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
(Ref)
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).