The Population of Mars-Crossers: Classification and Dynamical Evolution

Recent dynamical results (Gladman et al. 1997. Dynamical lifetimes of objects injected into asteroid belt resonances. Science277, 197–201) have pointed out that the bodies injected by collisions into the main resonances of the asteroid belt could not sustain the observed population of Earth-crossers of large diameter. In this paper, we present our numerical exploration of the dynamical evolution of Mars-crosser asteroids initiated by Migliorini et al. (1998. Origin of multikilometer Earth and Mars-crossing asteroids: A quantitative simulation. Science281, 2022–2024.) and improved by computing the evolution over 100 Myr of the orbits of the whole observed sample of this population. Mars-crossers are about 35 times more numerous than Earth-crossers (at least down to 5 km in diameter). On the basis of their current orbital elements, we show that this population can be divided in different groups which have well-characterized dynamical behaviors, lifetimes, and end-states. From the dominant group, the asteroids evolve to intersect the Earth's orbit on a median time scale of about 60 Myr. Then, they can interchange between the Earth-crosser state and an evolved solely Mars-crosser one before colliding with the Sun or being ejected outside Saturn's orbit. Based on these dynamical results, we show that Mars-crosser asteroids can sustain about half of the multikilometer Earth-crossing population, and the expected orbital distribution of Earth-crossers coming from Mars-crossers is computed. Then, from an estimate of the size distribution of the Mars-crosser population and the spectral analysis of its different groups, we derive expected numbers of Earth-crossers and ratios of different taxonomic types in the Earth-crosser population that are not in disagreement with the observed ones. We also confirm by spectral analysis the viability of the scenario, according to which the Mars-crosser population is sustained by asteroids which leak out from the main belt.

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