M. Fulchignoni

Classification of asteroids using G-mode analysis

Abstract A revised version of the G-mode multivariate statistics has been used to classify the 438 asteroids for which the eight-color photometric data and IRAS albedo are available. At a confidence level of 99.7%, seven taxonomic units of asteroids are separated by the method, while with higher values of the confidence level no separation occurs in the adopted sample. Decreasing the confidence level (i.e., accepting a higher probability of a wrong decision in classifying the asteroids), we obtain a more detailed grouping, which results in a successive subdivision of the first units found. At a confidence level of 97.5%, two groups are added to the original ones: C asteroids are in fact subdivided on the basis of their albedo into three different units. In total, 18 groups of objects can be distinguished, showing small deviations from the trend of the original groups. The method gives a quantitative estimate of the significance of the variables used. A comparison with the results obtained by D. J. Tholen (1984, Asteroid Taxonomy from Cluster Analysis of Photometry , Doctoral thesis, University of Arizona) shows that the nine principal units coincide with the Tholens A, S, C, D, M, E, BU, G, (Q, R, S) classes, while the other groups are subdivisions of these classes, particularly of the S, D, and B ones. The method allows us to interpret the obtained results in terms of natural processes which characterized the history of asteroid population.

Phase curves of meteorites and terrestrial rocks: Laboratory measurements and applications to asteroids

Abstract The measurement of the radiation reflected by a particulate surface as a function of the phase angle can be a powerful method for deriving information about the physical and chemical properties of the materials composing the surface layers of atmosphereless bodies of the Solar System. Several theoretical descriptions of the light scattering by a particulate surface have been developed to explain the observations of the phase functions for a number of Solar System bodies (asteroids, satellites of Jupiter, Moon, and Mercury). However, laboratory measurements on naturally occuring materials, both terrestrial and extraterrestrial, are needed to improve the understanding of the physics of the processes involved. The authors report the results of laboratory measurements carried out at the Teramo Observatory to study the reflectance of powdered meteorites and terrestrial rocks at various phase angles. We analyzed the reflectance of 17 meteorites: 14 chondrites (3 C-type, 5 L-type, 4 H-type, 2 LL-type), and 3 achondrites. In addition, a study of the effects on the phase curve of varying the size fraction and the compaction degree has been carried out on terrestrial rocks (peridotite, diabase, quartz-rich sand).

66 Maja and 951 Gaspra : possible flyby targets for Cassini and Galileo missions

Abstract The asteroids 66 Maja and 951 Gaspra, identified as possible flyby targets for Cassini and Galileo space missions, respectively, were observed during their apparitions in 1988. 66 Maja, a C-class asteroid with a diameter of 78 km, became the focus of a coordinated observing program carried out at Torino, Teramo, and Pic du Midi observatories using both photoelectric and CCD photometry. The computed synodic rotational period of Maja is 9.733 hr and the amplitude of the light variation was 0.45 mag. 951 Gaspra, an S-class asteroid with a diameter of 15 km, was observed about 2 months after opposition for two nights at the European Southern Observatory (La Silla, Chile) in August 1988. The best fit of the composite lightcurve was obtained with a rotational period of7.09 hr and a lightcurve amplitude of about 1.0 mag. An international program to coordinate Earth-based observations during the next oppositions of these asteroids is proposed.

Rotational properties of small asteroids : photoelectric observations

In 1984 we started an observational program on small asteroids (diameter lower than about 50 km) with the aim to enlarge the available dataset of the rotational periods of this size range objects. Here, we report the results obtained from photometric observations of ten small asteroids from August 1984 to January 1989 at the European Southern Observatory (La Silla, Chile). Reliable synodic rotational periods were determined for 8 asteroids.