Astronomy Picture of the Day

Explanation: This asteroid has a moon. The robot spacecraft Galileo on route to Jupiter in 1993 encountered and photographed two asteroids during its long interplanetary voyage. The second minor planet it photographed, 243 Ida, was unexpectedly discovered to have a moon. The tiny moon, Dactyl, is only about 1.6 kilometres across and seen as a small dot on the right of the sharpened featured image. In contrast, the potato-shaped Ida is much larger, measuring about 60 kilometres long and 25 km wide. Dactyl is the first moon of an asteroid ever discovered -- now many asteroids are known to have moons. The names Ida and Dactyl are from Greek mythology.