This is the Web Edition of "A Trip Into Space", a Coimbra-based electronic book on space science. Both the texts and the photos are by courtesy of National Aeronautics and Space Administration.

Asteroid Ida's Moon Dactyl
A Natural Satellite

This image is the most detailed picture of the recently discovered natural satellite of asteroid 243 Ida taken by the Galileo 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 kilometers (2,400 miles), just over 4 minutes before the spacecraft's closest approach to Ida. Each picture element spans about 39 meters (125 feet) on the surface of the moon.

More than a dozen craters larger than 80 meters (250 feet) 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 meters (1,000 feet) across. The satellite is approximately egg-shaped, measuring about 1.2 x 1.4 x 1.6 kilometers (0.75 x 0.87 x 1 mile). At the time this image was shuttered, Ida was about 90 kilometers (56 miles) away from the moon, outside this frame to the left and slightly below center. This image was relayed to Earth from Galileo on June 8, 1994.

The Galileo project, whose primary mission is the exploration of the Jupiter system in 1995-97, is managed for NASA's Office of Space Science by the Jet Propulsion Laboratory.


The International Astronomical Union (IAU) has approved the name Dactyl for the tiny moon discovered this year in orbit around the asteroid Ida by NASA's Galileo mission.

The IAU also approved names for surface features on another asteroid, Gaspra, which became the first asteroid ever visited by a spacecraft when Galileo flew by it on Oct. 29, 1991.

Dactyl is the first natural satellite of an asteroid ever discovered and photographed. The tiny moon, about one mile (1.5 kilometers) across, appeared in images obtained by the Galileo spacecraft during its flyby of the asteroid on Aug. 28, 1993.

Dactyl was discovered in data analyzed in March 1994 by members of Galileo's imaging and infrared science teams. The project recommended the name to the IAU, which is responsible by international agreement for the formal naming of Solar System bodies.

The name is derived from the Dactyli, a group of mythological beings who lived on Mount Ida, where the infant Zeus was hidden -- and raised, in some accounts -- by the nymph Ida and protected by the Dactyli. Other mythological accounts say that the Dactyli were Ida's children by Zeus.

Three regions on Gaspra were named for scientists associated with the asteroid. Neujmin Regio was named for G. Neujmin, the Ukrainian astronomer who discovered the asteroid in 1916. Yeates Regio honors the late Dr. Clayne M. Yeates, who was Galileo Science Manager and Science and Mission Design Manager until his death in 1991. Dunne Regio was named in honor of the late Dr. James A. Dunne, who served as Galileo Science and Mission Design Manager until late 1992.

"Clayne Yeates and Jim Dunne both contributed immensely to the Galileo project and to the Gaspra encounter in particular," said Galileo Project Manager William J. O'Neil at NASA's Jet Propulsion Laboratory (JPL), Pasadena, Calif.

The surfaces of Gaspra and Ida are covered with impact craters like those on Earth's Moon. Gaspra was named by Neujmin for a resort on the Crimean peninsula. Consequently, many of the asteroid's craters have been named for resorts and spas worldwide.

The Galileo spacecraft is on its way to Jupiter, where it will send a probe into the atmosphere on Dec. 7, 1995, and then go into orbit for a two-year scientific tour of the planet, its satellites and its magnetosphere. JPL manages the Galileo project for NASA's Office of Space Science, Washington, D.C.

Last Update: 2005-Nov-29