243 Ida was discovered by by the Austrian astronomer Johann Palisa on September 29th, 1884, and unlike most bodies in the main asteroid belt, this particular S-type asteroid has been studied at close quarters. Ida was visited by the Galileo spacecraft in 1993. It is one of the larger members of the Koronian family of asteroids, about 6000 strong, who orbit as a group, and are thought to be the remains of a collision involving a single, larger body. Thanks to Galileo, we have pretty good photographs of Ida, and we now know it has a small companion, Dactyl.
Dactyl is pretty teeny, measuring just over a kilometre across in most directions. It is the small dot on the right side of Ida in the photograph, and was the first moon discovered orbiting an asteroid. The name comes from a race of mythical beings who inhabited Mount Ida in Crete. They are credited with discovering how to use fire to work metal.
The asteroid itself is named for the nymph who, with her sister Adrasteia, was entrusted with caring for the infant god Zeus. The name was the idea of Moriz von Kuffner, an Austrian brewer, philanthropist and astronomer, who founded the Kuffner Observatory in Vienna. Kuffner eventually got his own named asteroid, but not until 2006 (number 12568).
Also as a result of the Galileo visit, I will be abandoning my usual use of the words “about” and “approximately”, and will be stating that Ida’s principal dimensions are 59.8 km by 25.4 km by 18.6 km, with a mean radius of 15.7 km (Belton et al, 1996).