology,
as well as in the sensitivity of ground based telescopes for early
discovery and follow-up, allows to show that missions toward
long-period and "new" comets are now in principle feasible. The LOCO
(Long-Period Comet Observer) proposal addresses this problem
and discusses the case of the two bright comets of 1996
and 1997 - Hale-Bopp and Hyakutake.
Long-Term Evolution of
Short-Period Comets. A.
Carusi, L. Kresak, E. Perozzi, G.B. Valsecchi. Adam
Hilger Ltd, Bristol and Boston, 1985. abstract
Comets Van Biesbroek and Neujimin 3. A. Carusi, L. Kresak, E. Perozzi, G.B.
Circular 3940, 1984.
The Long-Term Evolution Project. A.
Carusi, L. Kresak, E. Perozzi, G.B. Valsecchi. In proc.
'Dynamics of Comets', IAU Coll.83, A.Carusi & G.B.Valsecchi eds,
First Results of the
Integration of Motion of Short-Period Comets over 800 years. A.
Carusi, L. Kresak, E. Perozzi, G.B. Valsecchi. In proc. 'Dynamics of
Comets'; IAU Coll.83, A.Carusi & G.B.Valsecchi eds, 319-340,
of Halley-Type Comets. A.
Carusi, L. Kresak, E. Perozzi, G.B. Valsecchi. Astronomy
& Astrophysics 187, 899-905, 1987.
On the Past Orbital History of Comet
Carusi, L. Kresak, E. Perozzi, G.B. Valsecchi. Celestial
Mechanics 43, 319-322, 1988.
Satellite Missions to Long-Period Comets. E.
Perozzi & E.M. Pittich. In
proc. 'Small Satellites Systems and Services', CNES, 181-184, 1992
Small Satellite Missions to Long-Period
Comets: The Hale-Bopp Opportunity. E.
Perozzi, G. Rondinelli, G. Di Genova, E.M. Pittich, G.B. Valsecchi.
Acta Astronautica Vol.39 No 1-4, pp
On Targeting Long-Period and New Comets for
Small Satellite Missions. E.
Perozzi and V. Fabiani. In
proc. 'Space Exploration and Resources Exploitation - Explospace
Workshop. ESA WPP-151, P.3.1-P.3.6, 1998