ep comets

COMETS

Comet Hale-Bopp

Comets are 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..

LTEP

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.

references

Long-Term Evolution of Short-Period Comets. A. Carusi, L. Kresak, E. Perozzi, G.B. Valsecchi. Adam Hilger Ltd, Bristol and Boston, 1985. abstract
Periodic Comets Van Biesbroek and Neujimin 3. A. Carusi, L. Kresak, E. Perozzi, G.B. Valsecchi. IAU 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, 203-214, 1985.
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, 1985.
High-Order Librations 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 P/Halley. A. Carusi, L. Kresak, E. Perozzi, G.B. Valsecchi. Celestial Mechanics 43, 319-322, 1988.
Small 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 45-50, 1996.
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