Not Yet Explored

I he way some people tell it, NASA's mission to I Pluto started with a postage stamp. In 1991, the U.S. Postal Service chose NASA's Jet Propulsion Laboratory in Pasadena, California, as the site for the unveiling of a new set of twenty-nine-cent stamps, titled "Exploring the Solar System." The ten stamps in the set depicted all nine planets, plus Earth's moon, and every destination was paired with a space probe. Except one.

While nine of the stamps were adorned with a Voyager or a Mariner, a Pioneer or a Viking, a Lunar Orbiter or a Landsat, one stamp showed a simple globe—and the legend "Pluto: Not Yet Explored."

Two engineers at the Jet Propulsion Laboratory, Robert Staehle and Stacy Weinstein, saw that stamp as a challenge. They resolved to figure out how to get a spacecraft to the edge of the solar system, even if it meant shrinking the scientific payload down to a scale that didn't seem possible back then.

It's a great story—but it's only half true. Staehle and Weinstein didn't know it at the time, but a dozen astronomers who called themselves the Pluto Underground were already scheming to get NASA to send a probe to the same place. The timing was just right for the two groups to join forces and answer the challenge of a twenty-nine-cent stamp. Little did they know that it would take another fifteen years to get their idea off the ground.

The idea of sending a spacecraft past Pluto actually goes back almost fifteen years earlier than the stamp. When NASA launched the twin Voyager spacecrafts in 1977, mission managers had the option of sending Voyager 1 past a planet that at the time was known as little more than a faint sparkle in the sky.

Decision time came two years later, when Voyager 1 and its handlers faced a fork in the road: Go one way, and the spacecraft could sail past Pluto in the late 1980s. Go the other way, and it could get a good look at Saturn's rings and its cloud-covered moon, Titan. After the Saturn encounter, Voyager 1's course would take it above the solar system's plane, putting it out of range for any more planetary flybys.

Mission managers opted for Titan—a world that scientists see as an analog to primeval Earth, with hydrocarbons and complex organic molecules raining down through its thick, nitrogen-rich atmosphere. The findings from Voyager 1's 1980 flyby led NASA and the European Space Agency to start planning an even more ambitious mission to Saturn and Titan, called Cassini-Huygens, which was launched in 1997.

Pluto, on the other hand, lost out.

"Of course, at the time this decision was made, Pluto's atmosphere, its small satellites, its complex surface composition and the entire Kuiper Belt all remained undiscovered, perhaps rationalizing the Titan choice from today's perspective," said Alan Stern, the planetary scientist from the Southwest Research Institute who now leads the science team for NASA's Pluto mission.1

In 1989, Stern and eleven other experts on Pluto gathered in a small restaurant in Baltimore's Little Italy, after attending a seminar on their favorite scientific topic. Over pasta and wine they decided to press NASA to follow the road not taken by Voyager. The planetary scientists, who called themselves the Pluto Underground, started drumming up support for a mission devoted to the reconnaissance of what they considered the solar system's "last unexplored planet."2

The Pluto Underground soon had some new ammunition for the cause. In the summer of that year, Voyager 2 sent back some intriguing readings from Neptune's biggest and weirdest moon, Triton. Here was a moon that spun on a tilt, in a direction opposite from Neptune's. That suggested that Triton didn't evolve along with Neptune, but was formed somewhere else and only later was captured in a Neptunian orbit.

What was Triton most like? It had an icy, variegated surface of frozen nitrogen and methane—perhaps just like Pluto's. There were signs that geysers on Triton were spewing nitrogen and dust into its thin nitrogen-methane atmosphere— perhaps just like Pluto. The more scientists looked at Triton, the more they became convinced that Triton and Pluto were sundered cousins from the same celestial family.

On the strength of the findings from Triton, plus a flood of supportive letters from other planetary scientists, members of the Pluto Underground managed to get NASA to fund a study for a mission called Pluto 350. This concept called for a spacecraft weighing 350 kilograms, about half as big as Voyager.

Could it be done? So much downsizing would go against NASA's trend of building bigger, more expensive space robots for each succeeding mission. The two Voyager probes to the outer planets cost $865 million to build and launch. The Cassini mission to Saturn, planned as a follow-up to Voyager, cost more than $3 billion to prepare. In fact, NASA briefly considered doing a Cassini-style production, but a working group headed by Stern decided against that approach when the cost estimates ballooned beyond the $2 billion mark. The scientists favored the Pluto 350 concept, even though NASA saw it as the riskier option, and even though the trip would take twelve to sixteen years.

