Enhanced Ida and Dactyl
Date: 28 Aug 1993
This color picture is made with images taken by the Galileo spacecraft about 14 minutes before its closest approach to asteroid 243 Ida on August 28, 1993. The range from the spacecraft was about 10,500 kilometers (6500 miles). The images used are from the sequence in which Ida's moon was originally discovered; the moon is visible to the right of the asteroid. This picture is made through the 4100-angstrom (violet), 7560-angstrom (infrared) and 9680-angstrom (infrared) filters.
The color is enhanced in the sense that the CCD camera is sensitive to near-infrared wavelengths of light beyond human vision; a natural color picture of this asteroid would appear mostly gray. Shadings in the image indicate changes in illumination angle on the many steep slopes of this irregular body, as well as subtle color variations due to differences in the physical state and composition of the soil (regolith). There are brighter areas, appearing bluish in the picture, around craters on the upper left end of Ida, around the small bright crater near the center of the asteroid, and near the upper righthand edge (the limb). This is a combination of more reflected blue light and greater absorption of near-infrared light, suggesting a difference in the abundance or composition of iron-bearing minerals in these areas
Ida's moon also has a deeper near-infrared absorption and a different color in the violet than any area on this side of Ida. The moon is not identical in spectral properties to any area of Ida in view here, although its overall similarity in reflectance and general spectral type suggests that it is basically made of the same rock types. These data, combined with study of further imaging data and more detailed spectra from the Near Infrared Mapping Spectrometer, may allow scientists to determine whether the larger parent body (of which Ida, its moon, and some other asteroids are fragments) was a heated, differentiated object or made of relatively unaltered primitive chondritic material.