243 Ida is an S-type asteroid. It was discovered on September 29, 1884 by Austrian astronomer Johann Palisa. It was named after a nymph from Greek mythology.
Ida's orbit lies between the planets Mars and Jupiter. It's orbit is 1,767.644 days (4.84 years) and it's rotation period is 4.63 hours. 243 Ida is 2.74 A.U. away from the sun.
Apparently, 243 Ida is composed of two larger objects connected together. It's average diameter is 31.4 km and the size is 58 x 23 km. Ida is 2.35 times as long as it is wide and a "waist" separates it into two geologically dissimilar halves.
Galileo discovered Ida's moon Dactyl, the first known satellite of an asteroid. Dactyl and Ida share many characteristics, suggesting a common origin.
It is not likely that Ida will collide with Earth. No human in the past 1,000 years is known to have been killed by an asteroid or the effects of ones impact. The risk is very small but the risk increases with the size and impact of the comet or asteroid.
Ida is a heavily cratered, irregular shaped asteroid in the main asteroid belt. It is mostly composed of silicate rocks. Ida is one of the most densely cratered bodies that has been explored in the Solar System. The composition of the interior has not been directly analyzed, but is assumed to be similar to OC material based on observed surface color changes and Ida's bulk density.
Ida was the second asteroid visited by a spacecraft. In 1993 it was visited by the Jupiter-bound space probe Galileo.