Castalia was found to have a peanut shape, suggesting two 800m diameter pieces held together by their weak mutual gravity. Since then radar measurements of other asteroids has indicated that this contact binary arrangement appears to be common.
The asteroid Castalia was the first asteroid to be directly imaged, in August of 1989. Its orbit took it within 11 lunar distances of Earth, allowing it to be scanned with radar from the Arecibo radio telescope.
Orbital characteristics Orbit type Near-Earth Semimajor axis 1.063 AU Eccentricity 0.4831 Orbital period 1.10 years Inclination 8.89° Physical characteristics Diameter 1.8×0.8 km Mass 5.0×1011 kg Density 2.1 g/cm3 Rotation period 4h Spectral class S[?] Albedo ? History Discoverer Eleanor F. Helin[?], 1989