Pluto and its largest moon, Charon, could be considered a double planet. The balance point between them is between them in space, not within Pluto, so they're orbiting together around a point in space. This is our first mission to a double planet.
Science

Pluto really is a planet (probably): The meaning of the first close-up photos

Planetary scientist Alan Stern explains why he believes new close-up pictures of Pluto prove beyond doubt that it should have its planet status reinstated. Also, close-up photographs of Pluto!