That was right at the time when the "Not Yet Explored" stamp made the difference. At the Jet Propulsion Laboratory, Staehle and Weinstein labored over their low-budget concept for a mission to Pluto—not knowing that the Pluto 350 team was working on the same challenge, using a different approach.

This alternative concept, known as Pluto Fast Flyby, called for building a probe that would be less than half the size of Pluto 350—140 kilograms, to be exact—and yet would still be able to answer three key questions about Pluto and Charon: What did they look like? What were they made of ? What kind of atmosphere did Pluto have?

The clock was ticking on these questions. Every day, Pluto was receding farther and farther into the solar system's cold, dark depths. If the scientists waited too long, the trajectory would get trickier and the data transmissions would get dodgier. Pluto's southern winter would start setting in, leaving almost half the planet in darkness. Pluto's atmosphere might start freezing out as well. Based on computer projections, the drop-dead date would come sometime before 2020. If NASA's mission to Pluto didn't reach its destination by then, the trip probably wouldn't be worth it at any price.

So when JPL's engineers came up with a spacecraft concept in 1992 that could be launched for less money and get to its destination faster, NASA administrator Dan Goldin quickly embraced the idea. Goldin, who had been appointed the space agency's chief just months earlier, adopted the phrase "faster, cheaper, better" as his mantra for space exploration, and Pluto Fast Flyby sounded like the perfect embodiment of that philosophy. He gave the go-ahead for the development of two spacecraft that could get to Pluto in seven or eight years, at a cost of less than $500 million.

Unfortunately, Pluto Fast Flyby turned out to be the perfect embodiment of an old joke among engineers: If you're trying to make something faster, cheaper, and better, the best you can do is two out of three.

During the mission design phase, the projected weight of the spacecraft quickly exceeded the 140-kilogram target. The mission's price tag exceeded targets as well, going past the $1 billion mark. Making matters worse, NASA's planetary exploration program was thrown into disarray in 1993, when its $1 billion Mars Observer probe was lost just as it was preparing to enter Martian orbit. All these developments soured Goldin on his agency's not-so-cheap, not-so-easy mission to Pluto.

Pluto Fast Flyby went through one makeover after another. The mission was scaled back to one spacecraft instead of two. Stern worked out a deal with the Russians to launch the probe along with a Russian-built piggyback lander, but the deal crumbled when the Russians asked to be paid for the launch, which was forbidden under U.S. law. German scientists offered to step in and try to find a way around the financing snag. By then, however, Goldin had moved on to other priorities: for example, implementing a "faster, cheaper, better" approach to Mars exploration.

By 1999, NASA had spent ten years and $250 million on mission studies and hardware development, with no spacecraft to show for it. During all this time, more and more Kuiper Belt objects were being discovered. The scientific spotlight was widening to focus not just on Pluto but an entire frontier at the solar system's edge. The idea of exploring that frontier, as well as getting a close look at the "last unexplored planet," led NASA to reconsider its long-stalled Pluto plans.

This time, the mission was retooled as the "Pluto-Kuiper Express." Once again, scientific teams worked up proposals for a spacecraft that would send fresh observations of Jupiter as it flew by, then press on for an encounter with Pluto and Charon—and then keep sending back observations as it plunged deeper into the Kuiper Belt. But once again, the projected price tag spiraled well past the $1 billion mark.

For some at NASA Headquarters, that cost escalation was the last straw. The Pluto-Kuiper Express was abruptly eliminated from NASA's mission list in the autumn of 2000. The agency's associate administrator for space science, Ed Weiler, declared that the mission to Pluto was "over, canceled, dead."3

But was it really? Weiler didn't reckon on the persistence of planetary scientists—or the general public, for that matter. NASA's decision drew an outcry from experts who saw the exploration of Pluto and the Kuiper Belt as a top priority for future unmanned space missions. The Planetary Society, a space advocacy group cofounded by the late astronomer Carl

Sagan, collected more than ten thousand letters of protest and delivered them to lawmakers on Capitol Hill. That sparked a wave of questions from editorial writers and politicians.

