In 1991 I did a set of US postage stamps called the "Space Exploration" series. Each one featured a planet along with a spacecraft associated with it. Pluto, though, had to be all by itself and was labeled "Unexplored" on the stamp. This got a lot of space scientists in a swivet and pretty soon a Pluto space mission was proposed. This eventually evolved into the New Horizons probe. It was launched in 2006 and recently passed the halfway mark on its way to Pluto. One of the Pluto stamps was attached to the spacecraft before it was launched, which is very cool! My wife and I were invited to the launch of New Horizons, which was pretty exciting!.