(CNN) -- Technicians have started inspecting a third orbiter in the space shuttle fleet for frayed wiring like that which caused a short circuit on Columbia five seconds after its July launch.

Inspections were completed a week ago on Columbia and Atlantis and are now 25 percent complete on dozens of miles of thin wiring on the Discovery orbiter. One exposed wire was found on Atlantis like that on Columbia, only on the opposite side of the payload bay.

The discovery of the cause of the Columbia short circuit -- a burred screw head made contact with a wire with worn insulation -- and the mirror discovery on Atlantis prompted a review of the entire four-craft shuttle fleet and has put the upcoming launch dates for at least two shuttle missions on hold.

"The inspection process will include up-close inspection of the wiring in the mid-bodies of each shuttle orbiter," said Joel Wells, spokesman for NASA's Kennedy Space Center.

Inspectors are looking for damaged wires and wires near burred screws or with a lot of slack on the line that could lend them to being pushed against a potentially rough object, Wells said.

The July 23 short circuit knocked out power to two of the three engines on Columbia, but back-up systems took over. A second short circuit would have meant a permanent loss of power to at least one engine, forcing an emergency landing in Florida or Africa and putting astronauts' lives at risk.

There are no scheduled Columbia launches for the next several months, but Discovery had been set for an October 14 launch for a crucial Hubble Space Telescope repair mission. The launch date for that mission now is under review. Atlantis is set for a December 2 launch for an International Space Station assembly mission.

Technicians removed the lengthy, unwieldy Shuttle Radar Topography Mission payload from Endeavour's cargo bay on Wednesday. Endeavour was to launch on September 16 for a radar mapping mission of Earth, but now will lift off no earlier than October 7.

Inspections of Endeavour's wires will begin this weekend.

Shuttle managers are expected to announce new launch dates next week for the rest of the 1999 shuttle missions.

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NASA set to reveal next Mars landing site

(CNN) -- The target landing zone for NASA's Mars Polar Lander -- a site located in mysterious layered terrain near the martian south pole -- will be unveiled in a press briefing on Wednesday, August 25, at 1 p.m. EDT.

If all goes according to plan, the lander will set down gently on the red planet December 3 for the start of a three-month mission to help scientists study the planet's climate history.

The site was selected based in part on data gathered by the Mars Global Surveyor, which since March has been mapping and photographing the planet's surface from an orbit 240 miles (386 km) overhead.

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