October 17, 2025
The universe has spent billions of years making things that shine. On your birthday, we celebrate one of its quieter masterpieces.
Ida and Dactyl: Asteroid and Moon
Birthday · October 17, 2025
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APOD published October 17, 2025. Full image credits are on the original APOD entry below. A publication-date match, not a photo taken on your date. Independent of NASA.
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.