It was discovered on 9 August 1989, by American astronomer Eleanor Helin (Caltech) on photographic plates taken at Palomar Observatory in California.

[1] On 25 August 1989 Castalia passed 0.0269378 AU (4,029,840 km; 2,504,020 mi)[3] (within eleven lunar distances) of Earth, allowing it to be observed with radar from the Arecibo Observatory by Scott Hudson (Washington State University) and Steven J. Ostro (JPL).

The data allowed Hudson et al. to produce a three-dimensional model of the object.

[4] Castalia has a peanut shape, suggesting two approximately 800-meter-diameter pieces held together by their weak mutual gravity.

[5] Castalia is a potentially hazardous asteroid (PHA) because its minimum orbit intersection distance (MOID) is less than 0.05 AU and its diameter is greater than 150 meters.