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.