243 Ida is a 56 km long asteroid orbiting in the main asteroid belt with a number of notable features. Ida is an S-type asteroid, or stony asteroid, and is mostly composed of rock and iron from accretion during early solar system formation. Ida was a subject of study by the Galileo spacecraft in 1993, and much of what we thought we knew from telescopic observation has been overturned by the closer look provided by the flyby mission.
Ida orbits with a large number of other asteroids in what is known as the Koronis Asteroid Family. This is a collection of objects with similar orbits, which indicates that they likely were formed from the shattered remains of a single larger asteroid that was recently destroyed by an impact. However, the surface of Ida is heavily cratered, indicating that the asteroid is older than originally anticipated, and older than estimates for the Koronis breakup. It was also discovered that Ida has a natural satellite, Dactyl.
Dactyl is the first natural satellite of an asteroid discovered and scientists have sought to explain how the small asteroid with gravity 0.001 times that of Earth’s gravity could hold onto a satellite. With no atmosphere and such low gravity, it is very unlikely that Dactyl was captured by Ida, so it seems likely that Dactyl may have broken off Ida during a large impact, and then settled in a slow orbit around the asteroid. This theory has gained credibility because of the discovery of a relatively recent crater, named the Azzura crater that may be the result of the impact that created Dactyl. Additionally, ejecta blocks, which are large boulders laying on the surface of Ida, have been found primarily on the leading surfaces of the oblong asteroid as it rotates. This suggests that these boulders were ejected from the surface of Ida and then slowly settled back down to be swept up preferentially by leading surfaces of the quickly spinning asteroid.
Much of the information in this blog post came from Cosmic Pinball by Carolyn Somners and Carlton Allen and I would highly recommend reading it for more information about solar system collisions.