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.

2004 June 19
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Ida and Dactyl: Asteroid and Moon
Credit: Galileo Project, JPL, NASA

Explanation: This asteroid has a moon! The robot spacecraft Galileo destined to explore the Jovian system, encountered and photographed two asteroids during its long interplanetary voyage to Jupiter. The second asteroid it photographed, Ida, was discovered to have a moon which appears as a small dot to the right of Ida in this image from 1993. 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 from Greek mythology. Many other asteroids are now known to have moons.

Tomorrow's picture: solstice celebration

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