Today we know that the Kuiper Belt is the largest single structure in our planetary system. Beginning just beyond the orbit of Neptune, this ancient wilderness stretches over a space as broad the distance from here to Uranus.

And within that vast expanse are Pluto-Charon and over 100,000 other miniature worlds, billions of comets, and uncountable smaller flotsam-all of which are remnant material from the ancient era of planet building in our solar system that took place between 4 and 5 billion years ago.

It has been estimated that the Kuiper Belt contains hundreds of miniature planets bigger than the 1000-km diameter of the largest asteroid, Ceres. Moreover, the Kuiper Belt contains more water and more organic material than is present in the entire inner solar system-yes, in the Earth, Venus, Mercury, and Mars, combined.

And the KBOs themselves are amazing-some are red, some are blue; some appear spotted; some may soon be revealed to have atmospheres; a surprisingly large number, perhaps tens of thousands, have moons. The Kuiper Belt is a scientific wonderland of almost unparalleled proportions and promise in 21st-century planetary science.

The largest, the brightest, and the best known member of the Kuiper Belt is, of course, the double planet Pluto-Charon. Almost 2500 kilometers (1600 miles) in diameter, Pluto-Charon was discovered by American astronomer Clyde Tombaugh in 1930.

Pluto-Charon is a scientific treasure many planetary scientists consider unmatched for its unique combination of fascinating attributes, public appeal, and raw romantic attraction to school children and adults alike.

The just-announced discovery of Quaoar, itself fully half Pluto's size and probably about 1/10th Pluto's mass, reminds us of how much we have to learn about the Kuiper Belt and the KBOs themselves.

Indeed, consider what we can learn from Earth about KBOs. Their colors, their sizes, their rotation rates, and their orbits are all certainly tractable. We can detect the presence of large satellites, search for evidence of atmospheres, and in a few cases, derive their internal densities.

But we cannot, even with tools as exquisite as the Hubble and the Keck, even hope to map their surfaces at resolutions comparable to what a good pair of binoculars reveals of Mars.

We cannot reveal more than the crudest information about their surface composition, and we cannot hope to resolve details about their surface geology, determine if these worlds are internally active, or explore their internal workings. If we had to describe the Earth at this level of detail we would not have a clue it has 7 continents, clouds, active geology, and a vibrant ecosystem.

It has been the lesson of planetary science, from the first crude reconnaissance missions were set forth to Venus and Mars to the landmark explorations of Voyagers 1 and 2 to Uranus and Neptune, that groundbased exploration is incapable of revealing the real details and workings of distant worlds.

History is littered with examples of how our best efforts at groundbased exploration fell short: Before spacecraft exploration by probes named Mariner, Pioneer, Viking, and Voyager, no one expected Mercury to be a world whose core is half the planet's size.

Before spacecraft exploration by these probes, no one expected Venus to have been completely resurfaced in the recent geological past. Before spacecraft exploration, no one expected Mars to show signed of recent running water, or volcanoes larger than any on Earth.

And before spacecraft exploration, no one expected the great variety of satellite geologies we take for granted today, or the paradigm-shifting discovery that distant, cryogenic worlds can be and often are geologically active.

The Kuiper Belt is the so-called, Third Zone of our planetary system, after the region of the inner rocky planets and the region of the gas giants. Yet it remains wholly unexplored by spacecraft.

While this is understandable in the sense that the Kuiper Belt had not been discovered, even as recently as 1990, its potential for new insights is at its heart why so many planetary scientists place it at the top of their list for immediate spacecraft reconnaissance.

NASA's New Horizons Kuiper-Belt mission (see pluto.jhuapl.edu and www.plutoportal.net), selected as the "lowest cost, lowest risk, greatest scientific return" option offered up in a year-long industry-academia competition for such mission, is now deep into detailed design.

If the U.S House of Representatives provides the same funding as the Senate has already agreed to for this fiscal year, New Horizons will enter construction this coming spring, and launch less than three years later, in January 2006. If funding is cut off, then even optimistic estimates indicate that the launch of a replacement mission would lag by up to a decade.

New Horizons will then fly to Jupiter, making that almost half-billion mile crossing in less than one-quarter the time it took NASA's successful Galileo mission, to do the same. After exploring the Jupiter system and gaining a gravitational boost from it in early 2007, New Horizons will set out across the remaining, three billion mile (5 billion kilometer) expanse to Pluto.

Traveling at 100 times an average speed of a jetliner, 24x7, it will reach Pluto in 2015 or 2016 (depending upon the selected launch vehicle) for a five-month long suite of explorations. It will then set out to explore a variety of KBOs in the years that follow.

New Horizons carries a suite of six miniaturized imagers, spectrometers, and in situ measurement sensors with remarkable capabilities that no previous first-reconnaissance spacecraft could have matched.

When New Horizons completes its explorations of Pluto-Charon and the Kuiper Belt, it will surely be remembered as a beacon, shining brightly in the crown of space exploration achievements.

And in doing so, it will surely capture the imagination of humankind, reminding us all that NASA and the United States, and indeed our species, is still capable of something more than simply everyday battles, and even historic adventures: it will remind us that we in our time were capable of something wonderful.

Alan Stern is a planetary scientist at the Southwest Research Institute and the Principal Investigator of NASA's New Horizons mission to Pluto-Charon and the Kuiper Belt. The views expressed here are those of the author.

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