Leda is Jupiter's ninth known moon. Leda's orbit is about 6.8 million miles from Jupiter. Leda has a diameter of 10 miles. The name Leda comes from a queen of Sparta and the mother, by Zeus in the form of a swan, of Pollux and Helen of Troy. Leda was discovered in 1974 by Kowal. Io has a mass of 5.68e15kg.