Ida was the first asteroid proved to have a tiny moon, Dactyl. Like Gaspra, it is heavily cratered and probably composed of silicates, but it also has a scattering of large boulders and is probably covered with a deep layer of fine material. Until then, little was known about Mathilde. Its rotation period of 417 hours is one of the longest known so far. Mathilde is a C-type asteroid.
Some scientists believe it is little more than a giant rubble pile.
Named Braille shortly before the arrival of NASA's Deep Space 1 spacecraft in December 1998. Distant images show that it is very black and possibly similar in composition to the large asteroid Vesta. It may be a piece which broke away from Vesta during a past collision. Most asteroids have very irregular shapes, mainly because they have suffered many impacts over billions of years. These impacts have left large craters, as seen in this computer model which shows the shape of Eros 433 Eros.