NASA is taking another look at its recent decision to cancel Dawn, a spacecraft mission that would explore two large asteroids in the solar system.

The U.S. space agency canceled the mission March 2, citing an independent assessment that found over two dozen major issues in need of resolution before Dawn would be ready to launch.

The independent assessment also found that Dawn was on track to exceed its $373 million cost cap by 20 percent and miss its June 2006 launch window at least 14 months, NASA officials said at the time.

But within a week of that announcement, NASA said it had decided to further review the cancellation decision before issuing a final ruling on the mission. NASA Associate Administrator Rex Geveden, the agency’s number three official, has been asked to conduct a review of the cancellation decision “in light of additional information provided by NASA’s Jet Propulsion Laboratory (JPL),” the agency said in a March 9 statement. Geveden is expected to complete his review by the end of March.

Dawn is managed by the Jet Propulsion Laboratory in Pasadena, California with Orbital Sciences Corporation of Dulles, Virginia developing the spacecraft. Dawn was selected in December 2001 under NASA’s Discovery program of moderately-priced missions.

News that Dawn was on the chopping block stirred up anger in scientific circles, both in the United States and abroad, some of which surfaced here the week of March 13 at the 37th Lunar and Planetary Science Conference (LPSC).

When pressed by scientists to clarify whether or not the Dawn mission would be restored, NASA associate administrator for science, Mary Cleave, offered no hints, but said the agency was trying to be responsive to new cost-control guidelines Congress included in 2005 NASA Authorization Act. Cleave said the guidelines, modeled on the Nunn-McCurdy cost controls Congress imposed on Pentagon in the 1980s, dictate that “if we get to a certain percentage cost overrun we have to review a project. And if it gets to another percentage, Congress will zero the money going in and we will be in this limbo with no money going in.”

Under the guidelines, a project that exceeds its budget by 30 percent would have 18 months to make its case for continuation before Congress would freeze its funding.

Andrew Dantzler, director of NASA’s solar system division, was similarly opaque about what will become of Dawn.

“I really can’t get into the details on Dawn,” Dantzler told the LPSC gathering, but noted that the cancellation “is under review by our management.” Because of that fact, he added, he could not go into specifics.

Cleave was more forthcoming during a presentation she gave to an ad hoc committee of the Space Studies Board during a March 6 meeting in Washington. Cleave told the ad hoc committee that NASA has spent about $284 million on Dawn to date and would have to spend a further $162 million to complete the mission.

She said that by early 2005 it had become apparent that completion of critical hardware for the spacecraft and its ion propulsions systems had fallen behind schedule. She said the team was given an opportunity to present a plan for getting the project back on track for its June 2006 launch but “failed to bring a credible story forward.”

Late last year, NASA ordered the Dawn team to “stand down”, allowing only a minimal amount of activity to proceed while an independent assessment team had a chance to take a closer look at the mission. The team’s report, delivered to NASA in late January, found that the Dawn team had underestimated its additional funding needs by about $33 million and had “clear project management and planning issues,” according to Cleave’s presentation charts.

Her charts also say that “heritage basis for the spacecraft and ion propulsion system was overly optimistic.”

The Dawn mission was based in large part on technology demonstrated on Deep Space 1, an experimental spacecraft that launched in 1998 and used a first of its kind ion propulsion engine to fly by two asteroids before its retirement in 2001.

While it is not yet clear whether Geveden’s review will overturn the Cleave’s decision to terminate the mission, the fact that Dawn is on the chopping block has incensed scientists, not only those in the U.S. but in other countries taking part in the mission.

“It’s totally unacceptable what’s happening now,” said Gerhard Neukum, professor of planetary sciences at Freie Universitt Berlin in Germany and a member of the Dawn team. “NASA has responsibility to their cooperation partners first and foremost before they go to the last resort of canceling a mission,” he said..

The confusion over Dawn’s status was spotlighted by Jonathan Lunine, Professor of Planetary Sciences and of Physics at the University of Arizona in Tucson during a major talk at LPSC on the future of solar system exploration.

“Dawn — which actually did get canceled but is now on some kind of life-support…or there’s some religious ceremony in which it’s being resurrected — if it does go to two very large asteroids, Ceres and Vesta, it is a very important mission.”

Prior to the stand down, Dawn’s objective was to reach asteroids 4 Vesta in 2011 and 1 Ceres in 2015. By surveying those asteroids up close, scientists hope to glean clues about the formation of the solar system.

Brian Berger contributed to this story from Washington.