Dr. Derek Richardson, University of Maryland, will carry out research aimed at understanding the origin of binary near-Earth asteroids (NEAs). Current estimates from light curve analysis and radar imaging suggest that at least 15% of NEAs (asteroids that approach Earth within 0.017 AU) may be binaries, i.e., dynamical systems that consist of two asteroids orbiting a common center of mass. Moreover, roughly 50% of fast-spinning NEAs (having rotation periods between 2 and 4 hours) with diameters larger than 0.2 km appear to be binaries. The NEA binaries appear qualitatively different from other observed asteroid binary populations, namely the main belt, Trojan, and Kuiper belt binaries. In particular, the NEA binaries tend to have small separations and intermediate size ratios between components. These characteristics, and the frequency of doublet crater formation on terrestrial planets, may be explained if tidal disruption plays an important role in the creation of binary NEAs. However, for tidal disruption to be effective, the progenitor must be fragile (a gravitational aggregate or rubble pile) and therefore capable of being pulled apart by tides. Observational evidence has been mounting that such bodies exist: comet breakups, crater chains, doublet craters, truncated spins, low bulk densities, giant craters, surface grooves, and unusual shapes are all indications of weak, possibly rubble structure among comets and asteroids. Yet previous studies of tidal disruption of asteroids by terrestrial planets were either very simplified models or were concerned primarily with distinguishing between severe and mild disruption without specifically concentrating on binary formation.
Dr. Richardson will perform high-resolution numerical simulations of the tidal disruption of NEAs by the Earth to determine the frequency of binary formation and the typical characteristics of the resulting binary systems. The parameter space to be sampled includes progenitor size, shape, spin, encounter speed, and close-approach distance. The first year of the effort will include some software development since, although the main simulation software is already written and stable, auxiliary codes specific to the proposed project are needed for automation and analysis. In the second and third years the main exploration of parameter space will take place. The results of this study will be compared to existing and future observations of binary NEAs to establish whether tidal disruption can explain some or all of the existing population. Some simulations will be carried out for a longer interval to compare the evolution with analytical estimates, in particular to derive the tidal dissipation parameter Q for a rubble pile, a quantity that could be used to estimate the long-term evolution of a binary system without requiring explicit integration. The simulations will also be examined for slow or tumbling rotators in an effort to understand how such anomalous rotation states originate. Results from this research will give insight into the internal structure of NEAs and may have implications for hazard mitigation strategies. Overall, a better understanding of NEAs in particular will lead to a better understanding of asteroids in general, and thereby to a clearer picture of our origins.
Part of this work entails the development of a web-based interface for organizing and mining the large data sets that will be produced. An undergraduate student with experience in web design will assist in this development. In fact, as many as three undergraduate students and two graduate students will be involved directly in this project. Once the project is completed, the web interface will be available to the public for accessing our data freely. ***
