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APOD
Astronomy Picture of the Day
Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.
Ida and Dactyl: Asteroid and Moon
Explanation: This asteroid has a moon. The robot spacecraft Galileo enroute to explore the Jovian system in 1993, encountered and photographed two asteroids during its long interplanetary voyage. The second minor planet it photographed, 243 Ida , was discovered to have a moon. The tiny moon, named Dactyl , is only about 1.6 kilometers across and seen as a small dot to the right in the image . In contrast, Ida is much larger, measuring about 60 kilometers long and 25 km wide. In fact, Dactyl is the first moon of an asteroid ever discovered. But now many asteroids are known to have moons. Minor planet names Ida and Dactyl were taken from Greek mythology .
| Date | October 17, 2025 |
|---|---|
| Credit | Image Credit: NASA , JPL , Galileo Mission |
| Authors & editors: | Robert Nemiroff (MTU) & Jerry Bonnell (UMCP) |
| A service of: |
ASD at NASA / GSFC, NASA Science Activation & Michigan Tech. U. |