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The Perils of Pluto

A seemingly simple trip to the world at the edge of the solar system is looking more like a voyage into the perfect meteor storm.

By Corey S Powell
Mar 14, 2013 4:23 PMNov 14, 2019 8:47 PM
New Horizons will study Pluto and Charon (crescent) in detail -- if it survives the trip. | JHUAPL/SWRI


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Try this for a challenge. Your task is to thread a needle—but not your average sewing-kit variety. The needle in question is five miles away. No matter how fast you push the thread, it will take 9.5 years to get it there. 

When your thread finally dangles in front of the needle’s eye, perfectly aligned, don’t get cocky, because a dust speck blowing by could fray your thread, ruining all that you’ve done. And did I mention that if you fail, you will have blown $700 million?

Now you have a sense of the slow-motion cliffhanger that is New Horizons, a NASA space probe currently racing toward Pluto and its large moon Charon at 35,000 miles per hour. “We have to hit our arrival time window within 450 seconds,” says principal investigator Alan Stern, a former chief of NASA’s science mission directorate who currently hangs his hat at Southwest Research Institute in Boulder, Colorado. 

“In terms of distance, Pluto and Charon are orbiting 20,000 kilometers [12,000 miles] apart and we need to hit our aim point within 1 percent of that. And if we hit anything even the size of a rice grain, it could kill us.”

One might expect these to be restful times for the New Horizons team. Their spacecraft blasted off on January 19, 2006, and will not reach Pluto until July 2015. Right now, New Horizons cruises smoothly through interplanetary space. But nothing about Pluto is simple or predictable. 

A Dwarf Oddball

Pluto is, by any measure, a strange and exotic world. When the dogged sky sleuth Clyde Tombaugh discovered it in 1930, he expected a massive world capable of disturbing the orbits of the giant outer planets Uranus and Neptune. Instead, Pluto seemed like a modest interloper in the outer solar system. Then with each additional observation, its estimated size and heft got smaller. 

Today we know it is a pygmy, just 1,400 miles wide—two-thirds the diameter of our moon. For a while, many scientists even speculated that it was an escaped satellite of Neptune. 

In the 1990s, thinking about Pluto shifted again with the realization that it is just one member of the Kuiper Belt, a swarm of thousands (if not millions) of tiny icy objects that circle the sun in the outer regions of the solar system. Like the asteroid belt between Jupiter and Mars, the Kuiper Belt consists of leftover planetary material that never gathered into a single large world. 

In 2006, the International Astronomical Union stripped Pluto of its title and rechristened it a “dwarf planet.” Stern, who has ceaselessly championed Pluto’s importance, argues that dwarf planets are still planets. Hal Weaver of the Johns Hopkins University Applied Physics Laboratory, who collaborates with Stern on New Horizons, goes the other way and speaks of Pluto being “in some respects like a giant comet.”

The ongoing difficulty in classifying Pluto is understandable: It is not like any object humans have ever studied up close. It rotates on its side, meaning that one hemisphere bathes in sunshine for more than a century while the other is plunged in darkness. During this summer break, the distant sun, still more than 2.8 billion miles away, evaporates Pluto’s surface into a temporary atmosphere that probably freezes solid again a few decades later. 

It follows a peculiar, oval orbit—unlike the other planets, though typical of Kuiper Belt objects—that at times carries it closer to the sun than Neptune. Even Charon, Pluto’s main moon, is an outlier. It is more than half as wide as Pluto itself, so large that the pair is more properly described as a double planet—or a double dwarf planet, or double Kuiper Belt Object, or whatever astronomers decide to call it next. 

Trying to make more sense of this oddball system, Weaver and Stern joined up with a team of collaborators to book time on the Hubble Space Telescope and search for additional satellites around Pluto. In 2005 they found two, each about 50 miles wide, later named Nix and Hydra (partially to honor the New Horizons craft). 

“At that point the Pluto system went from being a pair to a quadruple system. It had become a lot more interesting,” Stern says. Even so, when New Horizons launched a year later, “you could probably put everything we knew about Pluto all on one piece of paper, or one short Wikipedia article.” 

The next breakthroughs—and the beginning of the nail-biting—came in the summers of 2011 and 2012 when further Hubble observations turned up two more satellites, still unnamed and designated P4 and P5.

