A mission to two of the largest asteroids in the solar system, which was being readied for a June 2006 launch, has been placed in what NASA is calling “stand-down” mode.

The mission, which is called Dawn, is intended to explore the two most massive asteroids known: Vesta and Ceres.

Dawn is designed to improve scientific understanding of how planets formed during the earliest epoch of the solar system by studying these two “baby planets,” which are very different from each other.

“Yes … NASA has asked us to stand down,” said Dawn’s principal investigator, Christopher Russell of the University of California at Los Angeles (UCLA). “None of us take this as any indication that [NASA Headquarters] does not want to launch Dawn,” he said, given “strong words of support” from space agency personnel in Washington.

Russell said that Dawn is an extremely robust mission. The particular launch window that the spacecraft mission is heading for is extremely long — over a year long, he noted.

NASA spokeswoman Erica Hupp, who works at the agency’s Washington headquarters, said the stand-down is the result of technical concerns in the program. As a result, the agency instructed the Jet Propulsion Laboratory (JPL) in Pasadena, Calif., to stop spending money on the Dawn program while a full assessment of the program is conducted at NASA’s Marshall Spaceflight Center in Huntsville, Ala.

Hupp said no decision on the fate of the Dawn program would be made until after the three-month long review at Marshall is completed.

“This is both a blessing and a curse,” Russell said. “Typically a planetary mission heading to a launch opportunity has a very limited time for any delays. In this case, we can tolerate delay in launch without science impact, and so when someone wishes to review an issue to gain more confidence that it is completely resolved then it may result in increased expenditures but not loss of the mission,” he added.

Russell said there are a number of technical issues that on a chemical launch would be examined in parallel to development, “but in this case we were asked to stand down while an independent assessment team reports back to headquarters. This has interrupted the final preparations for launch and we wish that they had not done this, but it is something we can tolerate.”

Dawn is a NASA Discovery-class mission, selected in December 2001. The goal of the Discovery program is to launch many smaller missions with fast development times, each for a fraction of the cost of NASA’s larger missions. The program is managed by JPL with Orbital Sciences Corp. of Dulles, Va., which is developing the spacecraft.

Brian Berger contributed to this article from Washington.