A NASA spacecraft built to explore two of the solar system's largest asteroids won't launch this year because the space agency is dealing with cost overruns and technical issues in the project.

The planned summer launch of the Dawn spacecraft has been indefinitely postponed, said Andrew Dantzler, director of NASA's solar system division.

Mission managers had been ordered to halt work on Dawn last fall while the project was assessed by an independent review team, which is expected to present its findings to NASA on Jan. 27.

Even if NASA gives Dawn the green light, it would take another year for engineers to finish routine testing of the spacecraft, said mission principal investigator Christopher Russell of the University of California, Los Angeles.

"It's like running a relay race," Russell said. "You're on your last leg and the judges suddenly say 'Stop.' You lose your momentum."

Dawn is part of a NASA program called Discovery that seeks to explore the solar system on what for NASA is considered a shoestring budget. The program includes the Stardust mission, which last week returned to Earth with samples of comet dust.

Dawn, however, has suffered several setbacks, including ruptures of two of its fuel tanks during testing, forcing engineers to reduce the amount of xenon gas that will be loaded into the tanks.

The project was capped at $371 million, according to Russell, and when project scientists asked for an extra $40 million last year, NASA ordered the standdown to figure out why it was going over budget.

Dantzler said that while Dawn is vital to advancing knowledge of the solar system's beginnings, overfunding it would hurt other missions.

"This is first-class science and we'll do everything we can to give it the go-ahead," Dantzler said in a recent interview.

Dawn was supposed to be launched from Florida in June. Powered by an ion engine fueled by the xenon gas, it was to make a nine-year journey to Ceres and Vesta, located in the main asteroid belt between Mars and Jupiter.

Asteroids are believed to be remnants from the solar system's formation about 4.5 billion years ago, and studying them could provide clues into how the sun and planets emerged.

Previous missions to asteroids include a Japanese probe that is believed to have landed on one last year to collect samples. Dawn would be the first spacecraft to spend months orbiting two large asteroids to study them in depth.

The two asteroids are believed to have formed in different parts of the solar system and to have undergone different evolutionary processes.

Ceres, the solar system's largest asteroid at about 600 miles long, appears to have a warm surface and evidence of a weak atmosphere. Vesta is about 320 miles long and appears to have been resurfaced by basaltic lava.