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A. W. Harris (Space Science Institute)
The population of Near-Earth asteroids down to ~ 1 km diameter (absolute magnitude H = 18) is reasonably well determined. However, in the size range of ``Tunguska event" NEAs (diameter ~ 50-75 m), estimates of population, or equivalently of impact frequency, range from once per couple hundred years to once per 10,000 years. The fact that one such event occurred just a century ago argues for a population closer to the former value. The LINEAR survey has now discovered ~ 30 NEAs in the ``Tunguska" size range (H ~ 24.0-24.5), thus a better estimate is possible. In order to make such an estimate, one can calculate the expected fraction of a synthetic population that should be detected given the known survey parameters, and then inflate the detected number by dividing by that fraction. However for small NEAs, the completion factor is a very small number and a very large simulation is needed in order to obtain even a few ``detections" in the model. I report here a new simulation from which I have obtained the relative detection efficiency in the size range 21.5 < H < 25.5. Taking the actual number of NEAs discovered by LINEAR in 0.5 mag. bins of H, and dividing by the relative completion factors, I obtain relative populations in each size bin. By normalizing the populations to agree with the absolute population estimates of Stuart (Science 294, 1691-1693, 2001) in his two smallest size bins, I extend his curve from H = 22.5 to H = 25.5, spanning the size range of ``Tunguska" objects. I find a population of the order of half a million objects in this size range (H ~ 24.0-24.5), corresponding to an expected impact interval of the order of once per thousand years. This estimate is uncertain by a factor of about 3, largely due to uncertainty in the actual size of the Tunguska event and uncertanty in relating absolute magnitude to size of object.
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.