Visual and radiometric photometry of 1580 Betulia

Broadband visual and 10.6-micron photometry of 1580 Betulia was obtained during its close approach to earth in May 1976. The photometry was analyzed by using the 'radiometric method' to derive the radius (2.10 + or - 0.40 km) and albedo (0.108 + or - 0.012) of Betulia. Radar and polarimetric results indicate a radius greater than 3.0 km and a geometric albedo of about 0.05. To be compatible with these results, Betulia was also modeled as having a surface with the thermal characteristics of bare rock rather than those of the 'lunar' regolith model used for previous analysis of radiometry of other asteroids. A 3.7-km radius and a geometric albedo of about 0.04 are compatible with all available observations. Betulia is the first Mars-crossing asteroid found to have such a low albedo, which may be indicative of carbonaceous surface material.