Twenty years ago, the existence of a distant wilderness beyond Neptune—seeded with tiny planets, dormant comets, and bits of ice and rock —was mere conjecture. There was Pluto, discovered practically by luck in 1930, and that was it. Astronomers photographed squares of the night sky and compared the images to see if anything was moving, but either their technology was not good enough, or they were searching in the wrong place. Or there was nothing more to find.

Then in 1992, after surveying the heavens for seven years, first with cameras and then with progressively more advanced digital imagers, University of Hawaii astronomer David Jewitt, with Jane Luu, one of his former graduate students, identified 1992 QB1. It was an icy object 125 miles in diameter (less than one-tenth Pluto’s size), orbiting the sun a billion miles beyond Pluto. “We really had no idea whether there was anything there,” Jewitt recalls. “But it was obvious as soon as we saw it.” They tracked the object all night, and then the next, before reporting the find to the Minor Planets Center at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. Long-time center director Brian Marsden was skeptical; he suggested they had found a wandering comet. Jewitt bet him $500 that, like Pluto, 1992 QB1 was an orbiting object beyond Neptune. Eventually, Marsden paid up.

Since then, scientists have found more than 1,300 objects in this remote, mysterious region, and researchers estimate  there are 70,000 of them with diameters of at least 60 miles. Pluto, it turns out, was neither alone nor unique. Today the Kuiper Belt, a region in space between 2.8 billion and 4.6 billion miles from the sun, is one of the hottest topics in astronomy. Named for Dutch-born U.S. astronomer Gerard Kuiper (rhymes with “viper”), who theorized its existence back in the 1950s, the Kuiper Belt defied detection for decades until scientists realized that Pluto was the first signpost.

Kuiper Belt Objects are leftovers from the whirling gas and dust disk that formed the solar system 4.5 billion years ago. Computer models suggest that the end of that process was marked by the outward migration of the planets, and as Neptune moved into its present orbit, its gravity tugged the remaining smaller bits with it deeper into space. Some of the pieces combined to form larger bodies like Pluto, but something stopped the planet-making process. The models say that in order for larger bodies like Pluto to have formed, there had to have been at least a hundred times more material in the Kuiper Belt than  there is now. Somehow that material disappeared, perhaps pulverized in collisions or flung into interstellar space by the gravity of the outer planets.

“Dwarf planets are embryos,” says Alan Stern, a planetary scientist at the Texas-based Southwest Research Institute and a former NASA associate administrator for science. “If you have a Pluto-sized object, it should grow to be an Earth-sized object, unless you remove Pluto from the food supply or remove the food supply from Pluto. It is not clear what happened here.”

Or where it started. As the conviction grows that Neptune is the reason why the Kuiper Belt is where it is, researchers have become interested in finding out more about the belt’s origin and how its chaotic birth prevented other planets from forming. The migration had to have occurred during the final phases of solar system formation and had to have started very far from the sun. Otherwise the ice in the Kuiper Belt’s comets would have evaporated, and Pluto’s meager atmosphere would have boiled off. But “we don’t know where the shipwreck took place,” says California Institute of Technology astronomer Michael Brown, who in 2003 discovered Eris, a Kuiper Belt dwarf planet bigger than Pluto. “Or what happened after that.” By the time the migration ended, however, gravitational interactions had brought many Kuiper Belt Objects, including Pluto, into orbital synchronization, with Neptune (Pluto orbits the sun twice in the time it takes Neptune to orbit three times). So whatever happened, Pluto witnessed it.

As the big picture of the Kuiper Belt comes into focus, a team of scientists led by Stern is preparing for a first close look. In 2006, NASA launched New Horizons, a piano-size spacecraft weighing 1,054 pounds, on a nine-and-a-half-year voyage to the solar system’s outback. So long is its transit and so new is the field of Kuiper Belt study that some of the objects it will examine have not yet been discovered.

