Estimated abundance of Atens and asteroids evolving on orbits between Earth and Sun

The so-called Aten asteroids, evolving on orbits with semimajor axis a < 1 AU and aphelion distance Q > 0.983 AU, spend most of their time inside Eart h's orbit. Currently, they account for about 7% of the observed near-Earth asteroid population and 13% of the Earth-crossing one. However, observation al biases play against their discovery; thus the present number of Atens is probably severely underestimated. Another still unobserved population of a steroids, which we call IEO (inner-Earth objects), could also exist and evo lve entirely inside Earth's orbit. Orbital numerical integrations of known source populations of Earth-crossers indicate that the real fraction of Ate ns and IEOs in the Earth-crossing population depends on the considered obje cts' diameters and could be close to 20% considering only the multikilomete r bodies. Moreover, the fraction of IEOs could be as large as half that of Atens. Since these asteroids can also impact Earth, their threat should ser iously be taken into account. (C) 2000 Academic Press.