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The launch of NASA's unmanned Dawn spacecraft to two asteroids thought to have been formed in the early days of the solar system went off at 4:34 a.m. Thursday.

The 3 billion-mile mission to Vesta, a large asteroid, and Ceres — one recently reclassified as a dwarf planet — will use ion propulsion and a gravity boost from Mars to reach Vesta in 2011 and Ceres in 2015. After studying the two asteroid-belt objects from orbit, it is hoped the spacecraft will continue sending back data about other objects it encounters.

For Mark V. Sykes, director of the Tucson-based Planetary Science Institute and a Dawn co-investigator — one of the project's science leaders — it was a great relief.

"I've been waiting for this over 14 years," Sykes said in a call from Cape Canaveral after the launch.

As of Thursday afternoon, Sykes said readings from the spacecraft were well within limits and one major event — deployment of Dawn's solar panels — had gone flawlessly.

After being pushed out of Earth orbit by its last stage, conventional-fuel engine, Sykes said, it would continue to "coast" for six days until the ion-propulsion is turned on.

By this morning Dawn should be outside the moon's orbit, project manager Keyur Patel said in a statement after the launch.

Sykes said NASA had been reluctant to use ion propulsion, a highly fuel-efficient but very slow means of propulsion, when it was first proposed for a mission outside Earth orbit. But he said it was later proved on another mission, clearing the way for its use on Dawn.

The system uses ions to gently push a spacecraft. It's been compared with a car that takes four days to go from zero to 60 mph. The payoff is that Dawn's system will need less than 1,000 pounds of xenon gas to get to its two destinations.

"It saves so much mass. It's so efficient. It just opens up possibilities for science missions, multiple-rendezvous missions," said Sykes.

Though Dawn won't get to Vesta until 2011 and Ceres until 2015, Sykes says it should be worth the wait to learn more about objects thought to date to the solar system's formation.

"We're going to have great (images)," Sykes said. "When we get to Ceres we're going to see a surface that's substantially different than anything else we've seen in the solar system. Same with Vesta."

Vesta, he said, has a volcanic history, and Ceres is thought to be covered in clay. "What's that going to look like? Are we going to see evaporite deposits like you see around Yosemite, where water evaporates and forms crystals?"