The largest of Pluto’s five moons, Charon, was discovered on June 22, 1978, by James Christy and Robert Harrington at the U.S. Naval Observatory in Flagstaff, Arizona.
Read the story: https://www.nasa.gov/feature/charon-at-40-four-decades-of-discovery-on-pluto-s-largest-moon
Charon was discovered only about six miles from where Pluto itself was discovered at Lowell Observatory. They weren’t even looking for satellites of Pluto – Christy, examining a series of grainy telescope images, trying to refine Pluto’s orbit around the Sun.
Christy and others tell the story of this amazing scientific find, which fueled Pluto’s transformation from a telescopic dot into an actual planetary system – and a source of many discoveries to come.