MIAMI, June 25: NASA has grounded the US space shuttle fleet indefinitely after finding small cracks in propellant lines on the main engines of two shuttles, US space agency officials said on Tuesday.

The discovery of the cracks on shuttles Atlantis and Discovery during recent inspections will delay the scheduled July 19 launch of shuttle Columbia, which was to carry the first Israeli astronaut, Ilan Ramon, and six others on a science mission.

The cracks were found in the metal liners used to direct flow inside the main propulsion-system propellant lines on Atlantis and Discovery, two of the four US space planes that routinely ferry astronauts into space and to the International Space Station, an orbiting laboratory funded by 16 countries.

“The concern is that ... if a piece were to crack off and go down into the engine, would that damage the engine and cause it to shut down,” said James Hartsfield, a NASA spokesman at the Johnson Space Center in Houston. “Whether that is something that could actually happen, we don’t know yet.”

NASA said the cracks “may pose a safety concern.” Engineers found the first one during a visual inspection of “flow liners” — thin pieces of metal that aid the flow in propellant lines — as they were installing engines in Atlantis more than a week ago.

Subsequent sonic vibration testing found other cracks measuring from 0.1 inches to 0.3 inches (0.25 to 0.75 cm) in both Atlantis, which is 17 years old, and Discovery, which is 19. Columbia, the oldest of the four shuttles in the fleet at 21, will be inspected.

Because it takes a week to 10 days to remove a shuttle’s engines and the same amount of time to reinstall them, Columbia’s July 19 mission has been delayed indefinitely.

No decision has been made whether to inspect 11-year-old Endeavour, which returned last Wednesday from a two-week mission to the International Space Station.

Hartsfield said NASA engineers will try to determine if the problem is a flaw introduced at the time the flow liners were installed in the shuttles years ago, whether the cracks are a problem of age or whether other factors are involved.

“We’ve never seen these (cracks) before,” he said. “The flow liners have been in the shuttles since day one. We have begun an analysis to try to understand it.”

Beyond the postponed July 19 mission of Columbia, shuttle Atlantis was scheduled for a construction trip to the International Space Station on Aug. 22.

The new space station crew members delivered by Endeavour this month, Russians Valery Korzun and Sergei Treschev and American Peggy Whitson, are expected to return to Earth in October.—Reuters