Pluto's great year will continue into 2016, when the U.S. Postal Service (USPS) will commemorate the dwarf planet with a new stamp. The exploration of Pluto by the New Horizons spacecraft was a historic event that completes NASA's exploration of the inner solar system. The views of the eight planets -- and dwarf planet -- that make up the solar system receive their own stamps in 2016. Last, but certainly not least, Earth's moon gets its own special edition stamp in 2016.
New Horizons completed its close flyby of Pluto on July 14. The spacecraft did not slow down or spend much time around Pluto, but the flyby delivered unprecedented views of the dwarf planet. Most famously, Pluto has a heart-shaped plain. There are also ice mountains on its surface. New Horizons also captured incredible photos of Charon and Pluto's smaller moons Hydra, Nix and Kerberos. The data collected by New Horizons will continue to refine our understanding of Pluto and planetary evolution for the next decade.
Pluto famously was featured in a set of stamps celebrating NASA's exploration of the solar system in 1991. At that time, Pluto was still a planet, but unexplored. The USPS stamp for Pluto was a greenish-organ circle with the words "Not Yet Explored" printed next to its name. New Horizons carried that stamp to Pluto as part of its payload. The mission's team proudly declared Pluto was now explored after the flyby. The team petitioned to have a stamp celebrating the achievement and the USPS agreed.
"Since the early 1990s the old, ‘Pluto Not Explored’ stamp served as a rallying cry for many who wanted to mount this historic mission of space exploration. Now that NASA’s New Horizons has accomplished that goal, it’s a wonderful feeling to see these new stamps join others commemorating first explorations of the planets," Alan Stern, New Horizons principal investigator, said in a statement.
On Jan. 19, 2006, New Horizons began its nine-year journey to the distant object. On Aug. 24, 2006, the International Astronomical Union reclassified Pluto as a dwarf planet after it changed the definition of a planet.
"A celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit," the IAU declared. Pluto passed the first two hurdles, but with other large objects in the Kuiper Belt, Pluto became a dwarf planet.
Pluto is no longer a planet, which is why the USPS "Views of Our Planets" stamps do not include the celestial object among the eight planets that NASA has explored over the last 50 years. Earth's moon is also celebrated in a Global Forever stamp.