Almost Time for Pluto's Close-Up

The pictures coming back from Pluto are still a bit fuzzy. But just wait.

As of Monday, NASA's New Horizons spacecraft was still almost 6 million miles away - about 25 times the distance from Earth to the moon - but it is closing in fast. About 7:50 a.m. Eastern time July 14, New Horizons is expected to pass less than 7,800 miles above Pluto's surface. Alan Stern, the mission's principal investigator, likes to say that if New Horizons were to view Manhattan from a similar distance, its telescopic lens would be able to pick out the ponds in Central Park.

Ever since a young astronomer named Clyde W. Tombaugh discovered Pluto 85 years ago, it has been little more than a dot in the night sky. This first ever spacecraft visit will bring Pluto into focus, illuminating mysterious dark regions on its surface and possibly erupting ice volcanoes. Weather patterns could swirl in Pluto's thin atmosphere of nitrogen and methane, with haze and snowfall.

Stern said he has made only one prediction about Pluto, at a conference in 1993. "I said we would find a Pluto that was two words: something wonderful," he said. "It's turning out to be completely true. The Pluto system is turning out to be completely amazing."

While Pluto was once thought to be a singular strange body in an otherwise dull, empty expanse of space, it is now the archetype of what astronomers call the "third zone" of the solar system. Beyond the rocky planets like Earth and the gas giants like Jupiter, there appear to be millions of icy worlds circling the sun in what is known as the Kuiper belt, named after Gerard Kuiper, an astronomer who had suggested that some comets originated from the outskirts of the solar system.

New Horizons is carrying sentimental mementos from Earth, including some ashes of Tombaugh, who died in 1997; a CD-ROM with 434,000 names of people who responded to a "Send Your Name to Pluto" request; and a couple of coins - the state quarters of Maryland (where the spacecraft was built) and Florida (where it was launched).

The latest surveys of Pluto's neighborhood show no signs of moons other than the five already known, nor of debris that could pose a danger to New Horizons. There could still be smaller objects - hitting something as small as a pebble could be catastrophic at New Horizons' speed of 31,000 mph - but the path appears to be clear. Computer simulations indicate that the spacecraft would travel through unscathed 4,999 out of 5,000 tries.

"I'm pretty confident," Stern said.

Images from New Horizon have already revealed the unexpected, like a dark region at the north pole of Charon, Pluto's largest moon. Usually, polar regions are bright with ice. Pluto also has a series of black spots on its surface, including a string of them, each about the size of Missouri, that the scientists have nicknamed "brass knuckles."

The $722 million mission had a scare Saturday when a flaw in commands caused the computer to restart. Engineers at the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland, which designed, built and operates the spacecraft, are confident that the problem will not recur, and New Horizons is to resume its work Tuesday.

It has been a long wait to get there - 14 years after the initial proposal and 9 1/2 years after New Horizons left Earth in January 2006.

Back then, Pluto was still a full-fledged planet, but the days for that designation were numbered. A year earlier, Michael E. Brown, a professor of planetary astronomy at the California Institute of Technology, had discovered something that was farther out than Pluto and appeared to be bigger. Either this world, which he named Eris, after the Greek goddess of chaos and strife, would also be a planet or Pluto would no longer be a planet.

Seven months later, the International Astronomical Union officially decided that to be a planet, an object had to push around everything else around its orbit. Pluto is too small to be a gravitational bully, and the organization invented a new label for Pluto, Eris and their ilk: dwarf planet.

As a planet, Pluto had always been an oddity. The inner four planets of the solar system - Mercury, Venus, Earth and Mars - are rocky spheres. The next four - Jupiter, Saturn, Uranus and Neptune - are giant balls of hydrogen and other gases. Then there was Pluto.

Pluto turned out to be small - about 1,465 miles wide, smaller than Earth's moon. Its orbit is an ellipse tilted askew compared with the other planets, and its composition is about one-third ice, two-thirds rock.

In 1978, James W. Christy spotted what appeared to be a bulge in the side of Pluto. That turned out to be a moon, which Christy named Charon. While Pluto was small, Charon was surprisingly big, about one-eighth the mass of Pluto. By comparison, Earth's moon has just one eighty-first the mass of Earth.

That pointed to a cataclysmic collision in Pluto's distant past, similar to the collision of a Mars-size body into Earth that is thought to have created the moon.

