Back in 1991 I did a series of postage stamps, one for each planet and the moon. There was a spacecraft associated with each world...except Pluto. It was labeled "Not Yet Explored". This rankled so many space scientists that a movement was started to launch a Pluto exploration mission. This eventually culminated in the New Horizons probe. As a kind of thank-you for the inspiration, Alan Stern---the mission's principal investigator---attached one of the stamps to the spacecraft (and invited my wife and me to the launch). That was very cool! My only fear is that the stamp probably wasn't canceled...if not, the spacecraft is going to wind up right back here in 15 years!

The stamp joined the ashes of Pluto discoverer Clyde Tombaugh on the New Horizons spacecraft (right).

In 2016, the  stamp was listed by Guinness World Record as having achieved the furthest distance ever travelled by a  postage stamp: over 3 billion miles.