NASA's Dawn spacecraft launched Thursday evening its eight-year, 3.2-billion-mile journey that scientists hope can help them understand the formation of the solar system.

The spacecraft, atop a Delta 2 rocket, took off from Cape Canaveral in Florida at 4:34 a.m. PDT. "We have our time machine up and flying," said UCLA space physics professor Christopher T. Russell, the lead scientist on the project.

Powered by slow-go ion engines that take four days to go from zero to 60 mph, the spacecraft will make two stops on an eight-year journey to the asteroid belt between Mars and Jupiter. Once the craft is there, scientists plan to study some of the oldest objects in the solar system, lumps of stone that are thought to have changed very little in 4.5 billion years.

Dawn's first destination is Vesta, a lump of rock with a big hole at its south pole. After seven months of orbiting the 350-mile-wide rock, it will travel to the dwarf planet Ceres, which some scientists think could have more fresh water than Earth, locked up in an icy underground vault.

By observing them close up, scientists at JPL hope to better understand the process that allows some objects to become planets while others remain stunted as solar system flotsam.

Scientists used to think that the asteroid belt, which contains hundreds of thousands of small rocks and a few big ones, was the remains of a planet destroyed in an apocalyptic collision.

Now the prevailing theory is that Ceres and Vesta, which together contain about half the mass of the entire asteroid belt, are the biggest chunks of a proto-planet that failed to form because of interference from Jupiter's strong gravity.

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