204 Kallisto is a fairly typical, although sizeable Main belt asteroid. It is classified as an S-type asteroid. Like other asteroids of its type, it is light in colour. It was discovered by Johann Palisa on 8 October 1879, in Pola, and was named after the same nymph Callisto in Greek mythology as Jupiter's moon Callisto. Photometric measurements during 2009 produced a lightcurve that indicated a sidereal rotation period of 19.489 h with a variation amplitude of 0.18 magnitudes. This result conflicted with previous determinations of the period, so the latter were ruled out.