Pluto was named by a child and associated with a children's cartoon character, so it was fitting that young people came to the tiny world's defense this time as well. A high school senior from rural Pennsylvania, Ted Nichols, created a "Save the Pluto Mission" Web site, collecting thousands of responses from around the world in a matter of days. "Don't break our kids' dream!" one posting from Osaka urged.4

After a few weeks of this, NASA gave Pluto another chance. Plans for a mission could go forward after all—provided that those plans posed no out-of-the-ordinary risk of failure, addressed the key goals for Pluto exploration, and cost no more than $500 million. Recalling Weiler's earlier announcement that the mission was dead, Stern quipped, "We are the undead."5

The Pluto project had indeed repeatedly risen from the dead, but the clock was still ticking. If the mission's backers wanted to have any chance of getting the probe to its destination no later than 2020, they had to act quickly. "Even with our best technology, it takes about 10 years to get there," Stern explained. "You can take longer, but you're not going to do it in much less time. . . . When you count the time to design it, build it and launch it, you're talking 15 years or more."

Following up on a suggestion from planetary scientists, NASA opened up a competition for the mission. Within three months, five groups of scientists and engineers submitted thick volumes setting forth their mission plans. Less than three months after that, NASA picked two of the groups to draw up more detailed proposals. For a while, it looked as if the space agency would back out of fUnding the Pluto mission even before a winner was named. But the mission still would not die: Congress ordered NASA to go ahead with the competition, and approved the money for Pluto over the space agency's objections.6

In November 2001, NASA gave the go-ahead to a mission proposal called New Horizons. Stern, a hard-driving researcher who had been in line during the mid-1990s to fly into space as a scientist-astronaut, was named the project's principal investigator. It looked as if Stern had finally succeeded in his quest to get a spacecraft launched to Pluto, twelve years after he and the rest of the Pluto Underground first gathered in a Baltimore restaurant.

NASA threw up one last hurdle in the mission's path, however. When its budget proposal for the 2003 fiscal year was released, there was no funding included for New Horizons. For the umpteenth time, Stern and his colleagues had to argue their case with Congress and NASA.

This time, the argument was cut short by a clear-cut scientific verdict—almost as if a god had descended to the stage in a Greek drama.

Every ten years, the influential National Research Council issues a "Decadal Survey" to serve as a guide for NASA on the top priorities for solar system exploration. When the new Solar System Exploration Decadal Survey came out in mid-2002, sending a probe to Pluto and beyond was the highest-ranked priority for a new mission—due in part to the Kuiper Belt discoveries that had piled up over the previous decade.

The council said the Pluto mission's findings could lead to a "new paradigm for the origin and evolution" of the solar system's little-known far frontier.7

That survey shattered any opposition to New Horizons, Stern said. The so-called Pluto War was finally over.8 "Both Congress and the administration actually said, 'Oh well, if the National Academy says this is at the top—not near the top, not in the top half, not in the top quartile, but at the top—then maybe we were wrong. We need to do this,' " Stern recalled. "Congress supported it unflinchingly from that point on. So did the Bush administration, by the way. Up until that time, they did not."

Alan Stern, principal investigator for NASA's New Horizons mission, and Patsy Tombaugh, Clyde Tombaugh's widow, attend ceremonies marking New Horizons' launch.

At last, the team behind New Horizons could set to work on the spaceship that would go to Pluto. Stern's home institution, the Southwest Research Institute, was one of the team's principal partners. The other partner was Johns Hopkins University's Applied Physics Laboratory (APL), a space operation engaged in a friendly rivalry with the Caltech-managed Jet Propulsion Laboratory. APL would build the spacecraft and, once the probe was launched from Cape Canaveral, would manage the mission on NASA's behalf from its Maryland headquarters.

The 478-kilogram (1,054-pound) New Horizons spacecraft turned out to be bigger than the Pluto 350 concept that the Pluto Underground first proposed back in 1989, but still just a little more than half the size of a Voyager probe. The spacecraft generates electricity using radioactive plutonium, which is currently the only feasible power source for spacecraft that travel far from the sun.9

Its scientific instruments weigh just 30 kilograms (66 pounds) and draw just 28 watts of New Horizons' electricity. There's a high-resolution telescope with a built-in CCD camera, an imager that will map the composition of Pluto and Charon, a spectrometer for studying Pluto's wispy atmosphere, a radio experimental package that doubles as a communications link, devices to measure the solar wind and its interactions with Pluto, and an interplanetary dust counter. A 2.1-meter-wide (7-foot-wide) dish antenna sends data back and forth across the ocean of space.

