The U.S. space agency NASA says the earliest it could launch the shuttle Discovery to the International Space Station would be next Tuesday, July 26. But officials concede that is optimistic, assuming that technicians find the technical problem that stopped Discovery's countdown last week.

Shuttle engineers have spent several days working around the clock wiggling wires to test connections and checking other electrical components. But they still do not know why a sensor showed incorrect hydrogen fuel levels during a test about two hours before Discovery's planned launch.

The agency says an improper low fuel reading could cause shuttle engines to shut down prematurely during the climb to orbit.

Deputy shuttle manager Wayne Hale says all the technical detective work so far has occurred at normal outside temperatures without frigid liquid hydrogen fuel in the tank, the actual condition under which the sensor failed.

"Hopefully in the next 24 to 48 hours, we will find the glitch that has got us all confused or frustrated or pick your adjective, and be able to go forward. But I think Tuesday, the 26, is probably the earliest day that we would be looking for a launch, even in that optimistic case," Mr. Hale says.

Mr. Hale says that if the problem is not solved by this coming Wednesday, the next step is to load Discovery's fuel tank again in the supercold state to emulate the temperature condition in which the problem occurred. If the trouble is then found and fixed, the tanking will have been done already to still allow a launch next week.

NASA is eager to send Discovery up by the end of this month. If not, it must wait until September, the next time the space station will be in the right orbital position to be reached with a daytime liftoff. The agency wants the daylight so new cameras around the launch site can scrutinize the shuttle from every angle to determine if launch debris poses a threat. Hard insulation foam fell off the external fuel tank and punctured the wing of the shuttle Columbia in 2003, dooming it to burn up in orbit and halting shuttle launches until the problem was corrected.

Wayne Hale sounds unfazed by the latest technical issue.

"I can tell you this team is persistent and it's energetic and we will conquer this problem. We have been working for two and a half years to return the shuttle to flight," Mr. Hale says. "A few days more when it's all said and done to make sure we're flying safely is not a problem in the bigger scheme of things."

The problem fuel level sensor is one of four such instruments that NASA requires in its fuel tank as backups to each other. In the first five years of shuttle operations, the agency allowed launches if only three worked. It added the requirement for all four to work after the orbiter Challenger blew up during launch in 1986.

Mr. Hale says mission managers are discussing whether to discard that rule if the difficulty cannot be traced.