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This paper presents the result of the first systematic investigation of very small Main-Belt Asteroids (sub-km MBAs) using the Subaru Prime-Focus Camera (Suprime-Cam) with an $8 \,\mathrm{K} \times 10 \,\mathrm{K}$ mosaic CCD array attached to the 8.2m Subaru Telescope atop Mauna Kea, Hawaii. We call this survey SMBAS (Sub-km Main-Belt Asteroid Survey). Observations were carried out on 2001 February 22 and 25 (HST) and a $\sim 3.0 \,\mathrm{deg}^{2}$ sky area near the opposition and near the ecliptic was searched. We detected 1111 moving objects down to $R \sim 26 \,\mathrm{mag}$ (including very slow Trans-Neptunian Objects). In this survey, we could not determine the exact orbits of the moving objects, because of their short observational arc of only 2 hours. Instead, we statistically estimated the semi-major axis ($a$) and inclination ($I$) of each moving object from its apparent sky-motion vector, and then obtained the size and spatial distributions of sub-km MBAs. The main results of SMBAS are: (1) The sky number density of MBAs is found to be $\sim 290 \,\mathrm{deg}^{-2}$ down to $R\sim 24.4 \,\mathrm{mag}$ (for MBAs) near the opposition and near the ecliptic. (2) The slope of the cumulative size distribution for sub-km MBAs ranging from 0.5 km to 1 km in diameter is fairly shallower ($\sim 1.2$) than that for large MBAs of more than $\sim 5 \,\mathrm{km}$ in diameter ($\sim 1.8$), which was obtained from past asteroid surveys. This means that the number of sub-km MBAs is much more depleted than a result extrapolated from the size distributions for large asteroids. (3) The depletion of sub-km MBAs is clearer in the outer main-belt than in the inner main-belt. (4) It seems that SMBAS asteroids distribute more widely in the $I$-direction in the outer zone ($a= 2.8 \hbox{--} 3.1 \,\mathrm{AU}$) of the main-belt than known large asteroids do. We also discuss the possible causes for the characteristics of the distributions of SMBAS-observed small asteroids.

Abstract. We present photometric observations of the near-Earth asteroid (25143) 1998 SF36 from the 2001 apparition campaign, and we discuss the corresponding physical model. The asteroid's photometric behaviour is consistent with an S-type object, it has a retrograde pole at λ = 355• , β = −84 • ± 5 • , and its sidereal rotation period is P = 12.132 ± 0.0005 hours. 1998 SF36 is elongated, with rough global dimension ratios a/b = 2.0, b/c = 1.3, but the elongation is not due to a bifurcated shape. The surface is not likely to contain major concavities. No significant albedo variegation was detected.