In 2015, the spacecraft will awaken from hibernation for a flyby that will take it within 6,200 miles of Pluto. It will also study Pluto’s largest moon, Charon, and two other tiny, recently discovered satellites, Nix and Hydra. New Horizons will take photographs, study Pluto’s wispy atmosphere, and analyze its surface, geology, dust, and temperatures. Scientists should then be able to draw at least some general conclusions about the nature of Kuiper Belt Objects. “I’m sure we’ll over-interpret,” says Stern, but the flyby will provide information that telescopes cannot get. The impact craters on Pluto’s relatively young surface, for example, should give researchers a better idea of the size and number of the objects now in the Kuiper Belt. Because Pluto’s surface ices, composed mostly of nitrogen, turn to gas when the planet comes closer to the sun, then refreeze when it moves farther away, the terrain changes seasonally and offers a fresh record of impacts. The surface is relatively “young.” Charon, with little or no atmosphere, has a much older surface, so it should have many more craters, and should provide a record of the size distribution of objects in the original belt. By counting the number and size of craters per unit of surface, scientists can determine the craters’ ages.

In planetary science, the mix of targets is about as good as it gets. Pluto is one of the larger known bodies in the Kuiper Belt, while Nix and Hydra are minuscule. “We have the bookends,” says project scientist Hal Weaver of Johns Hopkins University’s Applied Physics Laboratory in Baltimore, Maryland, which built the spacecraft. “Here’s an opportunity to get the first look at a completely different class of objects.” Weaver, who came to the project because he was interested in “frontier scientific research,” describes himself as a “comet guy,” and Pluto, he notes, “is nothing more than a big comet.”

Imaging of Pluto and preparations for the flyby will start five months before New Horizons’ closest approach, but the key portion of the journey is only the 12 hours before arrival and the 12 hours afterward. Downloading all the data to Earth, with a one-way transmission time of four and a half hours, will take nine months. By that time, New Horizons will be on its way to another object. No one knows yet what that will be, but by 2015 there should be a number of candidates.

Right now Pluto is backlit by the constellation Sagittarius, making it impossible to find nearby objects, but by 2011 or 2012 the background will be dark enough for astronomers to begin looking. Stern has not decided whether the team will conduct its own survey or hire a contractor. Telescopes of the new U.S. Air Force-funded Pan-STARRS (Panoramic Survey Telescope & Rapid Response System) project in Hawaii can image three-quarters of the sky four times a year, and the leader of the outer solar system search , Matthew Holman of the Harvard-Smithsonian Center for Astrophysics, predicts the survey will find between 5,000 and 10,000 Kuiper Belt Objects as big as or bigger than 1992 QB1.

New Horizons is the fastest spacecraft ever built. It left Cape Canaveral, Florida, on January 19, 2006, aboard an Atlas V-551 rocket, NASA’s biggest, with five strap-on boosters. When the last of three stages dropped away, 47 minutes after launch, the spacecraft was traveling 36,000 mph. It passed the moon late that evening, completing in nine and a half hours a trip that took Apollo astronauts three days. In February 2007, it flew by Jupiter for a gravity assist that hurled it onward at nearly 40,000 mph. Last spring, it passed Saturn’s orbit to join Voyagers 1 and 2 as the only functioning spacecraft to traverse the farthest reaches of the solar system.

Planning and executing New Horizons’ so-far flawless itinerary has required an unusual combination of rapid response and patience. The orbital mechanics are unforgiving. Pluto is 2.8 billion miles from the sun at its closest approach, reached in 1989. Since then, Pluto has been outward bound, and by the time the spacecraft arrives, it will be about 3 billion miles away. But planners needed to ensure that the spacecraft would reach Pluto by 2020; otherwise there was a risk that Pluto would be so far from the sun that its atmosphere—a blend of nitrogen, carbon monoxide, methane, and perhaps other gases—would freeze and the pieces fall to the surface, rendering atmospheric analysis, one of New Horizons’ primary objectives, impossible. With an orbit 248 Earth years long, Pluto does not offer optimum geometry very often. “The Founding Fathers had the last opportunity,” Weaver says. “We didn’t want to miss ours.”

New Horizons has few moving parts. It carries 170 pounds of hydrazine for thrusters that maneuver the spacecraft and point its instruments, and the instruments draw power from a 200-watt—the equivalent energy of two light bulbs—thermoelectric generator fueled by 24 pounds of radioactive plutonium dioxide. Five months before arrival, the spacecraft will begin taking pictures of Pluto and Charon. At its closest approach, New Horizons will be able to discern features as small as 80 feet.

Scientists today know more about Pluto than any other object in the Kuiper Belt. With a 1,475-mile diameter, it is about a third the size of Earth’s moon. Its orbit is tilted 17 degrees from the orbital plane of the other planets in the solar system. Pluto and Charon, only 12,200 miles apart, are a binary system, locked in synchronous embrace. Both rotate once every 6.4 Earth days. Charon is about half Pluto’s size, while Nix and Hydra, in orbit around Pluto and Charon and between 24,000 and 37,000 miles away, are 10 to 20 percent the size of Charon.

