Astronomy Picture of the Day

Discover the cosmos! Each day we feature a different image or photograph of our fascinating universe, along with a brief explanation written by a professional astronomer.

June 30, 1995



Ida and Dactyl: Asteroid and Moon

Picture Credit: NASA, JPL, Galileo Project

Explanation: An asteroid with a moon! The robot spacecraft Galileo whose primary mission is to explore the Jupiter system, has encountered and photographed two asteroids during its long journey to Jupiter. The second asteroid it photographed, called Ida, was discovered to have a moon which appears as a small dot to the right of Ida in this picture. The tiny moon, named Dactyl, is about one mile across, while the potato shaped Ida measures about 36 miles long and 14 miles wide. Dactyl is the first moon of an asteroid ever discovered. The names Ida and Dactyl are based on characters in Greek mythology.

For more information see NASA, Jet Propulsion Laboratory press release.