The U.S. stamp traveled further than any stamp has traveled before…

July 25th, 2015 got to know exciting info from the world of philately. A 1991 U.S. stamp featuring Pluto is headed to the dwarf planet on board NASA's New Horizons robotic probe, which will soon render the phrase featured on the stamp, "Pluto: Not Yet Explored".

The phrase, which first appeared in 1991 along the bottom of a 29-cent U.S. postage stamp depicting Pluto, is now affixed – in the form of one those 24-year-old stamps – on NASA's New Horizons robotic probe, which as of July 7 is just one week out from flying by the mysterious dwarf planet.

The encounter will make space history – completing the initial reconnaissance of the classical solar system – and, because the stamp is onboard, it will make some postal history, too. Not only is it thought to be the first time that a U.S. postage stamp has been present for the event that effectively made its design outdated, but it is also the farthest that any stamp has ever traveled before.

"We're honored to have the 'Not Yet Explored' stamp travel the billions of miles to Pluto," Mary-Anne Penner, the U.S. Postal Service's acting director of stamp services said. "We are hoping that the New Horizons mission provides Pluto with a 'stamp of approval', regardless of its status in the solar system". Not like the others

The United States Postal Service (USPS) didn't just issue a stamp in 1991 to highlight the fact that Pluto had never been explored. Instead, it was part of a 10-stamp "Space Exploration" set that was first proposed by Howard Paine, the postal service's art director, and artist Ron Miller.

As released, each stamp depicted one of the planets (the 10th stamp was for Earth's moon) and a NASA robotic spacecraft that had been sent to study it.

"Paine had been a fan of my first space art book, 'The Grand Tour', and thought the subject of the planets would make an interesting stamp set", Miller recalled in an email to "The original idea was to just focus on the worlds themselves. It was only later that spacecraft were added [because] the stamp committee thought there should be an American connection with each planet".

And so Miller went about designing the art for the stamps that, for example, paired Mariner 10 with Mercury, Viking with Mars and Voyager with Saturn, Uranus and Neptune.

All of the planets (and Earth's moon) had a partner probe to go with them – all, that is, except Pluto.

On Oct. 1, 1991, when the stamps were released for sale, Pluto was still some 15 years from losing its planethood in a decree by the International Astronomical Union (IAU). But even then it was the odd planet out, having never had a spacecraft visit.

The U.S. stamp traveled further than any stamp has traveled before…