Charon

A distinct lump observed in Pluto's photographs was identified as its satellite, Charon. This discovery contributed significantly to understanding Pluto's composition and celestial positioning. Pluto was measured to be about 2000 miles in diameter, and Charon 750 miles, forming something like a double planet.
 James W. Christy United States 1978 Charon

A distinct lump observed in Pluto's photographs was identified as its satellite, Charon. This discovery contributed significantly to understanding Pluto's composition and celestial positioning. Pluto was measured to be about 2000 miles in diameter, and Charon 750 miles, forming something like a double planet.