Less than an hour later, all three stages of the booster rocket worked as planned, and the spacecraft separated from them and sprinted away toward deep space. The robot ship sped away at about 36,000 miles per hour, the fastest flight of any spacecraft sent from Earth, allowing it to pass the Moon in about nine hours.

"This is a historic day," said Alan Stern of the Southwest Research Institute in Boulder, Colo., the mission's principal scientist and team leader.

Speaking at a news conference at the Kennedy Space Center in Florida, Dr. Stern said the timing assured that the New Horizons would arrive for its closest approach to Pluto on July 14, 2015 -- the 50th anniversary of the first flyby of Mars by the Mariner 4, the mission that began the exploration of the planets.

Yesterday's liftoff also paid homage to Pluto's discoverer, the astronomer Clyde W. Tombaugh, who in 1930 became the only American to find a planet in the solar system. (He died at 90, in 1997.) His widow, Patricia Tombaugh, 93, and other family members were present at the cape, and some of his remains were among the commemorative items aboard the spacecraft.

"Some of Clyde's ashes are on their way to Pluto today," Dr. Stern said.

The New Horizons is to reach Jupiter's gravitational field in 13 months. The trip to Pluto will take eight more years, most of which the craft will spend in electronic "hibernation" to save power and wear on the equipment needed for its seven experiments.