After a nine-year journey, a NASA space probe is nearing one of the solar system’s final frontiers.
Photo composite by Lon Tweeten for TIME. Sun: SDO; Mercury: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington; Earth, Neptune: NASA; Mars, Jupiter, Saturn: Universal History Archive/UIG/Getty Images; Uranus: NASA/JPL-Caltech
July 9, 2015 11:36 AM EDT

Alan Stern was still a grad student at the University of Colorado back in 1989 when NASA’s Voyager 2 space probe completed its grand tour of the outer solar system. Its sister ship Voyager 1 had gone before it, returning the first close-up images and scientific measurements of the giant planets Jupiter and Saturn, along with their moons. But then it veered off, and Voyager 2 went on alone to explore Uranus as well, and then Neptune, at the outer edge of the solar system. And with that remarkable feat, the first reconnaissance of our sun’s planetary family was complete.

Except, of course, for Pluto. At the time, there simply wasn’t enough fuel to get Voyager 2 there. Surely NASA’s strategic vision included a later trip by a different ship to visit the tiny planet–and for the record, nobody doubted back then that it was a planet. But through failure of both will and wallet, NASA had no such plans. So a young Stern decided he’d lead the charge to make a Pluto mission happen.

On July 14, more than a quarter-century later, his dream will finally be fulfilled. Around noon on that date, after a nine-year, 3 billion-mile journey, NASA’s 1,000-lb., grand-piano-size, $700 million New Horizons probe will streak past tiny Pluto at a blistering 31,000 m.p.h. The spacecraft is so remote now that radio communications–traveling at the speed of light–require a nearly nine-hour round-trip. Ultimately, New Horizons will come to within just 6,000 miles of the icy world, furiously snapping pictures and recording data on the temperature, structure and composition of Pluto, its five known moons and anything else that might be there–more moons, perhaps, or a system of rings.

But it’s Pluto that’s the real prize. The little world has intrigued astronomers since it was first discovered more than 85 years ago. Until Pluto showed up, all the outer planets were known to be gas giants. What was this pip-squeak doing out there all alone? What was it made of? Why did it even exist?

For a long time, those questions seemed destined to go unanswered. Stern and other scientists appealed to NASA a half-dozen times during the 1990s with one proposal after another for a Pluto mission. They trudged to and from conference rooms, armed with charts and graphs and spreadsheets, with technical specs demonstrating why this latest version of a Pluto plan would be better, faster and cheaper than the one before it. And each time, NASA said no–usually on the grounds of budgetary poverty.

“I was counseled many times by senior people to give it up,” Stern says. “They said things like ‘You have better things to do with your career’ and ‘There’s a fine line between being in the groove and stuck in a rut.'” He ignored them.

Fortunately, he had a very nonscientific card to play: human sentiment. People just love Pluto. From the moment of its discovery in 1930 by Clyde Tombaugh, an Illinois farm boy who’d talked his way into a job at Arizona’s Lowell Observatory even though he’d never been to college, the tiny world fired the public imagination. At first, it was known simply as New Planet, but it was soon given its familiar name at the suggestion of an 11-year-old British girl named Venetia Burney, who thought the Roman god of the underworld nicely captured the feeling surrounding the dark and remote world. A few months later, Walt Disney capitalized on the public’s fascination by naming Mickey Mouse’s pet dog after the new planet. In 1955 the first Frisbee was marketed as the Pluto Platter.

In the decades after Pluto’s discovery, better observations made it clear that the new world wasn’t anywhere near the size of the 7,926-mile-diameter Earth but just 1,471 miles across, or smaller than the moon. And starting in the ’90s, astronomers began to realize that Pluto isn’t alone at all. It’s merely the brightest member of an enormous swarm of ice-covered objects that make up what’s known as the Kuiper Belt, which orbits the sun out beyond Neptune.

In 2000, that led Neil deGrasse Tyson, director of the Hayden Planetarium in New York City, to leave Pluto out of a display in the planetarium’s newly renovated main gallery. And in 2006, the International Astronomical Union (IAU), the cosmic court of last appeal, agreed. With the discovery of many more Kuiper Belt objects (KBOs), some rivaling Pluto in size, the IAU realized it was faced with the possibility of a solar system that could include dozens of planets.

So the term planet, which had never been formally defined, was reframed in a way exquisitely tuned to exclude Pluto and its kin. Not only would something that aspired to be a planet have to orbit the sun and be spherical but it would also have to “clear its neighborhood” of any other bodies of similar size. Pluto failed that last test spectacularly and would thus be busted down to dwarf-planet status. Pluto lovers steamed.

