One asteroid down, one to go.

After spending a year gazing at Vesta, NASA's Dawn spacecraft was set to cruise toward the most massive space rock in the asteroid belt between Mars and Jupiter -- a voyage that will take nearly three years.

Firing its ion propulsion thrusters, Dawn had been slowly spiraling away from Vesta for more than a month until it was to pop free from its gravitational grip. Since its antenna was pointed away from Earth during this last maneuver, engineers would not know until Wednesday how it went.

The departure was considered ho-hum compared with other recent missions -- think Curiosity's white-knuckle "seven minutes of terror" dive into Mars' atmosphere.

"It's not a sudden event. There's no whiplash-inducing maneuver. There's no tension, no anxiety," said chief engineer Marc Rayman of the NASA Jet Propulsion Laboratory, which manages the $466 million mission. "It's all very gentle and very graceful."

Launched in 2007, the Dawn mission is on track to become the first spacecraft to rendezvous with two celestial bodies in a bid to learn about the solar system's evolution.

- Chief scientist Christopher Russell

Dawn slipped into orbit last year around Vesta -- about the size of Arizona -- and beamed back stunning close-ups of the lumpy surface. Its next destination is the Texas-size Ceres, also known as a dwarf planet.

Vesta and Ceres are the largest bodies in the asteroid belt littered with chunks of rocks that never quite bloomed into full-fledged planets. As cosmic time capsules, they're ideal for scientists trying to piece together how Earth and the other planets formed and evolved.

During its yearlong stay at Vesta, Dawn used its cameras, infrared spectrometer, and gamma ray and neutron detector to explore the asteroid from varying altitudes, getting as close as 130 miles above the surface.

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