On July 14, NASA's New Horizons spacecraft will make our world's first encounter with Pluto. But this highly anticipated flyby will carry a special passenger you might not have heard about: A stamp.

Collect Space reports that the stamp -- which was produced in 1991 -- is affixed to the probe. The 4.67 billion miles to Pluto will be the farthest a USPS stamp has ever traveled, and as soon as the probe arrives, the words on the stamp -- "not yet explored" -- will become untrue. That's one paradoxical keepsake to have on board.

When the stamp was designed as part of a planetary set, Pluto was the only planet that lacked a robotic companion -- no spacecraft had ever been sent to explore it, so there was no American robot to show off in the stamp illustration.

Some say that the words on the stamp actually fueled scientists' efforts to make them untrue.

“It was my idea to send it," New Horizons principal investigator Alan Stern told Astronomy Magazine of the stamp. "For many years, people had waved that stamp around as sort of a call to arms — as a motivating graphic — ‘Not yet explored.' That stamp had been in so many presentations by that point, I knew it would please people to have it go along.”

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

Now that the Pluto stamp is about to be quite untrue, perhaps we'll finally see an update. In 2012, the New Horizons team petitioned for a stamp commemorating the mission, but USPS has yet to design one.

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