“If those people had been around in 1610 when Galileo discovered there’s an uncountable number of stars,” grumbles Stern, now at the Southwest Research Institute, in Boulder, “they would have restricted it to a number you could remember.”

But while Pluto partisans lost that round, they had already won a much bigger one. In 2001, after Tyson booted Pluto from his main hall, an impromptu “Don’t mess with Pluto” campaign began in astronomy circles. When NASA rejected yet another mission, Stern says, “we mounted an insurrection on every level from the press to little kids to the science community.” He and his team lobbied Congress as well–and that same year, New Horizons got the final thumbs-up.

“You could tell even when we were in grad school together that it was Alan’s destiny to make a Pluto mission happen,” says MIT planetary scientist Richard Binzel, a member of the New Horizons team. “The project is lean and mean, and he runs it with incredible efficiency.”

Tyson, who remains firmly opposed to granting Pluto the planet honorific, agrees and looks forward as eagerly as anyone to the spacecraft’s encounter. “Alan and I are not always on the same side of the argument,” he says, “but then that’s what makes the frontier of any endeavor a vibrant place to be.”

The very thing that reduced Pluto’s official standing–the abundance of other KBOs–actually makes this endeavor even more tantalizing. We used to think there were two zones to the solar system: the rocky inner worlds and the gaseous outer ones. “We now know there’s a third zone,” says Stern, “and Pluto is part of it.”

Given all of Pluto’s Kuiper Belt company, its surface is likely pocked with craters created when smaller KBOs slammed into it; a count of those craters could reveal the frequency of impacts and the sizes of the objects that made them, providing an indirect census of what’s actually in the Kuiper Belt.

Or maybe there won’t be many craters after all. That would suggest that Pluto, long assumed to be totally inert, is geologically active, with slush or even water erupting from underground to create a fresh surface every so often.

There’s also the mystery of how Pluto acquired the largest of its five known moons, Charon. Discovered in 1978, Charon is relatively big–about half the size of Pluto–leading some scientists to consider the pair a double-planet system, not a planet plus a moon. One possible explanation: a large body slammed into Pluto long ago, creating a debris cloud that coalesced to form Charon. Planetary scientists think a similar scenario created Earth’s moon, which means New Horizons could help them understand the Earth-moon system better.

The observations that will yield these answers will be conducted by a suite of onboard instruments measuring Pluto’s atmosphere, surface chemistry, gravity field and more–all with a spacecraft only half the size, two-thirds the weight and about half the cost of Voyager 2 yet with far more powerful capabilities. “We’re going to a whole new world and to a new region of the solar system,” says Stern. “And we’re doing it with 21st century technology. I think it’s as sexy as it gets for a robotic spaceflight.”

But sexy doesn’t mean foolproof, and on July 6, New Horizons had a near death experience when NASA briefly lost contact with the probe, a radio silence that lasted a heart-stopping hour and 21 minutes. The problem was caused by a timing anomaly in one of the spacecraft’s automatic commands–a command that fortunately won’t need to be repeated for any part of the Pluto encounter.

Whatever problems the spacecraft may have had will surely be forgotten once the pictures from New Horizons start flowing in. The images that have arrived so far are still blurry, but Pluto and Charon are already looking sharper than in anything seen before.

The latest surprise: four dark spots evenly spaced along Pluto’s equator, each about the size of Missouri. At closest approach, New Horizons will be able to spot objects as small as a football field, revealing details Tombaugh couldn’t have imagined. (Tombaugh died in 1997; at Stern’s insistence, some of his ashes are onboard New Horizons.)

Once the probe passes Pluto, it will pivot around and continue taking pictures and readings until it is out of range. New Horizons may then set its course for a 2018 or 2019 encounter with a second, smaller KBO. When that is done, the probe could last for decades, just like Voyagers 1 and 2, sending back data from the edge of the solar system.

With the encounter just days away, the New Horizons team has had plenty of time to think about the questions the mission may answer. What they’re more excited about is the possibility of ones they don’t even know enough to ask yet.

Back in 1993, Stern convened a group of planetary scientists to talk about what a mission to Pluto should look like. At dinner, he asked each of them to think of something that the still unnamed probe might find. They wrote their predictions on pieces of paper, then sealed them away. Most of what the scientists wrote concerned highly technical matters–the thermal structure of Pluto’s atmosphere or the mix of isotopes in the surface ice. All of the forecasts–technical and fanciful–will be unwrapped after the encounter and compared with what New Horizons actually finds.

On his paper, Stern simply wrote, “We’ll find something wonderful.” Predictions are not what he does, he explains. “My policy is ‘Let’s just go see what’s there.'”

This appears in the July 20, 2015 issue of TIME.

Contact us at letters@time.com.

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