A MOON FOR PLUTO
In 1978, James Christy discovered that Pluto has a moon. He named it Charon, after the mythological boat-man who ferried the souls of the dead across the River Styx to Hades. When astronomers determined Charon's orbit, they were startled to note that between 1985 and 1990, observers on Earth could see the Pluto-Charon system essentially edge-on, and watch the two bodies eclipse and occult one another every 3.2 days.
By taking measurements during these eclipses, scientists worked out that Pluto has a diameter of 1,425 miles (2,300 km), and Charon is fully half its size: 760 miles (1,220km). They are separated by 12,100 miles (19,500 km), making almost a
"double planet." Scientists also estimate the masses of Pluto and Charon together amount to less than 1/400 Earth's mass.