Pluto
Discovered in 1930 by Clyde Tombaugh. Charon discovered at USNO in 1978.
Pluto: radius = 1145km, mass = 0.002 Earths, density = 2.1x water
Charon: radius = 600 km, mass = 0.1-0.2 Plutos
Pluto was closer than Neptune in the late 1990s, but never crosses Neptune’s actual path (2:3 orbital resonance, tilted).
Adaptive Optics image
Image on left from HST, on right from AO on ground. Pluto goes around twice while Neptune goes around 3 times, but inclination means they won’t collide.