Pluto remains the only planet not yet visited by a spacecraft either flying past it or in orbit about the planet. Its great distance from the Sun makes telescopic observations of Pluto a technical challenge. Earth-based telescopic photographs (upper left) have considerable difficulty distinguishing the planet and its recently recognized moon, Charon. The Hubble Space Telescope (HST) image (upper right) resolves both objects, as well as illustrating that Charon is actually almost half the size of Pluto. In many respects it would be valid to consider this a binary planet rather than a planet and moon. The diagram below shows how Charon orbits Pluto in just slightly more than six days.
Hubble Space Telescope image (Press Release 90-HC-598).