None of this was known in 1929, when Clyde Tombaugh, a young astronomer from Kansas, arrived at Lowell Observatory in Flagstaff, Arizona. His task: photograph the night sky through a telescope in hopes of finding a so-called Planet X—so massive that it was thought to be wiggling the orbits of the giant planets Uranus and Neptune. There was only one way to do this: take long photographic exposures of the same piece of sky over several nights and compare the plates in hopes of finding a faint pinpoint of light that moved. The chore, as any astronomer who has done it can attest, is mind-numbing drudgery, but every Kuiper Belt discovery ever made has used a variation on the theme. The advent of digital imaging in the last 20 years has made the task dramatically easier, but even so, sky surveys are tedious work.

Tombaugh was incredibly lucky. He found Pluto on February 18, 1930, after less than a year. The discovery was heralded worldwide, but it was based on a flawed premise. Astronomers learned later that there was nothing wrong with the orbits of Uranus and Neptune—the orbital wobbles were due to errors in measurement—so there was no reason to assume Pluto was massive. And if Pluto was small, there was no reason to think that it had gobbled everything around it. “They might have realized that Pluto was not the only thing out there, but only the first of a large number of unusual objects we had never seen before,” says Jewitt. “That opportunity was lost for 60 years.”

But scientists were slowly advancing their understanding of the outer solar system. In 1950, Dutch astronomer Jan Hendrik Oort theorized that there was a huge spherical cocoon of icy debris surrounding the solar system beyond Pluto and extending as far as a light-year away. These pieces, flung out by the giant planets, were the source of  “long-period” comets, which enter the solar system from all angles and have orbits of  thousands of years. During the 1980s, astronomers learned a lot more about Pluto, especially during a series of mutual eclipses between Pluto and Charon that lasted five years. The study was made possible because the Pluto-Charon orbital plane could be seen edge-on from Earth, a phenomenon that happens only twice during Pluto’s 248-year orbit of the sun. Pluto’s color, astronomers found, was yellowish pink, tending toward scarlet. The surface had methane ice, contrasting areas of dark and light, and one or maybe two polar caps. At this point Pluto began to acquire a new identity: a tiny, intriguing outlier. “We began to ask why we should believe that everything stopped at Neptune,” says Jewitt. “If there was Pluto, why not something else?”

In 1985, Jewitt and Luu started hunting. They began as Tombaugh had—comparing photographic plates by eye—then switched to charge-coupled devices and digital imaging. “Our first few searches produced nothing, but we kept on looking because the technology kept evolving,” Jewitt recalls. “We’d get burned out, but then we’d get a new CCD and get all charged up again.”

Meanwhile, in 1989, Stern, a doctoral candidate and Pluto buff, joined several colleagues in what they called a “Pluto underground” to urge NASA to explore the only planet in the solar system that a spacecraft had not visited. Stern says he was convinced that scientists had “pretty much used up our [astronomical] bag of tricks,” and were reaching the limit on what they could determine from telescopes. “There was only one way to learn more about Pluto,” he says. “We had to go there.” Stern pushed hard through the 1990s, as NASA planned and scuttled four separate Pluto projects before finally awarding the mission to New Horizons in 2001. For $700 million, the project would use what Weaver calls the “elephant gun” approach: Take the biggest rocket available, make the payload as small as possible, point the gun at Pluto, and pull the trigger.

In 2006, with New Horizons en route, the International Astronomical Union stripped Pluto of its “planet” status, created a new class of celestial body called “dwarf planet,” and made Pluto and the newly discovered Eris the first two members. A dwarf planet had enough gravity to be round, the Union said, but was not massive enough to suck up the material in the area around its orbit. Cal Tech’s Brown, who had hopes of joining Tombaugh as the only Americans to discover a planet, instead became the second person to discover a dwarf planet. “We’re cleaning up a scientific mistake,” Brown says. “It’s hard to look at the solar system and not quickly come to the conclusion that there are eight large objects, and Pluto is not one of them.” Pluto, it turns out, was something altogether different: the gateway to the Kuiper Belt.

Guy Gugliotta is a former space reporter for the Washington Post.
 

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