The new decision arrived in a letter from Rex. D. Geveden, the agency's associate administrator for science missions, to Charles Elachi, director of NASA's Jet Propulsion Laboratory, which was developing Dawn. It followed an appeal of the decision by JPL.

Officials had named the mission Dawn because it was designed to study objects dating from the beginning of the solar system. Its planned trajectory would have taken it to Vesta and Ceres, two of the largest asteroids orbiting the Sun between Mars and Jupiter, including planned orbits of each object via an advanced electric ion-propulsion system.

The mission originally was approved in December 2001 and set for launch this June, but technical problems and other difficulties delayed the projected launch date to July 2007 and pushed its cost from the original estimate of $373 million to $446 million.

NASA Administrator Michael Griffin decided to cancel Dawn on March 2, after about $257 million already had been spent. Officials estimated that about $14 million more needed to be spent to terminate the project.

The reinstatement resulted from a review process established by Griffin and intended to "help ensure open debate and thorough evaluation of major decisions regarding space exploration and agency operations," NASA said in a statement.

"We revisited a number of technical and financial challenges and the work being done to address them," Geveden, who chaired the review panel, said in the same statement. "Our review determined the project team has made substantive progress on many of this mission's technical issues, and, in the end, we have confidence the mission will succeed."

Based on the decision, Geveden's letter ordered JPL to restaff the Dawn project immediately, based on "full funding," and to resume planning operations for the mission.

The letter said Geveden's Science Mission Directorate would develop a detailed replan with JPL, "including an integrated master schedule and updated cost analysis with appropriate confidence factors. JPL will undertake Propulsion Power Unit 500-hour life testing as soon as possible and will report progress and outcomes to SMD within 90 days."

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