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IN THE 85 YEARS that humankind has known of Pluto’s existence, the dwarf planet has yet to complete a single revolution around the sun. So far is it from the sun and from Earth that even the most powerful telescopes see it as a blurry disk. Decades of calculating—and speculating—by astronomers have created only a sketchy portrait of the icy rock.
That all changes on Tuesday morning.
At 7:49:58 Eastern time, NASA’s New Horizons spacecraft will come within 8,000 miles of Pluto—close enough to capture features as small as the docks of Manhattan. Pluto’s exotic ices, nitrogenous atmosphere, and mysterious dark spots will no longer exist merely in the minds of astronomers. They’ll be real.
Not since Voyager 2 in the 1980s have humans explored an entirely new planetary world. “It’s the kind of thing kids will read about in textbooks in the 22nd century,” says Alan Stern, New Horizons’ principal investigator, who has led the mission since NASA chose to embark on it 15 years ago.
Nine and a half of those years were taken up by New Horizons’ 3 billion-mile journey across the solar system. When the little spacecraft launched, Pluto was still a planet. Stern and many Pluto researchers bristle even now at its 2006 demotion to the rank of dwarf planet—an indignity of cosmic proportions. But it’s precisely Pluto’s oddball status that makes it so fascinating.
Unlike the four rocky planets and four gas giants of the solar system, Pluto belongs to the unexplored “third zone” on its fringes, called the Kuiper Belt. Here lie dwarf planets and millions of other icy bodies, frozen remnants of the solar system’s formation. New Horizons has traveled so far, in space and back in time.
A Race Against Time
That New Horizons has made it so far is thanks to two things: the Pluto Underground and how the planets aligned, quite literally.
The Pluto Underground is a group of researchers who gathered over dinner at a Baltimore restaurant in 1989. (It’s been so long they can’t agree over whether the restaurant was seafood or Italian.) Planetary scientists were excited by new Pluto maps, made possible by a once-in-a-century event. For five years beginning in 1985, Pluto and its biggest moon, Charon, passed back and forth in front of each other within Earth’s line of sight. By measuring the light Charon blocked as it eclipsed Pluto, scientists could infer patchy regions of light and dark on the surface of the dwarf planet.
Those maps only made the astronomers more curious: What the hell were those dark regions? “The obvious next step was sending a spacecraft to Pluto, but NASA wasn’t talking about it,” says Marc Buie, a member of the Pluto Underground who is now a co-investigator on New Horizons. The group drew up a strategy to promote a Pluto mission, and Stern became its unofficial leader.
In the next decade, the US rejected four Pluto missions, mostly for budget reasons. And time was running out. Pluto travels in an oval orbit, so from 1989 onward, it has been moving away from the sun—so far away that its atmosphere may freeze, rendering a large part of the scientific mission moot. The next window would not open until 2200. This was the opportunity of a lifetime.
Plus, there was the matter of Jupiter. If positioned right, the astronomers could use the gas giant as a gravity slingshot to speed up the journey to Pluto. Without a gravity assist, the nine-year voyage would be 50 percent longer. A few more years of dawdling and Jupiter would be out of alignment.
What ultimately got New Horizons off the ground was old-fashioned political maneuvering. At one point, Bill Nye the Science Guy lugged postcards bearing notes from thousands of Pluto mission supporters into the US Senate. The biggest breakthrough came in 2002, when the mission appeared at the top of the National Research Council’s Decadal Survey for planetary science—no small thanks to Stern’s lobbying of fellow scientists. New Horizons was officially funded.
With the political battle over, Stern took on the technical challenge of getting New Horizons off the ground—literally—in time for Jupiter’s gravity assist. “A lot of people said, ‘You can’t do this,'” he says. New Horizons had to deal with a punishingly small budget of less than $600 million and a tight schedule (on NASA time, four years is nothing to get a mission launched). Then there was the difficulty of building a probe reliable enough to travel 3 billion miles.
But they did. On January 19, 2006, New Horizons took off atop a huge Atlas V rocket. It is a lean, fast machine—the fastest spacecraft ever launched from Earth. The rocket was flying at 36,373 mph when it left Earth’s atmosphere, and New Horizons reached the moon in just nine hours—Apollo 11 took three days to get there. Exuberance among scientists was equally intense. The wonder inspired by the mission hasn’t waned over the last nine and a half years, even for those not working on New Horizons. “This is really exciting!” says Mark Sykes of the Planetary Science Institute. “I keep using that word, but what else do you call it?”