Still, there was nothing else like Pluto and Charon in the solar system - until 1992, when David Jewitt, then at the University of Hawaii, and Jane Luu, then at the University of California, Berkeley, discovered a small icy body beyond the orbit of Neptune. Soon, more icy objects were spotted. Today, more than a thousand Kuiper belt objects have been discovered, and astronomers estimate there might be millions more orbiting out there. The region beyond Neptune is now by far the most populous part of our solar system.

It is like an attic of memorabilia, with clues of how dust and gas coalesced into planets. "I still think it's one of the most fascinating regions of the solar system that holds clues to planet formation and the early dynamical evolution of the solar system," said Harold F.

Levison, a planetary scientist at the Southwest Research Institute's space studies department in Boulder, Colorado.

A Moon as a Night Light

During the flyby, the seven instruments on New Horizons will take detailed measurements of Pluto and its moons. By photographing a surface from multiple vantage points, scientists will be able to construct a topographic map. An instrument called a spectrometer will be able to identify some of the substances on the surface of Pluto and Charon.

After the closest approach, New Horizons will pass through the shadows of Pluto and Charon.

The arrival of New Horizons was timed so Charon will be over the night side of Pluto - that is, someone standing on Pluto would see Charon in the dark sky - and that moonlight, reflected off Charon to Pluto, is enough to illuminate the surface for photographs. The scientists will be able to compare the night view on Pluto with how the same region appeared in sunlight three days earlier. (A Plutonian day - the time to rotate on its axis - is 6.4 days.)

That, for instance, could show whether it snows on Pluto and whether nitrogen and methane in the atmosphere condense in the frigid nighttime. "It's our only way to map that part of Pluto," Stern said.

After New Horizons emerges from the shadows, the geometry of backlighting will enable the study of haze or clouds, similar to how in the late afternoon, "you see these dust motes in the air which you couldn't see earlier in the day," Stern said.

New Horizons will also measure the reflectivity of a radar signal off its surface. New Horizon's transmitter is not powerful enough, but a 70-meter radio dish at NASA's Goldstone Deep Space Communications Complex in California will be aimed at Pluto, and its pings of radio waves will bounce off Pluto to a receiver on New Horizons. "The spacecraft will be busy all day," Stern said.

For people back on Earth, that Tuesday will be one of largely uninterrupted waiting. Partly for simplicity, partly for fiscal economy, the instruments and antenna are bolted to the main spacecraft body and not on a rotating platform. That means New Horizons cannot take pictures and talk to Earth at the same time. It will take only a brief pause to send back a message to tell the scientists and engineers that it has survived the encounter and provide some engineering details about the day's events.

Four and a half hours later, the time it takes light to travel the 3 billion miles from Pluto, that status message will arrive at mission control. "That doesn't mean we won't be on the edge of our seats on the 14th," Stern said. "It's just human. We're going to be waiting for that signal at 9 p.m., and that'll be our moment of suspense."

The first photographs will be sent back the next day. Because of the sheer volume of data, the slow trickle of communications and the need to share NASA's Deep Space Network with other spacecraft, it will take a year and a half for all of the information to be radioed back to Earth.

"This is a mission of delayed gratification," said Hal Weaver, the project scientist.

The long work of analyzing data, digesting the unexpected and putting together a new understanding of Pluto will take years.

Kuiper Belt Bound

After Pluto, New Horizons is to be aimed toward one of two smaller Kuiper belt objects, 20 or 30 miles wide, both belonging to what astronomers call the cold classical Kuiper belt. Unlike that of Pluto, the orbits of these objects are circular and in the plane of the planets, seemingly undisturbed since the formation of the solar system 4 1/2 billion years ago. "They are probably the only things that were there at the beginning," said Levison of the Southwest Research Institute.

What New Horizons will not discover is what might lie beyond the Kuiper belt; its camera is not designed to scan the sky for dark objects. Before Brown discovered Eris, he found Sedna, a smaller object with a much weirder 11,400-year elliptical orbit. At its closest point to the sun, it is still beyond the Kuiper belt. At its farthest point, it is 31 times as far from the sun as Neptune.

Sedna remained a singular oddity until a few years ago when other astronomers found a second object with a Sedna-like orbit. In addition, they suggested that Sedna, the new object and other bodies at the outer edges of the Kuiper belt seemed to be nudged by the gravity of something big, several times the mass of Earth, farther out.

That idea is still controversial, although Brown, for one, is looking. "I think there is a super-duper Earth out there," he said. "We are pretty convinced that there is something large enough out there that we can quit worrying about wannabe things like Pluto and start worrying about the real ninth planet."

The age of planetary discovery may not yet be over.