In 1978, careful Earth-based observation indicated that the image of Pluto had a slight bulge. This was interpreted as evidence for a previously unknown moon, named Charon. The adjacent image shows subsequent higher quality ground-based observation, and early Hubble Space Telescope observations that show conclusively the existence of Charon. The orbit is show in the inset.

The figure shown below shows a more recent Hubble Space Telescope image of Pluto and Charon.

Pluto (left) and Charon (right)

With the presence of a moon, it was now possible to determine the mass of Pluto to much better precision than before because of the gravitational interaction between the moon and planet. This caused a drastic decrease in the previously assumed value for the mass of Pluto (previously the mass had been assumed to be as large as 10-100% of that of the Earth).

Information supplied by: http://csep10.phys.utk.edu