After all the instruments and fuel were packed aboard the spacecraft, there was still room for nine mementos. Two

U.S. flags were put on board, as well as two compact disks—one encoded with 434,738 names from a "Send Your Name to Pluto" promotion, and the other containing digital pictures of the project team. Some of Clyde Tombaugh's cremated remains were placed in a canister, inscribed with a tribute to the "discoverer of Pluto and the solar system's 'third zone.'" A 100-gram piece of the SpaceShipOne rocket plane, the first privately developed space vehicle, was attached to the spacecraft. The engineers also affixed two state-themed quarters—one for Maryland, where the spacecraft was built, the other for Florida and its launch site.

And then there was the stamp: Stern made sure one of those "Not Yet Explored" stamps from 1991 was tucked inside the spacecraft.

"Pluto may not have been explored when that stamp set came out, but we were going to conquer that," he told an interviewer. "I wanted to fly it as a sort of'in your face' thing."10

After seventeen years, the Pluto Underground's dream finally took flight on January 19, 2006, when an Atlas 5 rocket blasted the New Horizons probe from its Cape Canaveral launch pad into space. Among the dignitaries invited to the launch were members of Clyde Tombaugh's family and Venetia Burney Phair, the woman who gave Pluto its name when she was eleven years old. To return the favor, the dust counter on the New Horizons probe was named "Venetia."

"I feel quite astonished, and to have an instrument named after me is an honor," she said. "I never dreamt, when I was 11, that after all these years people would still be thinking about this and even sending a probe to Pluto. It's remarkable."11

It was a fitting sendoff for the probe, and a fitting memorial to Venetia Burney Phair as well. On April 30, 2009, while New Horizons and its Venetia dust counter were traversing the celestial emptiness between the orbits of Saturn and Uranus, Phair passed away at the age of ninety at her home south of London.12

To make its appointed rendezvous with Pluto and Charon in mid-2015, New Horizons took the fastest space ride NASA ever devised. Relative to Earth, its top speed was 36,250 mph (58,338 kilometers per hour), making New Horizons more than twice as fast as the space shuttle. On the way to Pluto, the spacecraft got a big gravitational boost from its encounter with Jupiter, and sent back stunning pictures of the giant planet as a bonus.

Stern and his colleagues won another bonus as well, even before New Horizons' launch. During the preparations for the mission, they were given some time on the Hubble Space Telescope to focus on Pluto and Charon—and two mysterious points of light turned up on the resulting photos. The Hubble team determined that the spots were two previously undetected moons of Pluto, now named after two creatures of the mythological underworld, Nix and Hydra.13 As New Horizons comes nearer to its destination, still more of Pluto's dark secrets will likely be revealed. "I think it's exciting that all the textbooks will have to be rewritten," Stern said.14

If so much about Pluto and the Kuiper Belt is out there still to be learned, why did it take so long to get the mission off the ground?

Stern is uniquely placed to consider that question. In addition to his role as principal investigator for New Horizons, he was appointed NASA's associate administrator for the science mission directorate in 2007, filling the very post that Weiler held in 2000 when he declared that the Pluto mission was "over, canceled, dead." Stern left the space agency just a year later, after dealing with the same kinds of budgetary limitations he faced for seventeen years on the other side of the desk.

In retrospect, Stern said any mission to the outer planets faces a hard sell because of all the years of travel required to get to its destination, and the delayed gratification that is required as a result. "Bureaucracies like to do things that can take off in the time scale of those who start them," he explained. "And a Pluto mission isn't that. . . . A mission to Mars is much easier because you launch it, and next year it's there."

More than anything else, NASA's mission to Pluto came about when it did because the pace of discovery couldn' t move any faster. The Pluto Underground had to wait until the revelations about Pluto's place in the Kuiper Belt finally sank in with the scientific community and created a ground-swell of support.

"If we had known in 1989 what we know today—that the most populous class of planets are Pluto-like—there wouldn't have been any argument about whether it was important to go," Stern said. "We' ve done all this exploration, and we haven't yet been to the most populous class of planets? That's a no-brainer! We need to do that."

But would NASA have felt the same way if Pluto lost its primacy as the "last unexplored planet"? What if the International Astronomical Union had struck Pluto off the list of major planets while New Horizons' fate was still in doubt? The Pluto Underground would have lost one of its most emotional arguments for sending a probe to the solar system's edge. And even though the scientific wonders of the Kuiper Belt would still be beckoning, the public support might have faded.

" I am convinced you're on to something," Stern said, "and if the IAU had acted prior to 2003, we would probably not be en route today."

As it is, however, New Horizons is safely speeding toward its 2015 encounter with Pluto—whether you call it a planet or not.

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