Ida and Moon

Ida is an asteroid which became much more familiar with its close encounter with the planetary explorer spacecraft Galileo. In 1993, Galileo's rendevous with Ida brought the surprise discovery that Ida had its own satellite: a tiny moon about 1.5 km in diameter orbiting at a distance of about 90 km. The moon was named Dactyl.

This color picture is made from images taken by the imaging system on the Galileo spacecraft about 14 minutes before its closest approach to Ida on August 28, 1993. The range from the spacecraft was about 10,500 km. The images are from the sequence in which Ida's moon was originally discovered. The maximum dimension of Ida is about 60 km.

This picture is made from images through the 410 nm (violet), 756 nm (infrared) and 968 nm (infrared) filters. The color is enhanced in the sense that the camera is sensitive to near-infrared wavelengths of light beyone human vision; a natural color picture of this asteroid would appear mostly gray. Shadings in this 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 the small bright crater near the center of the asteroid, and near the upper righthand limb. This is a combination of more reflected blue light and greater absorption of near infrared light, suggesting a difference in the abundance of iron-bearing minerals in these areas.

From the analysis of the binary orbit of the moon Dactyl around Ida, Petit* determined the mass of Ida to be about 2.2 x 10^-14 solar masses or 4.3 x 10¹⁶ kg and its density to be 2.7 +/- .4 g/cm³.

Sources

• Naval Observatory posting on asteroid masses
• NASA/ Jet Propulsion Laboratories information

Closeup of Asteroid Ida

The Galileo imaging system captured this picture of the limb of the asteroid 243 Ida about 46 seconds after its closest approach on August 28, 1993, from a range of only 2,480 kilometers. It is the highest-resolution image of an asteroid's surface ever captured, and shows detail at a scale of about 25 meters per pixel.

Prominent in this view is a 2-kilometer-deep "valley" seen in profile on the limb. This high-resolution view shows many small craters and some grooves on the surface of Ida, which give clues to understanding the history of this heavily impacted object.

Ida's moon is approximately egg-shaped, measuring about 1.2x1.4x1.6 kilometers (.75x.87x1 miles). The moon was about 90 kilometers away from Ida at the time of this image.

This image is the most detailed picture of the recently discovered natural satellite of asteroid 243 Ida taken by the Galileo spacecraft's solid state imaging camera during its encounter with the asteroid on August 28, 1993. Shuttered through the camera's broadband clear filter as part of a 30-frame mosaic designed to image the asteroid itself, this frame fortuitously captured the previously unknown moon at a range of about 3,900 km (2,400 miles), just over 4 minutes before the spacecraft's closest approach to Ida. Each picture element spans about 39 meters (125 ft) on the surface of the moon.

More than a dozen craters larger than 80 meters (250 ft) in diameter are clearly evident, indicating that the moon has suffered numerous collisions from smaller solar system debris during its history. The larger crater on the terminator is about 300 m (1,000 ft) across. The satellite is approximately egg-shaped, measuring about 1.2x1.4x1.6 km (0.75x0.87x1 miles). At the time this image was shuttered, Ida was about 90 km (56 miles) away from the moon. This image was relayed to Earth from Galileo on June 8, 1994. These images were provided by Jet Propulsion Laboratories via the Internet.