Numerical simulations of particles placed in orbital resonances in the main asteroid belt show that the typical dynamical lifetimes of objects that could become near-Earth asteroids or meteorites are only a few million years, with the majority destroyed by being transferred to Jupiter-crossing orbits or being driven into the sun. Particles that fortuitously migrate to the terrestrial planet region may be pushed to high-inclination orbits by resonances but are still dynamically eliminated on time scales of -10 million years. These shorter lifetimes may require a reassessment of our qualitative understanding about near-Earth asteroids and meteorite delivery.T h e main asteroid belt is considered to be the source of most meteorites and probably the majority of near-Earth asteroids (NEAs). Our understanding of the mechanisms that transport objects from the main belt to Earth has evolved over the past three decades. We have moved from the idea that it was difficult, if not impossible, to remove obiects from the belt. to an understanding that resonant phenomena can increase eccentricities to the point where Earth collisions are possible (1 ). This progress is due to advances in analytical modeling of resonant phenomena and the use of numerical ex~eriments as comDutational power has increased.Analvtic theories of the dvnamics in the main beit (2) indicate that'many secular and mean-motion resonances are ca~able of transporting particles to Mars-crossing, and often Earth-crossing, orbits. The gravitational attraction of a planet can then pull these ~articles out of the resonances, and subsequent close encounters modify the orbits of the extracted particles. A qualitative understanding has emerged (3) that the resonant phenomena tend to operate quickly and result in orbits that become either sun-grazing (4) or Jupiter-crossing. Particles that escape the resonances by means of planetary close encounters wander more slowly until they find a strong resonant region in which their dynamical lifetimes are short, unless they fortunately escape by another extraction event. This general picture was illustrated with numerical integrations for time scales on the order of a few million years (3), but because the particles chosen for the integrations were the most potentially unstable ones, it was unclear how generic the effects might be for particles deep inside the main belt. To address this question, we numerically integrated =I500 particles initially placed in the most important main-belt resonances. Initial Conditions and MethodThe original intent of this project, known as GAPTEC (5), was to dynamically associate NEAs of known taxonomic type with asteroid families near main-belt resonances with Jupiter. Examining proper orbital elements of the familv members and the location of the borders of neighboring resonances (6) shows that some asteroid families are truncated by a resonance. This implies that the familv-creation event iniected kilometer-scale fragments (now gone) into the resonance. To estimate their original distribution,...

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