WASHINGTON: NASA's Dawn spacecraft rocketed away Thursday toward an unprecedented double encounter in the asteroid belt. Scientists hope the mission sheds light on the early solar system by exploring the two largest bodies in the belt between Mars and Jupiter: an asteroid named Vesta and a dwarf planet the size of Texas named Ceres.

Dawn's mission is the world's first attempt to journey to a celestial body and orbit it, then travel to another and circle it as well. Ion-propulsion engines, once confined to science fiction, are making it possible.

"To me, this feels like the first real interplanetary spaceship," said Marc Rayman, chief engineer. "This is the first time we've really had the capability to go someplace, stop, take a detailed look, spend our time there and then leave."

The 3 billion-mile trip began a little after sunrise. The Delta II rocket thundered through a clear blue sky and headed southeast above the thick clouds over the horizon. A harvest moon was faintly visible in the west.

"Dawn, you're on your way. Good luck," Launch Control said once Dawn separated from its third rocket stage an hour later, right on cue. Already, the spacecraft was 4,000 miles from Earth.

Dawn won't reach Vesta, its first stop, until 2011, and Ceres, its second and last stop, until 2015.

Scientists chose the two targets not only because of their size but because they are so different from one another.

Vesta, an asteroid about the length of Arizona and not quite spherical, is dry and rocky and appears to have a surface of frozen lava. It's where many of the meteorites found on Earth came from. Ceres, upgraded to a dwarf planet just last year, is nearly spherical, icy and may have frost-covered poles. Both formed around the same time some 4 1/2 billion years ago.

Spacecraft have flown by asteroids before — albeit much smaller — and even orbited and landed on them, and more asteroid missions are on the horizon. But none has attempted to orbit two on the same mission, until Dawn.

"While these other asteroid missions are, I think, very exciting, I hope one doesn't confuse the kind of asteroids that Dawn is going to with the near-Earth asteroids and these other small bodies," said Rayman, who is based at Jet Propulsion Laboratory in Pasadena, Calif. "I think many people think of asteroids as kind of little chips of rock. But the places that Dawn is going to really are more like worlds."

Dawn has cameras, an infrared spectrometer and a gamma ray and neutron detector to probe the surfaces of Vesta and Ceres from orbit. It also has solar wings that measure nearly 65 feet from tip to tip, to generate power as it ventures farther from the sun.

Most importantly, Dawn has three ion engines that will provide a gentle yet increasingly accelerating thrust. Electrons will bombard Dawn's modest supply of xenon gas, and the resulting ions will shoot out into space, nudging the spacecraft along.

Even "Star Wars" had only twin ion engines with its T.I.E. Fighters, Rayman noted with a smile earlier in the week.

The mission costs $357 million, excluding the unpublicized price of the rocket.