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.
Cowing said that earlier in NASA's history, "You didn't want to be the guy who stood up and said, `We shouldn't fly.' There's been a slow-motion change in that culture, and that's good."
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.
"This is the safety attitude from Apollo that some program managers forgot prior to Challenger," Oberg said. Apollo was the NASA program that landed American astronauts on the moon.
Grounding the fleet to find answers to a problem, Oberg said, "is a refreshing reminder that there is a backbone of integrity within mission operations that is critical to success in space."
Hartsfield said Tuesday that NASA engineers are just beginning to search for solutions to the fuel-line cracks, and it could be weeks before the shuttle fleet is cleared to fly again.
That means possible delays for missions scheduled for July, August and October. The October flight is particularly critical since it involves resupplying the international space station and rotating crew members from the orbiting laboratory.
The fuel-line problem was discovered during a routine inspection of space shuttle Atlantis, a single, tiny crack in the lining of one of 12 fuel lines. That prompted inspectors to use magnification and, eventually, a sort of sonic sonar machine to search for more cracks. They found three, all minute.
Next, NASA engineers checked the fuel lines on space shuttle Discovery and again found cracks. That was enough to ground the fleet of four space shuttles.
Hartsfield said the main rocket engines on space shuttle Columbia will be removed and checked, a process that will take at least three weeks. An inspection also is planned for Endeavor, the space shuttle that landed in California last week after a 14-day flight.
Engineers plan to cut out some of the cracks from one of the space shuttles and send the pieces to a metallurgical lab for atomic-level examination. They will look for such anomalies as metal fatigue or manufacturing flaws. That test could take days.
"The leading priority right now is to determine what caused the cracks," Hartsfield said. After that, engineers will decide if the cracked linings can be repaired or the lines must be replaced. That would be a problem, since there are no spare parts.