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.
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.
Dawn will accelerate at a snail's pace, reports , taking four days to go from zero to 60 mph, in order to conserve fuel.
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.
"They're two of the largest asteroids in the main asteroid belt, but also they're every different from each other," NASA engineer Jennifer Roca said.
Ceres is so big - as wide as France - that it has been reclassified a dwarf planet. The spacecraft will spend a year orbiting Vesta, about the length of Poland, from 2011 to 2012, then fly to Ceres and circle there in 2015.
"We should learn a lot about the evolutionary history, the geologic makeup, and, of course, take stunning visual images throughout the orbital phases," Roca told King.
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.
"The idea behind ion propulsion is to use a very low level of thrust over a very long duration to achieve a very long velocity change over the course of the mission," said Roca. "The acceleration that we impart from our spacecraft is equivalent to about the acceleration you get from holding a piece of paper in your hand, so when I say low thrust, I mean very low thrust."
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.
"We're very excited. I can tell you it's a very emotional state around the lab," Roca said.