NASA Counting Down to Asteroid-Probe Launch

NASA is about to embark on an unprecedented asteroid-belt mission with a spacecraft aptly named Dawn.

The 3 billion-mile, eight-year journey to probe the earliest stages of the solar system will begin with liftoff, planned for just after sunrise Thursday.

Rain is forecast, however, and could force a delay.

• Click here for the Dawn probe Web page.

Scientists have been waiting for Dawn to rise since July, when the mission was put off because of the more pressing need to launch NASA's latest Mars lander, the Phoenix. Once Phoenix rocketed away in August, that cleared the way for Dawn.

"For the people in the Bahamas, on the 27th will be one day where they can say that Dawn will rise in the west," said a smiling Keyur Patel, project manager from the Jet Propulsion Laboratory in Pasadena, Calif.

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Dawn will travel to the two biggest bodies in the asteroid belt between Mars and Jupiter — rocky Vesta and icy Ceres from the planet-forming period of the solar system.

Ceres is so big — as wide as Texas — that it's been reclassified a dwarf planet. The spacecraft will spend a year orbiting Vesta, about the length of Arizona, from 2011 to 2012, then fly to Ceres and circle there in 2015.

Dawn's three science instruments — a camera, infrared spectrometer, and gamma ray and neutron detector — will explore Vesta and Ceres from varying altitudes.

"In my view, we're going to be visiting some of the last unexplored worlds in the inner solar system," chief engineer Marc Rayman said Tuesday.

Because Vesta and Ceres are so different, researchers want to compare their evolutionary paths.

No one has ever attempted before to send a spacecraft to two celestial bodies and orbit both of them. It's possible now because of the revolutionary ion engines that will propel Dawn through the cosmos.

Dawn is equipped with three ion-propulsion thrusters. Xenon gas will be bombarded with electrons, and the resulting ions will be accelerated out into space, gently shoving the spacecraft forward at increasingly higher speeds.

"It really does emit this cool blue glow like in the science fiction movies," Rayman said.

NASA tested an ion engine aboard its Deep Space 1 craft, which was launched in 1998. Ion engines have been used on only about five dozen spacecraft, mostly commercial satellites.

Dawn also has two massive solar wings, nearly 65 feet from tip to tip, to generate power as it ventures farther from the sun. Ceres is about three times farther from the sun than Earth.

NASA put the cost of the mission at $357 million, but said that does not include the Delta II rocket. Officials refused Tuesday to provide the cost of the rocket, saying that was proprietary information.