Jet Propulsion Laboratory's Dawn mission to the asteroids Ceres and Vesta, canceled earlier this month, was given a reprieve Monday when NASA officials said they would reinstate the project despite cost overruns and technical problems.
The cost overruns are not unprecedented given the mission's complexity, and the technical hurdles are well on the way to being overcome, NASA Associate Administrator Rex Geveden said in a teleconference.
"We have confidence the mission will succeed," he said.
"I am thankful for this support and excited to be moving ahead," said JPL Director Charles Elachi.
Dawn is the first space probe designed to orbit two solar system bodies, and the first scientific mission to use an ion propulsion system, tested earlier on the Deep Space 1 probe.
That motor, which requires a relatively small amount of propellant, will allow the craft to spiral into a low orbit around Vesta, accelerate away from the asteroid and then spiral into a similar orbit at Ceres.
But engineers found abnormalities when they tested the motor and fuel tank, necessitating a delay in the summer 2006 launch date and ultimately leading to cancellation of the mission.
Later testing indicated that the problem had more to do with temperatures during testing than with the components themselves.
"It's not a flight hardware issue, but a test configuration issue," Geveden said. "We still have work to do [on those issues], but the way forward looks pretty clear."
The delays have led to a 20% cost overrun, raising the cost of the mission from $373 million initially to $446 million.
The entire amount was included in the agency's 2007 budget, just in case the mission went forward, said Colleen Hartman, NASA deputy associate administrator. "But obviously it [the overrun] has to come from a pot of money, and the pot doesn't change, so that means there is less money for something else," Hartman said.
The Dawn craft was designed and built by Dulles, Va.-based Orbital Sciences Corp.; the mission is being managed by JPL in La Canada Flintridge.
The launch is now scheduled for the summer of 2007. Because of celestial mechanics, the delay in the launch will not affect the craft's scheduled arrival times at Vesta in 2011 and Ceres in 2015.
Dawn's targets lie in the asteroid belt between Mars and Jupiter.
They are among the largest asteroids in the solar system but are radically different in composition, though both were formed more than 4.6 billion years ago when Earth and other planets were condensing from the disk of dust surrounding the sun.
Ceres is the larger of the two and was formed under conditions that allowed it to retain large quantities of water. There is evidence of frost or vapor on its surface and, possibly, liquid water under the surface.
Vesta's origin was hot and violent and its interior was once molten with lava flows on its surface.
Both have survived relatively intact since then, Hartman said, adding that scientists should be able to learn a great deal more about the conditions that were present at the creation of our own world.