Pluto’s largest moon, Charon, is half the size of Pluto.

 

Pluto’s largest moon, Charon, is half the size of Pluto.  

The same surfaces of Charon and Pluto always face each other, which is known as mutual tidal locking. 

Charon was discovered in 1978 when sharp-eyed astronomer James Christy noticed images of Pluto were strangely elongated. The blob seemed to move around Pluto. The direction of elongation cycled back and forth over 6.39 days―Pluto's rotation period. Searching through their archives of Pluto images taken years before, Christy found more cases where Pluto appeared elongated. Additional images confirmed he had discovered the first known moon of Pluto.

At half the size of Pluto, Charon is the largest of Pluto's moons and the largest known satellite relative to its parent body. Pluto-Charon is our solar system's only known double planetary system. The same surfaces of Charon and Pluto always face each other, a phenomenon called mutual tidal locking. Charon orbits Pluto every 6.4 Earth days.