Cape Canavarel, Fla. ? An unmanned NASA spacecraft the size of a piano is set to lift off today on a nine-year journey to Pluto, the last unexplored planet in the solar system.

Scientists hope to learn more about the icy planet and its large moon, Charon, as well as two other, recently discovered moons in orbit around Pluto.

The $700 million New Horizons mission also will study the surrounding Kuiper Belt, the mysterious zone of the solar system that is believed to hold thousands of comets and other icy objects. It could hold clues to how the planets were formed.

“They finally are going! I can’t believe it!” said Patricia Tombaugh, 93, widow of Clyde Tombaugh, the Illinois-born astronomer who discovered Pluto in 1930.

Patricia Tombaugh, her two children, and the astronomer’s younger sister planned to witness the launch of the New Horizons spacecraft at the Cape Canaveral Air Force Station this afternoon.

Pluto is the only planet discovered by a U.S. citizen, though some astronomers dispute Pluto’s right to be called a planet. It is an oddball icy dwarf unlike the rocky planets of Mercury, Venus, Earth and Mars and the gaseous planets of Jupiter, Saturn, Uranus and Neptune.

NASA has sent unmanned space probes to every planet but Pluto.

“What we know about Pluto today could fit on the back of a postage stamp,” said Colleen Hartman, a deputy associate administrator at NASA. “The textbooks will be rewritten after this mission is completed.”

New Horizons will lift off on an Atlas V rocket, which was rolled to the launch pad Monday, and speed away from Earth at 36,000 mph, the fastest spacecraft ever launched. It will reach Earth’s moon in about nine hours and arrive in 13 months at Jupiter, where it will use the giant planet’s gravity as a slingshot, shaving five year off the 3-billion-mile trip.

The launch had drawn protests from anti-nuclear activists because the spacecraft will be powered by 24 pounds of plutonium, which will produce energy from natural radioactive decay.

NASA and the U.S. Department of Energy have put the probability of an early-launch accident that could release plutonium at 1 in 350. The agencies have brought in 16 mobile field teams that can detect radiation and 33 air samplers and monitors.

“Just as we have ambulances at football games, you don’t expect to use them, but we have them there if we need them,” NASA official Randy Scott said.