Next Stop, Pluto

When NASA first considered a mission to Pluto more than 15 years ago, the idea was to visit the last and most distant of the nine planets, an oddball whose icy composition, tilted orbit and tiny size made it unlike anything else in the solar system. But when the New Horizons probe finally takes off from Cape Canaveral—as early as next week, if all goes well—it will be heading for something else entirely. “This little misfit is now central to our understanding of the origin of our solar system,” says Alan Stern of the Southwest Research Institute in Boulder, Colo., and lead scientist for New Horizons.

Reason: Pluto, astronomers have learned, is no oddball. It’s one of thousands of icy bodies in a diskshape swarm known as the Kuiper Belt that orbits the sun in the dark, frigid realm beyond Neptune. Since the discovery last summer of an object called 2003 UB313, Pluto is not even the biggest. And because those little worlds have been in deep freeze since the solar system was formed more than 4 billion years ago, they represent a frozen record of what conditions were like back then.

Those primordial conditions are what Stern and his colleagues will be trying to understand when New Horizons reaches Pluto and its three moons (two were found just this past fall) in 2015. As the probe zips by, cameras will snap pictures of surface features about the size of a football field, analyze Pluto’s thin atmosphere and measure its temperature.

All of that will add enormously to our knowledge. But it won’t help scientists decide whether Pluto should keep its status as a planet, a debate that only intensified when 2003 UB313 was discovered; if Pluto is a planet, then its bigger cousin must be as well. The International Astronomical Union promises a decision, but Stern doesn’t know when it will come. For now, he’s not thinking much about that. He has a spacecraft to launch. [The following text appears as part of a complex diagram]

THE MISSION Using Jupiter’s gravity to speed it on its way, New Horizons will be the first probe to take close-up images of Pluto and analyze its atmosphere, thus enabling astronomers to understand how the icy bodies of the Kuiper Belt came to be

• SUN • EARTH LAUNCH – Between Jan. 17 and Feb. 14 • SATURN • JUPITER Jupiter gravity assist – February to March 2007 • URANUS • NEPTUNE • PLUTO PLUTO-CHARON ENCOUNTER July 2015. During flyby, the probe will pass within a mere 6,000 miles (10,000 km) of Pluto—40 times as close as the Earth is to our own moon • KUIPER BELT Voyage into Kuiper Belt 2016-2020 What is the Kuiper Belt? Named for Gerard Kuiper, who predicted its existence in the 1950s, it is a vast, disk-shaped cloud of thousands of icy bodies that starts near Neptune and reaches to about 4.5 billion miles (7.5 billion km)from the sun

ORBIT OF PLUTO • From Pluto, the sun appears about 1,000 times as dim as it does from Earth • Pluto and its largest moon, Charon, are locked in synchronous orbit, always keeping the same face toward each other

THE SPACECRAFT New Horizons is about the size of a grand piano, packed with highly sensitive instruments

• Antenna • Heat shield • Thruster • Star tracking cameras • PEPSSI – Detects molecules escaping from Pluto’s atmosphere • SWAP – Looks for magnetic fields and measures how fast the atmosphere is escaping • RTG – Powers the craft with a tiny amount of plutonium. Because the probe will travel so far from the sun, solar power was not an option • LORRI – A high-resolution telescope and camera capable of detecting features about the size of a football field • REX – Uses radio waves to analyze the atmosphere and determine night-side temperature • ALICE – Analyzes ultraviolet light to determine atmospheric composition • RALPH – Makes color maps of the surfaces of Pluto and Charon and uses infrared measurements to determine surface composition • SDC – Built by students in Colorado, this instrument will count and measure dust particles in space throughout the journey

MYSTERIES OF AN ICY WORLD • Pluto • Charon • If humans lived by Pluto time, they would never see a second birthday. The planet orbits the sun once every 248 Earth years • And if humans lived on Pluto, they wouldn’t have to diet. Pluto’s gravity is so weak that a man weighing 300 lbs. (136 kg) on Earth would weigh just 20 lbs. (9 kg) on Pluto • Unfortunately, breathing would be impossible. In addition to being intolerably cold, Pluto has a thin—and temporary—atmosphere of nitrogen molecules, with traces of carbon monoxide and methane. When the planet moves farther from the sun, the atmosphere freezes back onto the surface • Pluto is one of only two planets that rotate on their horizontal axis. Uranus is the other. A day on Pluto is equal to 6.4 days on Earth. • A radio signal moving at the speed of light takes about 4½ hours to reach Pluto from Earth