The kilometer-sized Main Belt asteroid population revealed by Spitzer

Aims. Multi-epoch Spitzer Space Telescope 24 mu m data is utilized from the MIPSGAL and Taurus Legacy surveys to detect asteroids based on their relative motion. Methods. Infrared detections are matched to known asteroids and average diameters and albedos are derived using the near Earth asteroid thermal model (NEATM) for 1865 asteroids ranging in size from 0.2 to 169 km. A small subsample of these objects was also detected by IRAS or MSX and the single wavelength albedo and diameter fits derived from these data are within the uncertainties of the IRAS and/or MSX derived albedos and diameters and available occultation diameters, which demonstrates the robustness of our technique. Results. The mean geometric albedo of the small Main Belt asteroids in this sample is p(V) = 0.134 with a sample standard deviation of 0.106. 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 and a 3 sigma deviation from the fitted size-frequency distribution slope is found near 8 km. Completeness limits of the optical and infrared surveys are discussed.