The Infamous “Pluto Not Yet Explored” Stamp

A couple years ago, I needed some postage stamps.  To my delight, the local post office had a wide selection of space-themed stamps to choose from, including this four-stamp sheet commemorating the New Horizons mission to Pluto.

But it wasn’t until I read Chasing New Horizons by Alan Stern and David Grinspoon that I realized the full significance of the stamps I’d purchased.  Those stamps were, in fact, a major part of New Horizon’s history.

The story begins in 1991, shortly after Voyager 2 completed its flyby of Neptune.  The U.S. Post Office issued a set of stamps honoring the first NASA space probes to visit each of the planets and also the Moon.  Pluto was also included in the set, with the caption “Pluto not yet explored.”

Apparently some in the planetary science community took this as a challenge.  That set of stamps, including the “Pluto not yet explored” stamp, became a symbol of work that had been left unfinished, of a job that still needed to be done.  The stamp was used in proposals and presentations arguing for a Pluto mission.  It was part of the public outreach campaign once New Horizons was underway. And the day New Horizons reached Pluto, a poster-sized version of the stamp was help up for the press with the words “not yet” crossed out.

So naturally, following that 2015 flyby mission, the Postal Service had to issue new stamps.  That day when I went to get stamps, so I could pay my rent and bills and other mundane things, I had no idea how much meaning and significance was packed into that little stamp sheet.  Even the elongated dash in “Pluto—Explored!” feels significant, as though it’s a reminder of the words that were crossed out.

I’ve never been a stamp collector, but as it so happens I still have at least one sheet of Pluto stamps left, and once I knew the full story behind them I went and did a little shopping online.  Now both sets of stamps—the 2015 stamps for New Horizons and the original set of stamps from 1991—are part of my modest collection of space exploration memorabilia.