remnants of a destroyed precursor body. Ida's reflection spectrum was measured on 16 September 1980 by astronomers David J. Tholen and Edward F. Tedesco as part of the eight-color asteroid survey (ECAS). Its spectrum matched those of the asteroids in the S-type classification. Many observations of Ida were made in early 1993 by the US Naval Observatory in Flagstaff and the Oak Ridge Observatory. These improved the measurement of Ida's orbit around the Sun and reduced the uncertainty of its position during the Galileo flyby from . Exploration Galileo flyby Ida was visited in 1993 by the Jupiter-bound space probe Galileo. Its encounters of the asteroids Gaspra and Ida were secondary to the Jupiter mission. These were selected as targets in response to a new NASA policy directing mission planners to consider asteroid flybys for all spacecraft crossing the belt. No prior missions had attempted such a flyby. Galileo was launched into orbit by the Space Shuttle Atlantis mission STS-34 on 18 October 1989. Changing Galileo's trajectory to approach Ida required that it consume of propellant. Mission planners delayed the decision to attempt a flyby until they were certain that this would leave the spacecraft enough propellant to complete its Jupiter mission. Galileo's trajectory carried it into the asteroid belt twice on its way to Jupiter. During its second crossing, it flew by Ida on 28 August 1993 at a speed of relative to the asteroid. The onboard imager observed Ida from a distance of to its closest approach of . Ida was the second asteroid, after Gaspra, to be imaged by a spacecraft. About 95% of Ida's surface came into view of the probe during the flyby. Transmission of many Ida images was delayed due to a permanent failure in the spacecraft's high-gain antenna. The first five images were received in September 1993. These comprised a high-resolution mosaic of the asteroid at a resolution of 31–38 m/pixel. The remaining images were sent in February 1994, when the spacecraft's proximity to the Earth allowed higher speed transmissions. Discoveries The data returned from the Galileo flybys of Gaspra and Ida, and the later NEAR Shoemaker asteroid mission, permitted the first study of asteroid geology. Ida's relatively large surface exhibited a diverse range of geological features. The discovery of Ida's moon Dactyl, the first confirmed satellite of an asteroid, provided additional insights into Ida's composition. Ida is classified as an S-type asteroid