Discovery sits on Launch Pad 39B at Kennedy Space Center at Cape Canaveral, Florida

The U.S. space agency NASA says a launch of the shuttle Discovery will not occur before Sunday, and most likely will take place some time after that. The agency has begun an intensive effort to figure out why a crucial fuel level sensor is not working correctly, delaying the first shuttle mission since the loss of the orbiter Columbia over two years a go.

Deputy shuttle manager Wayne Hale says NASA is beginning a nationwide effort among its various centers and contractors to troubleshoot the fuel sensor problem that canceled Wednesday's planned shuttle launch to the International Space Station.

Mr. Hale says that if the problem is relatively simple and technicians are extremely lucky, a Sunday launch is possible, but far from guaranteed.

"This represents a very optimistic, good luck scenario, which I think is not very credible. What are we are more likely into is several days of troubleshooting. That's the best I've got right now," he said.

The problem sensor is one of four in the shuttle's fuel system that monitor levels of liquid hydrogen. Four others show levels of liquid oxygen fuel. If levels are low, properly working sensors signal a computer to shut off the main engines before internal damage can occur.  NASA rules require all eight sensors to work for launch to take place. The faulty unit incorrectly indicated low hydrogen levels immediately after the tank had been filled.

Mr. Hale could not say if the sensor difficulty can be fixed at the launch pad or would require a slow rollback of Discovery into its hangar.

If Discovery is rolled back, Mr. Hale says it is still theoretically possible to have a July launch.  If not, September would be the next opportunity to return to flight after the long moratorium for safety upgrades. This is based on the space station's position and the agency's requirement to launch during daytime so cameras can record any launch damage that might occur of the kind that doomed Columbia in 2003.

After all NASA has invested in making shuttles safer to fly, Mr. Hale is philosophical about the fuel gauge difficulty.

"Going into space is right at the limits of human technology right at the start of the 21st century," he explained.  "We're doing something that is extremely difficult. If you think this is routine, you surely don't understand what it is we're trying to do here."

When Discovery does fly, it will carry vital supplies and equipment to the space station. Its astronauts will also test new methods to repair shuttle damage in orbit, a capability NASA did not have when launch debris punctured Columbia's surface after liftoff.