Mission to Pluto Is Long-Awaited

Dec. 19, 2005 — -- Next month, if all goes according to plan, a spacecraft called New Horizons will be launched from Cape Canaveral, Fla., riding on an Atlas V rocket.

In 2015, it will fly past Pluto, the only remaining known planet never to have received a visit from Earth.

And the spacecraft will not stop -- over the next five years, it will wind its way through the Kuiper Belt, a vast band of rocky, icy asteroids, far beyond the known planets, that may tell us volumes about the solar system, even the origins of life on our planet.

It's an ambitious, 15-year mission. But to Alan Stern, the principal investigator for New Horizons, time has taken on new meaning. He's been trying to fly to Pluto since 1988.

"If I and my 'Pluto underground' colleagues had known then what it would take, how many meetings, proposals, presentations, reversals, setbacks, and outright cancellations it would take," Stern said, "we probably would not have had the courage to take on the task."

Stern is not a NASA employee; he's on the staff of the Southwest Research Institute in Boulder, Colo. He was 31, just a few years out of graduate school, when he began lobbying for the mission. He'll be in his mid-60s when New Horizons finishes its primary mission and hurtles toward interstellar space.

10-Billion Mile Mission

Right now New Horizons is going through final checks before it can be mounted atop its booster -- a heavily updated version of the one that launched John Glenn into orbit back in 1962. Atlas rockets have been a workhorse of the space age.

After launch, it will make a long, lonely dash across the solar system, with most of its systems turned off. That is partly to protect them from breakdown, partly to reduce the expense of having controllers on Earth monitoring them. The mission budget is $650 million.

In February 2007, according to the primary flight plan, the ship will swing past Jupiter, picking up speed as if Jupiter's gravity were a giant slingshot. Without this "gravity-assist" maneuver, the trip would be about five years longer.

The encounter with Pluto and its largest known moon, Charon, is expected in July 2015.

Pluto follows a long, lopsided elliptical orbit, so distant that it takes 248 Earth years to circle the Sun once. Even at its closest point -- which it passed shortly after Stern began his quest -- it is 3 billion miles away. In digital images from the Hubble Space Telescope, it appears as only a few hundred pixels.

Pluto is only 1,430 miles in diameter, considerably smaller than the Earth's moon, and Charon is half its size. It is probably a dark, rocky, icy sphere where sunlight is only about a 1,000th as bright as it is on Earth. It appears to have a thin atmosphere, but it is so far from the Sun that as it moves away from the low point in its orbit, scientists believe the atmosphere freezes. Stern was afraid, as he faced one delay after another, that New Horizons would not get there in time to measure it.

The probe itself weighs about 1,000 pounds on Earth. If it could land on Pluto, it would weigh only about 67 pounds. But it is not designed to land; its five instruments will be very busy as they scan Pluto and Charon and the ship moves on at 30,000 mph.

Kuiper Belt

From there, the mission is not fully planned. Astronomers believe the Kuiper Belt may include 100,000 objects at least 62 miles across -- but no objects, other than Pluto and Charon, had been spotted before the 1990s. Before New Horizons gets there, astronomers hope to find one close enough to the ship's trajectory to make a flyby possible.

The Kuiper Belt is named for Gerard Kuiper, the astronomer who proposed its existence in 1950. Stern calls it the single most vast "structure" in the solar system.

"Its discovery," he wrote, "has revolutionized our understanding of the architecture of our home solar system and forced us to confront the jarring, but exciting new fact that miniature planets like Pluto are more numerous than the conventional ones like Mercury, Venus, Earth, Mars and the giant planets."

It may also be the home of countless comets, occasionally sent shooting toward the sun by the gravity of a larger asteroid or meteoroid. If comets contain ice and organic molecules -- carbon compounds that are the building blocks of life -- some scientists believe they may explain the beginnings of life on Earth. In the early ages of the solar system, comets crashing into the Earth's surface may have deposited those building blocks here.

That is one reason Stern says it is important to explore the dark, frozen outer reaches of the solar system.