A. Dell’Oro

Statistics of encounters in the trans-Neptunian region

The inventory of the populations of trans-Neptunian objects (TNO) has grown considerably over the last decade. As for other groups of small bodies in our solar system, TNOs are expected to have experienced a collisional evolution owing to their mutual impacts. The knowledge of the statistics of collisions, including determination of the rate of mutual collisions and the distribution of the impact velocity, is indeed a fundamental prerequisite for developing models of collisional evolution. We revised the evaluation of those statistical parameters for TNOs provided more than ten years ago on the basis of a much more limited sample of objects than currently available. We used the Canada-France Ecliptic Plane Survey (CFEPS) L7 model to extract an unbiased sample of orbits for TNOs, while the statistical parameters of impact are computed using a statistical tool. We investigated the statistics of impacts among TNOs for the whole population and for different dynamical subgroups. Moreover, we investigated the statistics of collisions between subgroups with crossing orbits. The peculiar dynamical behavior of objects in resonant orbits is taken into account. Our present computation of the probabilities of collision are 20% to 50% lower than previous estimates, while mean impact velocities turn out to be about 70% higher. For instance, the rate of collisions among Plutinos, expressed in terms of the so-called mean intrinsic probability of collision, results to be (3.90 ± 0.01) × 10 −22 km −2 yr −1 and the mean impact velocity is 2.46 ± 0.01 km s −1 .W e also fi nd that the distributions of impact velocities seem to be quite different from pure Maxwellian distributions. These results can be useful in developing models of the collisional evolution in the trans-Neptunian region.

Asteroid Dynamical Families

Asteroid dynamical families are extremely important for our understanding of the origin, evolution, and general physical properties of the asteroid population. First identified on the basis of their dynamical properties, families have been soon recognized as the products of well-defined physical processes, namely the disruption of single parent bodies as the consequence of energetic collisional events. The identification of dynamical families has opened important perspectives in all fields of research in asteroid science. The “paradigm” of interpretation of family data has been quickly evolving during the last decade and is now based on the evidence of a complex interplay of different physical and dynamical processes, some of which only recently have been fully recognized. In this chapter, we attempt to give a general and comprehensive review of the subject.

(121514) 1999 UJ7: A primitive, slow-rotating Martian Trojan

Aims. The goal of this investigation is to determine the origin and surface composition of the asteroid (121514) 1999 UJ7, the only currently known L4 Martian Trojan asteroid. Methods. We have obtained visible reflectance spectra and photometry of 1999 UJ7 and compared the spectroscopic results with the spectra of a number of taxonomic classes and subclasses. A light curve was obtained and analysed to determine the asteroid spin state. Results. The visible spectrum of 1999 UJ7 exhibits a negative slope in the blue region and the presence of a wide and deep absorption feature centred around ~0.65 microns. The overall morphology of the spectrum seems to suggest a C-complex taxonomy. The photometric behaviour is fairly complex. The light curve shows a primary period of 1.936 d, but this is derived using only a subset of the photometric data. The asteroid may be in a non-principal axis rotational state, but our observational coverage is insufficient to draw definitive conclusions. Conclusions. Although the observed spectral absorption is wider and deeper, this finding may be compatible with the 0.7 microns spectral feature exhibited by some Ch-type asteroids and could possibly be interpreted as diagnostic of the presence of hydrated minerals. The inferred composition of 1999 UJ7 as a primitive object can be consistent with a volatile-rich object originally accreted beyond the snow line of the solar system and subsequently evolved to reach the inner regions of the solar system.