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.

2023 May 28
A pair of asteroids are shown with a large, elongated and cratered
one on the left and a much smaller one on the far right.
Please see the explanation for more detailed information.

Ida and Dactyl: Asteroid and Moon
Image Credit: NASA, JPL, Galileo Mission

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.

Tomorrow's picture: sea blue sky


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