Washington ? Choosing safety over schedule, NASA has grounded the space shuttle fleet while engineers try to determine why tiny cracks are developing in the fuel line feeding the main rocket engines.

The announcement put a crimp in NASA’s efforts to satisfy a tight schedule for building and supplying the international space station. Solving the problem could take weeks or more, and people who have criticized the space agency in the past praised what they saw as a new emphasis on success and safety over speed.

“These days, the value of safety is higher in the NASA culture than it has ever been,” Keith Cowing, editor of NASA Watch, a watchdog Web site and frequent space agency critic, said Tuesday.

NASA engineers said they aren’t even sure that the space shuttle problem is a threat to safety. They found minuscule cracks in the metal liner of fuel lines that carry supercold hydrogen to the main rocket engines.

What caused the cracks and the extent of threat they represent are unanswered questions, said NASA spokesman James Hartsfield, but that, alone, is enough now to ground the fleet.

“When there is something that we don’t understand, it is a safety concern for that reason,” Hartsfield said. “We need to get answers.”

James Oberg, a veteran space engineer, author and well-known NASA watchdog, applauded the decision to ground the shuttles. Ignoring such a cautious, careful approach, he said, contributed to the 1986 explosion of space shuttle Challenger that killed seven astronauts, and to the loss in 1999 of three unmanned spacecraft sent to Mars, he said.