Opposition effect of asteroids

The opposition effect behavior is analyzed for 33 asteroids with well-measured magnitude-phase curves. It is shown that surface albedo is the main factor influencing the amplitude and width of the asteroid opposition effect. The amplitude of opposition effect depends on asteroid albedo in a non-monotonic way with the maximum for moderate albedo asteroids decreasing both for dark and for high albedo asteroids. This can be explained by the combined influence of both shadow hiding and coherent-backscatter mechanisms. The relative contribution of the coherent-backscatter effect to the asteroid magnitude-phase dependence close to opposition (alpha = 0.3 degrees) is estimated to be about 20-60% for low albedo asteroids and 80-90% for high and moderate albedo asteroids. At alpha > 5 degrees the shadow-hiding effect has the dominant influence. A tendency was found for asteroids with larger U-B colors to show larger values of the opposition effect. We also compare the opposition effects of asteroids with those of some atmosphereless satellites and meteorites. (C) 2000 Academic Press.