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The Kilometer-Sized Main Belt Asteroid Population as Revealed by Spitzer

Multi-epoch Spitzer Space Telescope 24 micron data is utilized from the MIPSGAL and Taurus Legacy surveys to detect asteroids based on their relative motion. These infrared detections are matched to known asteroids and rotationally averaged diameters and albedos are derived using the Near Earth Asteroid Model (NEATM) in conjunction with Monte Carlo simulations for 1835 asteroids ranging in size from 0.2 to 143.6 km. A small subsample of these objects was also detected by IRAS or MSX and the single wavelength albedo and diameter fits derived from this data are within 5% of the IRAS and/or MSX derived albedos and diameters demonstrating the robustness of our technique. The mean geometric albedo of the small main belt asteroids in this sample is p_V = 0.138 with a sample standard deviation of 0.105. The albedo distribution of this sample is far more diverse than the IRAS or MSX samples. The cumulative size-frequency distribution of asteroids in the main belt at small diameters is directly derived. Completeness limits of the optical and infrared surveys are discussed.