Presentation #312.01 in the session “WISE About Asteroids!”.
The sky-plane density of main-belt minor planets (MBAs) at various bandpasses and observing geometries is of great importance to solar system astronomers. At faint optical magnitudes (R ~25) there can be over 1000 objects per square degree at opposition (A. Heinze et al). For infrared surveys, particularly those conducted at smaller solar elongations, the sky-plane density of MBAs has yet to be measured. In preparation for the NEO Surveyor Mission, we are using the Reference Small Body Population Model (RSBPM) to estimate the number of objects observed at thermal wavelengths from ~45 to 120 degrees solar elongation. Preliminary results suggest a significantly smaller number of MBAs, ranging from ~100 per square degree at 45 degrees solar elongation to ~400 objects per square degree at 120 degrees solar elongation. The main reason for the decrease in density is the increased geocentric distance and hence the fainter flux received for the target MBA as compared to the same at opposition. We will present not only estimates of the background MBA sky-plane density, but also the sky-plane density of NEOs greater than 140 meters in diameter at similar observing geometries.