NASA readies for Pluto voyage

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NASA's spacecraft have explored all of the planets in the solar system except for distant Pluto -- until now. The New Horizons spacecraft, scheduled for launch next month, will be the first to visit the tiny icy world at the outer edge of the solar system.

"This is in a very real sense the capstone of the initial reconnaissance of the planets that the United States has led for the world since the 1960s," NASA scientist Alan Stern said.

Spacecraft visited Venus and Mars in the early days of the space race in the 1960s. Mercury was visited by the Mariner 10 spacecraft in 1974. Jupiter and Saturn were first visited by the Pioneer spacecraft in the 1973 and 1979. The Voyager 2 spacecraft flew past Uranus in 1986 and Neptune in 1989.

Because of the distance from the sun, the Pluto probe will be powered by a radioisotope thermoelectric generator (RTG) with 24 pounds of plutonium dioxide. RTGs have been used to power all of the spacecraft to the outer planets, the Mars Viking missions, scientific experiments placed on the moon by the Apollo astronauts and a handful of spacecraft in orbit around the Earth.

A three-stage Atlas rocket will accelerate New Horizons to 8 miles per second making it the fastest spacecraft ever. But even with that speed record, it will take New Horizons a decade to reach Pluto. If New Horizons launches between Jan. 17 and Feb. 1, it will pass Jupiter in 2007 -- using that giant planet's gravitational field to pick up extra speed -- and encounter Pluto in 2015.

But if the spacecraft launches between Feb. 2 and Feb. 14, it will have to take a slower route to Pluto without the Jupiter flyby, arriving between 2016 and 2020. After mid-February it will not be possible to launch New Horizons for another year.

New Horizons will be racing Pluto's winter, when the temperature drops to minus 380 degrees. Pluto was at its closest to the sun in 1999 and is gradually moving farther away from the sun. As the distance increases the temperature drops and at some point it will be too cold for the thin atmosphere of nitrogen and methane to remain a gas.

If the New Horizons spacecraft remains in good condition and additional funding becomes available, the spacecraft will be aimed toward one or two "Kuiper Belt objects," mysterious icy worlds which can barely be seen in the largest Earth-based telescopes. Mr. Stern called the Kuiper Belt "the largest structure in our solar system, dotted with almost a half-million worlds and worldlets that are 4 billion years old."

New Horizons will speed by Pluto at more than 30,000 miles per hour. That limits the amount of time to take the highest quality pictures. The New Horizons project will cost taxpayers roughly $650 million.