On July 16, the en-route-to-Pluto team celebrated an important milestone: Their New Horizons spacecraft had arrived at a point in space just one year shy of Pluto flyby. Mission scientists met for two days of detailed encounter planning at the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland, and threw a party at the Smithsonian National Air & Space Museum in nearby D. C. In exchange for agreeing to speak at the "One Year and Counting" event, I got to sit in on some of the science meetings and tour certain non-restricted areas of the Lab -- most notably the Mission Operations Center that controls New Horizons. The spacecraft had recently been awakened from hibernation to perform a slight course correction. Now, having smoothly executed the maneuver, it would go back to sleep for a few months while coasting toward its destiny.

The Mission Operations Center looks like Apollo Mission Control, only much smaller and less densely populated with anxious faces. Even though New Horizons is performing perfectly, and every phase of the mission has gone according to plan thus far, the spacecraft's handlers live with a constant concern for its welfare. They also admit, good-naturedly, to their own superstitious behavior. On Mondays, for example, when New Horizons signals home with a status update report, some staffers dress in green, the color that signifies optimal conditions. No one requires them to wear green, of course, but they are expressly forbidden to wear red, the color of danger. All shades of red, pink, and even fuchsia, are proscribed.

Toys also help control anxiety in mission control. New Horizons has a small Earthbound mascot affectionately known as the hibernation bear. When the spacecraft is hibernating, so does the bear, bedded down in the tiny nightshirt, cap, and blanket that have been lovingly hand-sewn for him. When New Horizons wakes up for a trajectory correction or instrument test, the bear is propped into a sitting position and dons his party hat.

In my remarks for the public program at the Air & Space Museum, I recalled my time on the Planet Definition Committee and considered the continuing controversy over Pluto's status among the bodies of the Solar System. Whether we call it a planet or a dwarf planet, Pluto remains an unexplored world.

We will start to come face to face with Pluto and its five known moons next January, when New Horizons rouses to begin its reconnaissance. By the time of closest approach in July, we'll see what kind of world Pluto is.


Confessions of a definer (the "p" word)

NASA's New Horizons has traveled far enough in the seven years since its launch to see the target at the end of the trajectory. The spacecraft's highest-resolution telescopic camera recently beamed back an image of Pluto and its largest moon, Charon, discernible for the first time as two separate worlds.

At least one announcement of this milestone referred to Pluto as a planet, re-igniting the debate over the definition of the "p" word and Pluto's contested status as a member of that category.

Having played a (small) part in the Pluto affair, I would like to share some of the lessons in humility it taught me.

The first came from my older brother Michael, who, upon learning that the International Astronomical Union (IAU) had appointed me to its "Planet Definition Committee," asked, "Why do they want you?"

Maybe it was folly to try to redefine a term already laden with significance. A hand-me-down from the ancient Greek  planetai, meaning "wanderers," the word had referred of old to the Sun and Moon as well as to Mercury, Venus, Mars, Jupiter, and Saturn. Modern astronomers needed a more detailed, comprehensive definition, informed by new discoveries of the Solar System's most distant objects plus several hundred "exoplanets" circling stars beyond the Sun. A strict definition, it was hoped, would also decide what to do about Pluto, whose planethood had come under question.

Our committee's draft proposal in the summer of 2006 defined a planet as a body in orbit around a star, and massive enough to be globe-shaped. By our definition, Pluto remained a planet, while several other bodies became planets. These included the asteroid Ceres and a few so-called Kuiper Belt Objects beyond the orbit of Pluto. The census of Solar System planets thus rose to twelve -- with a chance of climbing higher in the light of future discoveries.

Of all the objections raised against our suggestion, the one that surprised me most was the argument that children could not be expected to memorize an unlimited number of planet names.

When our proposal came to a vote at the IAU general assembly in Prague a few weeks later, another qualifier was added by some of the discussants: To be a planet, a round body orbiting a star had to dominate its neighborhood. This was Pluto's downfall, as it orbits in thrall to the planet Neptune, and has failed to clear its path of other small fry.

The official definition reduced the Solar System to eight planets. Some outsiders thought Pluto had been bodily ejected, when in fact it was merely relegated to a separate class of "dwarf planet." But unlike a dwarf star or a dwarf galaxy (each a diminutive version of its type), a dwarf planet is not a planet. This seems a harsh -- even an illogical distinction. As one astronomer lamented, "Is a dachshund not a dog?"

An unfortunate change in the wording of the final definition replaced the word "star" with "the Sun." This means that after years of (still unsettled) debate, we have defined only the worlds of our own Solar System, when a goal of the re-definition process had been the expansion of our vocabulary to embrace the myriad other worlds abounding throughout the galaxy.

Surely the definition needs retooling. Maybe by the time New Horizons arrives in 2015, Pluto will have morphed into a planet again.

Postage to Pluto

The status of Pluto is about to change again. A century ago it went from an unknown entity to a new discovery, hailed as the ninth planet. A few years ago it traded that title for a designation as the first dwarf planet. Now, thanks to the New Horizons spacecraft currently en route to Pluto, the small world's distinction as "unexplored" is about to fall away.

Stamp enthusiasts may recall a block of postage stamps, issued in 1991, celebrating the planets paired with their spacecraft visitors, such as Viking with Mars and Pioneer 11 with Jupiter. One stamp in the series called attention to the aloneness and  unknown-ness of Pluto. Its message, "Pluto not yet explored," struck some astronomers as a call to action.

Several mission designs and more than a decade later, New Horizons lifted off from Cape Canaveral in January 2006. Pluto was still a planet then. But in August of that year, the International Astronomical Union approved a resolution that defined the word planet in terms that excluded Pluto.

Triumphant Pluto-deniers and disgruntled Plutophiles still debate the Pluto decision, but both camps anticipate the findings that New Horizons will report upon its 2015 arrival. Already, the possibility of a new postage stamp is in the offing, drawn by space artist Dan Durda.

If you like the idea of a philatelic return to Pluto, please help the New Horizons team win over the U.S. Postal Service by signing their petition before March 13. Even if you communicate solely by e-mail and social networks, I hope you'll agree that "New Horizons First Spacecraft to Explore Pluto" deserves to find its place on the corners of envelopes "Forever."