At issue, reports CBS News Space Consultant Bill Harwood, is the health of metal liners mounted inside the propellant lines that feel super-cold liquid oxygen and liquid hydrogen to a shuttle's three main engines.
Recent inspections of the fuel flow liners aboard Atlantis and Discovery revealed cracks measuring up to three-tenths of an inch long. Should a liner rupture or break apart during ascent, the high-speed flow of propellants would be disrupted at the very least. Whether any fragments could be ingested by an engine, possibly with catastrophic results, is not yet clear.
But the cracks clearly pose a safety risk and shuttle program manager Ronald Dittemore wants to find out whether the flow liners aboard Columbia have experienced similar problems. At the same time, engineers are assessing what must be done to fix the already discovered cracks.
"These cracks may pose a safety concern and we have teams at work investigating all aspects of the situation," said Dittemore in a NASA statement. "This is a very complex issue and it is early in the analysis. Right now there are more questions than answers."
NASA had planned to haul Columbia to the launch pad in the next few days to begin final preparations for launch July 19. But work to ready the ship for rollover from its hangar to the Vehicle Assembly Building for attachment to a pair of boosters and its external tank was put on hold over the weekend.
Atlantis is scheduled for launch Aug. 22 on the next space station assembly flight. Depending on how the flow liner issue plays out, Atlantis could end up launching before Columbia. But at this point, trying to guess how the launch schedule might work out is pretty much an exercise in futility.
NASA's quick decision to ground the space shuttle fleet was applauded by experts who said it shows the agency is putting safety above schedule.
"These days, the value of safety is higher in the NASA culture than it has ever been," frequent space agency critic Keith Cowing said Tuesday.
Cowing, the editor of NASA Watch, a watchdog Web site, 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."
James Oberg, a veteran space engineer, author and NASA watchdog, also 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 the answers "is a refreshing reminder that there is a backbone of integrity within mission operations that is critical to success in space," Oberg said.
CBS News Space Consultant William Harwood has covered America's space program full time for more than 15 years, focusing on space shuttle operations, planetary exploration and astronomy. Based at the Kennedy Space Center in Florida, Harwood provides up-to-the-minute space reports for CBS News and regularly contributes to Spaceflight Now and The Washington Post.