Origin and Evolution of Near-Earth Objects

Asteroids and comets on orbits with perihelion distance q < 1.3 AU and aphelion distance Q > 0.983 AU are usually called near-Earth objects (NEOs). It has long been debated whether the NEOs are mostly of asteroidal or cometary origin. With improved knowledge of resonant dynamics, it is now clear that the asteroid belt is capable of supplying most of the observed NEOs. Particular zones in the main belt provide NEOs via powerful and diffusive resonances. Through the numerical integration of a large number of test asteroids in these zones, the possible evolutionary paths of NEOs have been identified and the statistical properties of NEOs dynamics have been quantified. This work has allowed the construction of a steady-state model of the orbital and magnitude distribution of the NEO population, dependent on parameters that are quantified by calibration with the available observations. The model accounts for the existence of ~1000 NEOs with absolute magnitude H < 18 (roughly 1 km in size). These bodies carry a probability of one collision with the Earth every 0.5 m.y. Only 6% of the NEO population should be of Kuiper Belt origin. Finally, it has been generally believed that collisional activity in the main belt, which continuously breaks up large asteroids, injects a large quantity of fresh material into the NEO source regions. In this manner, the NEO population is kept in steady state. The steep size distribution associated with fresh collisonal debris, however, is not observed among the NEO population. This paradox might suggest that Yarkovsky thermal drag, rather than collisional injection, plays the dominant role in delivering material to the NEO source resonances.