The pairing of Pluto and its moon Charon is unique in our solar system because Charon is nearly half the size of the planet it orbits, whereas the diameters of most moons are just a few percent of those of their parent bodies. Findings published today in the journal Science shed new light on how the pairing formed and support the hypothesis that a large cosmic collision was to blame.

In 2001 astronomers successfully simulated how a crash between a Mars-size body and our planet near the end of its formation could have generated the moon. A leading theory for the Pluto-Charon system is that a similar crash is responsible for Pluto's satellite. In the new work, Robin Canup of the Southwest Research Institute in Boulder, Colo., designed a computer model that is the first of its kind to demonstrate that the impact theory is quite plausible.

The results indicate that the blow of an object measuring between 1,600 and 2,000 kilometers in diameter traveling about one kilometer per second and crashing into Pluto could have spawned its moon. Alternatively, the smash-up may have created a disk of debris around Pluto from which Charon later accumulated. The best chance for a definitive answer to the question of Charon's creation may lie with NASA's New Horizons mission to Pluto, which is currently scheduled to a take off next year, although it won't reach its target until 2015.