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Pluto (smaller sphere) and its moon Charon are the first guideposts of the Kuiper Belt. They may help reveal why planets long ago stopped forming in the outer solar system. (Illustration by Ron Miller)

Where the Wild Things Are

We’re about to get a peek at the solar system’sfinal frontier.

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.

About Guy Gugliotta

Guy Gugliotta, a former national reporter for the Washington Post, is a freelance science writer based in New York.

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