NASA has practically grounded its space shuttle fleet because inspections have uncovered a startling number of damaged wires that could endanger a mission.

Technicians discovered that an exposed wire caused a short circuit that knocked out two engine computers during the most recent shuttle launch, that of Columbia in July. Similar flaws were found elsewhere on Columbia and in the wiring on Endeavour and Discovery.

Now all flights are off until at least mid-October.

Both Endeavour and Discovery have dozens of wires that must be repaired or reinforced. Endeavour should have gone up in mid-September with radar equipment to map Earth's surface, Discovery in mid-October with crucial replacement parts for the Hubble Space Telescope.

The trouble began July 23, when a short circuit five seconds into Columbia's launch knocked out computers for two of its three main engines. Because each engine has two computers and only one is needed, Columbia made it to orbit with five astronauts and the powerful Chandra X-ray Observatory.

It was believed to be the first time in 95 shuttle missions that a short circuit occurred during liftoff, the most dangerous part of the flight. If an engine had shut down, the shuttle would have had to attempt an unprecedented emergency landing in Florida or in Africa.

But shuttle circuit breakers are much more sensitive than those in a home or even on a plane; they immediately pop when a short occurs, halting the flow of electricity.

The half-second short was caused by an exposed wire that had lost its insulation in one spot, allowing the wire to come in contact with a rough screw. Workers evidently had stepped on the wire or brushed a tool or work platform against it.

The damage was done at least four years ago, the last time Columbia underwent an extensive overhaul. It's possible the damage was there since Columbia's first flight in 1981; the wiring is original.