Pluto Up Close
For such an important astronomical explorer, New Horizons is tiny—just the size of a baby grand piano. “All seven instruments on New Horizons weigh less together than just the camera on the Cassini Saturn probe,” says Stern.
As New Horizons flies by Pluto, those seven instruments will execute a choreographed dance, scooping up as much data as possible. For the craft’s earthly observers, the moment of its closest approach will be mostly quiet: New Horizons will be too busy observing to communicate much with mission control. The mission’s astronomers, gathered at the Johns Hopkins University Applied Physics Laboratory in Maryland—and all of the people watching from afar—will simply have to wait.
“After nine and a half years, so much is riding on the last day,” says Hal Weaver, the mission’s project scientist. “It makes for some nail-biting.” With New Horizons zipping by at 31,000 mph, a piece of debris as small as a grain of rice could punch a fatal hole in the spacecraft. So yes—nail-biting.
If all goes well, the data will begin trickling back to Earth, crossing a void so vast it will take more than four hours for each transmission to reach Maryland. A schedule of what happens exactly when fills four manuals, just one of which runs 50 pages. It’ll take more than a year for the probe to send all that data home: the downlink speed is only two kilobits per second and New Horizons has to share the antennas of NASA’s Deep Space Network with other spacecraft.
But the juiciest stuff will come first. Weaver says a close-up of Pluto from the camera on the Long Range Reconnaissance Imager (LORRI) will be among the first items on the list. Another camera, Ralph, will take color photographs of Pluto’s surface (which, by the way, is reddish-brown—not blue, as decades of textbooks would have you believe).
“New Horizons will transform Pluto from an object of the astronomical world to one of the geological and physical world,” says Richard Binzel of MIT, another co-investigator on the mission. That blurry disk of light will soon become a dynamic three-dimensional object. New Horizons might find weather in the form of methane or nitrogen snow. It might find an explanation for the dark spots on Pluto’s surface—possibly the work of cosmic rays hitting methane. And it might gather evidence of a liquid ocean beneath all that ice. Other instruments will also be attuned to Pluto’s atmosphere, analyzing its composition and how it interacts with solar winds.
New Horizons will also observe Pluto’s moons, including Charon, which, at half the diameter of Pluto is so bizarrely big that they form a binary planet system. The two bodies orbit around a single point in space between them. Charon was the only known moon of Pluto when New Horizons launched. Now, scientists have found a total of five.
For Marc Buie, who has been staring at Pluto for decades, the one piece of data he is most excited about is almost laughably basic: size. Measurements of Pluto’s diameter from afar leave about 20 kilometers or so of wiggle room, and that uncertainty affects just about everything else scientists think they know about Pluto, from the structure of its atmosphere to its temperature. “In all of the analyses I’ve done in past the 30 years, my software has something with how big Pluto is,” says Buie. “As soon as we figure it out, the first thing I’ll have to do is rerun everything.”
That’s just how little scientists know about Pluto, pre-New Horizons.
Beyond the Dwarf Planet
New Horizons’ job won’t be done once it passes Pluto. In the fall, mission control will fire up its thrusters and point the spacecraft at one of two objects in the Kuiper Belt—one closer and easier to reach, the other farther away but potentially larger and thus most interesting to visit. Both are smaller than the dwarf planet, and belong to another group of Kuiper Belt objects called the Cold Classicals.
These objects have been undisturbed since the birth of the solar system 4.5 billion years ago; an expedition there has been likened to an archeological dig in space. “This is maybe the most primitive material out there in the solar system,” says Weaver. The mission’s scientists are working out the last details before they make the call on which object to visit.
From there, New Horizons will keep flying and observing until it reaches interstellar space—like the Voyager missions that preceded it. For many of the scientists working on New Horizons, Voyager was the singular inspiration for the Pluto mission. As young students, they watched planets one by one go from dots of light to swirling, celestial bodies. It’s finally Pluto’s turn now—the most remote destination in humanity’s quest to make known the unknown. Says Stern: “We’re going exploring.”
TL;DR
New Horizons' probe flying by Pluto on Tuesday to take pictures.
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