The best existing Pluto image reveals little besides extreme colors.| NASA

“After P4 was discovered, very quickly I said to the New Horizons team, ‘I’m worried. Do we now have a hazard issue on our plate?’ ” Stern recalls. The danger lies not with the moons themselves, which are pikers (on the order of a dozen miles across), but with the rough lives they lead. 

Any piece of junk floating around in the Kuiper Belt that happens to strike one of these moons can kick up a cloud of debris. Because the satellites are so small, the debris would fly right off their surfaces and enter into orbit around Pluto. Over billions of years, the dwarf planet could have acquired a thick shroud of shrapnel.

The scientists and engineers who designed New Horizons didn’t anticipate anything like that. And with the probe barreling in at nearly 10 miles per second—one of the highest speeds ever achieved by a spacecraft—any encounter with a speck of shrapnel could spell disaster. “If it cuts a fuel line, that’s bad; if it gets in the main computer and strikes a circuit board, that’s not good; it’s not good if it knocks out a camera. There’s almost no place you want to take a hit,” Stern says with the practiced nonchalance of someone who has been thinking about this list. A lot.

Failure Is Not an Option

So two full years before the craft reaches its destination, the New Horizons team is hard at work. One way to deal with the debris threat is to look ahead, literally, through the spacecraft’s cameras for additional moons around the dwarf planet. Should they find any, mission scientists can plot a new flight path past Pluto (they’ve already worked out several), and New Horizons can alter its path as little as 10 days before the scheduled closest encounter on July 14, 2015. 

If conditions look especially grim, the probe can strike a defensive pose, pointing its main radio antenna forward to act as an improvised meteor shield. Doing so would sacrifice a number of exciting observations during the closest part of the flyby, but Stern strikes a Solomonic tone: “If you’re presented with a choice between a degraded science encounter that would still blow our door off, and complete loss of mission, utter failure—which would you take?” 

As a final fail-safe, he and the team have also reprogrammed New Horizons’s onboard computers to transmit large downloads of data back to Earth two days and one day before the main encounter, ensuring that researchers will receive a flood of information about Pluto and its moons even if the probe gets blasted to kingdom come. 

The happy irony is that the same discoveries that make the New Horizons mission so treacherous also make it scientifically thrilling. All of those small bodies orbiting Pluto mean that it must have had a dynamic history. Perhaps Charon and the smaller moons formed during a tremendous collision, similar to the way theorists believe Earth’s moon formed after our planet collided with a Mars-size body 4.5 billion years ago. 

The Kuiper Belt probably started out much more crowded, but thinned out as gravitational interactions with the outer planets ejected many of the bodies orbiting there. Some of those wayward objects crash-landed on Earth, delivering water that might have helped fill our planet’s oceans. (Even today, the Kuiper Belt is the source of some of the comets that streak past our skies.) We might be alive in part because of the same celestial pinballing that carved up Pluto and its moons. 

Perhaps that wild history also explains why Pluto looks so peculiar today. The limited observations we have so far show a landscape that is heavily mottled, varying widely in brightness and color from place to place. In fact, Pluto has more extreme contrasts than any planet in the solar system. 

Although Charon is close in size to Pluto, it appears covered with water ice, whereas Pluto appears much redder and is blanketed in frozen nitrogen, methane, and carbon monoxide. Somehow, they evolved in very different directions. Weaver holds out hope that Pluto may even remain geologically active, its surface sculpted by cryovolcanism: eruptions powered not by magma, but by melting or boiling ices. 

Assuming New Horizons survives its main assignment, further adventures await. If the team can find a suitable target (and if there is enough fuel left), the probe will swing past one or more small Kuiper Belt objects—Pluto’s cousins—in the years ahead. As it goes, the spacecraft will make long-range scans of 10 or more of these denizens of the outer solar system, looking to see if they, too, have crowded systems of moons like Pluto does. 

There is a strong incentive to keep the mission going as long as possible. The lineup of NASA planetary missions looks meager after 2015; a successor to New Horizons is decades away at the least. Stern, however, has little stomach for gloom. He expects the results from the Pluto flyby to be like the world’s greatest stash of Christmas gifts. “Will I be depressed the week after when there are no more presents to open?” he asks incredulously. “No, I’ll be playing with those presents for years to come.”

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