 Ida 's moon , Dactyl , was discovered by mission member Ann Harch in images returned from Galileo . It was named after the Dactyls , creatures which inhabited Mount Ida in Greek mythology . Dactyl , being only 1 @.@ 4 kilometres ( 4 @,@ 600 ft ) in diameter , is about one @-@ twentieth the size of Ida . Its orbit around Ida could not be determined with much accuracy . However , the constraints of possible orbits allowed a rough determination of Ida 's density , which revealed that it is depleted of metallic minerals . Dactyl and Ida share many characteristics , suggesting a common origin . 
