4769 Castalia is an Earth-crossing and potentially
hazardous asteroid that is a member of the Apollo
group, discovered by Eleanor Helin (Caltech) and colleagues. Castalia
became the first asteroid for which astronomers were able to generate an
image. In August 1989, when it passed 5.6 million km from Earth (11 times
the distance of the Moon), Steven Ostro and his group, at the Jet Propulsion
Laboratory, bounced radar beams off Castalia using the Arecibo radio telescope.
The three-dimensional model built from the resulting data was of a double-lobed
object – a peanut shape – nearly 1.8 km across at its longest.
This suggests that Castalia consists of two 800-m pieces resting together:
the first known contact-binary object in the solar system. Radar echoes
from other near-Earth asteroids have since shown that this configuration is quite common.
||1.8 × 0.8 km
ASTEROIDS, CENTAURS, AND KUIPER BELT OBJECTS