One way to explain the existence of the Moon is through a giant
collision, one that tore off enough material to build a
satellite in a planetary orbit. Can Pluto and its moon Charon be
explained the same way? Robin Canup thinks so. Canup is
assistant director of Southwest Research Institute’s Department
of Space Studies; she argues the case in the January 28 issue of
Science. The Moon may seem large in our skies, but it makes up
only about 1 percent of Earth’s mass. Charon, on the other hand,
is 10 to 15 percent the mass of Pluto, which suggests to Canup
that the corresponding collision must have been with an object
almost as large as Pluto itself. She also believes that Charon
probably formed intact as a result of the collision. “This work
suggests that despite their many differences, our Earth and the
tiny, distant Pluto may share a key element in their formation
histories. This provides further support for the emerging view
that stochastic impact events may have played an important role
in shaping final pl