Asteroid Dactyl

Within seconds of its closest approach to the asteroid 243 Ida on August 28, 1993, the Galileo spacecraft's Solid State Imaging camera caught this glimpse of Ida's previously unknown moon orbiting the asteroid. One frame of a 15-image set designed to capture a view of Ida at the highest possible resolution landed by chance with its edge right on the little moon. The range from the spacecraft was about 2.400 Km and each picture element spans about 24 meters (80 feet) on the surface of the moon. Only a small sliver of the sunlit crescent is visible at the edge of the frame (which was shifted inward toward the center in this frame). Dactyl is approximately egg-shaped, measuring about 1.2 x 1.4 x 1.6 Km (0.75 x 0.87 x 1 mile). At the time this image was shuttered, Ida was about 90 Km (56 miles) away from the moon, outside this frame to the left and slightly below center. The smoothly curving shape of the dark edge of Ida's moon can be seen on the left. The moon's observed darkside was just barely detectable.