The U.S. space agency NASA has reversed itself and reinstated a mission to send a spacecraft to orbit two asteroids later this decade. Scientists are happy about the decision, but say it does not overcome the planned drop in spending on planetary and other science missions in President Bush's proposed 2007 national budget.

A mission called Dawn is again scheduled to visit two of our solar system's largest asteroids, Ceres and Vesta, in the asteroid belt between Mars and Jupiter.

The launch had originally been planned for June, but NASA put it on hold in December because of technical problems and resulting cost overruns, and canceled it altogether earlier this month. The cancellation dismayed planetary scientists. Since the two asteroids are considered almost baby planets, researchers are eager to see what the Dawn spacecraft could reveal about the origins of our solar system.

So NASA reviewed the technical issues. An agency associate administrator, Rex Geveden, says he decided to reinstate the $446-million mission because NASA can absorb the 20 percent higher costs and overcome the technical problems.

"I think that the risk posture on this mission is not atypical for this type of mission," said Rex Geveden. "When you are doing deep planetary missions and dealing with the environments and temperature regimes and the complications of integrating a suite of instruments, there are always pretty tall challenges, and it looks like Dawn is prepared to take those on and beat them."

Some of the technical problems involved the spacecraft's revolutionary type of propulsion system, ion engines. Instead of using the traditional heavy solid or liquid hydrogen and oxygen as fuel, they employ positively charged, or ionized, xenon gas. The gas generates thrust as it is pulled backward by a negatively charged plate at the rear of the fuel chambers. The initial thrust of an ion engine is 10,000 times weaker than conventional rocket engines, but it builds thrust over time, using 10 times less fuel in the process.

Rex Geveden says some of the biggest troubles with Dawn's ion engines were products of the testing process and are not expected to occur in flight. The tests are being revised.

The new target launch date is between June and August next year. The woman in charge of NASA's science missions, Colleen Hartman, says the one year delay will not set back the date Dawn will begin orbiting the asteroid Vesta in 2011 and Ceres four years later because of the nature of orbital mechanics.

"The Science Mission directorate never lost site of the exciting science that would come from Dawn and will come from Dawn," said Colleen Hartman. "What we had here was a very gut-wrenching decision and significant management and technical hurdles to overcome, and we're happy to be going forward."

The reaction from planetary researchers to the reinstatement of the Dawn mission is positive. But one of them, Bruce Betts of the Planetary Society near Los Angeles, says his joy is tempered by reductions in other NASA science projects.

"The president's budget for fiscal 2007 slashes and burns all sorts of science missions and science research, unfortunately," said Bruce Betts. "Dawn is a nice success amongst that with it coming back to life, but there still are other missions and other concerns, amongst them cutting basic planetary science research by 15 to 50 percent, cutting astrobiology by 50 percent, and cutting some of the big missions of the future to study our solar system and others."

Several scientists recently told a U.S. Senate hearing that the proposed budget for science at NASA would set a pattern that would make it difficult to attract and retain space researchers. The agency's associate administrator for science, Mary Cleave, promised to review the relevant parts of the proposal.