august: the smallest moon?

The 56x24x21 km (35x15x13 mile) asteroid Ida has harboured an interesting secret since its discovery in 1884. In 1993 is was found to have a satellite, now named Dactyl (after the Dactyli who inhabit Mt Ida in Greek mythology). The tiny moon is about 1.2 by 1.4 by 1.6 km (0.8x0.9x1.0 mile) across.

Dactyl is made of much the same material as Ida - silicate rocks. The two may have formed at the same, or possibly Dactyl was formed from the material ejected by an impact on Ida. Dactyl's orbital velocity is the same as a slowly thrown baseball, and it almost certainly cannot have been gravitationally captured as it passed by.

In the inset picture of Dactyl, the large crater on the terminator is about 300 meters (1,000 feet) across.

Image: Galileo Project, JPL, NASA.