Studies into the rotational history of the minor planets

Abstract

Three aspects of the rotational history of the minor planets are considered - taxonomy, mathematical modelling and observation. Following an introductory review of the basic characteristics and formation, the classification of asteroids is critically assessed and a new scheme based on the spectral characteristics is formulated using numerical taxonomy. It is noted that the asteroids within the basic S-type of other taxonomies can be distributed as two separate zones. the archetypal Juno-type S are found in the middle and outer parts of the main belt whilst a discrete Flora-type S class predominates in the inner portion of the main belt. The groups appear to be two easily distinguishable types probably representing compositional differences. The observed distribution of spin rates for different classes, and hence different surface compositions, is considered. The collision evolution of the spin rates and spin axis orientations has been considered using a new model derived in this work. From this simple model the rotation geometry of precessing bodies can be derived and consideration of the nucleus of Comet Malley suggests that its spin axis is not aligned with any of the geometrical principal axes. Consideration of the geometry of rotation of 1220 Crocus suggests that it might be a binary asteroid. It is found that a random spin axis orientation for the asteroid belt would be expected to be consistent with the observed spin rate distribution. Using the model and the equilibrium shapes of asteroids, calculations have been carried out to investigate the observability of free-precession of asteroids in terms of the maximum light curve amplitude due to free-precession. This shows that only large displacements of the angular velocity vector from the angular momentum vector would produce an amplitude sufficiently great to be detectable. Using a particle in a box model the expected number of examples of free-precession for different sizes of asteroid in the main visible at this time is estimated. New observational data measured as part of this work is presented and analysed. Rotational light curves are examined to determine spin characteristics and whether complex rotations can be seen. As an example new data for 8 Flora has been obtained and, when combined with the digitised information from the international database, has allowed a new model for the shape and variegation to be Proposed by combining the data measured as part of this project with the international data set. From this evidence it is clear that there is no evidence of free precession arising from collisions amongst asteroids in the recent past. In particular it is noted that a shortage of data constrains the accuracy of statistical data and many more quality measurements will be needed before a satisfactory picture of the evolution of minor planets can be presented.