Two astronauts will climb on the Discovery shuttle's robotic arm on Saturday in a daring spacewalk to test the instrument's ability to serve as a platform for possible spacecraft repairs.
British-born US astronaut Piers Sellers and his colleague Mike Fossum will simulate repairs on the shuttle while hanging at the end of the robotic arm and a boom extension that together measure 30 metres (100 feet) long.
The pair will step out of the International Space Station (ISS), to which Discovery docked on Thursday, to begin their six-and-a-half hour excursion.
The test is part of an intense effort by NASA to dramatically improve safety for its astronauts since seven of them died in the Columbia accident in February 2003.
New repair techniques were tested in the first post-tragedy flight in 2005.
NASA is aiming at resuming regular shuttle missions to finish building the ISS before the fleet retires in 2010, but it must first show that it has made space flights safer.
The ability to make in-orbit repairs is among several measures taken by the US space agency to ensure that the shuttle can be safely flown back to Earth.
{{/usCountry}}The ability to make in-orbit repairs is among several measures taken by the US space agency to ensure that the shuttle can be safely flown back to Earth.
{{/usCountry}}"As part of the return to flight effort, we want to make sure we have the capability to repair any area on the orbiter thermal protection system if required," shuttle flight director Tony Ceccacci told reporters at the Johnson Space Centre in Houston, Texas.
The first of three planned excursions could actually serve as a practice run for repairs in the current shuttle mission, as analysts reviewed images of Discovery's heat shield for potential damage.
Analysts reviewing hundreds of images found two gap fillers sticking out from between thermal tiles on Discovery's underside. In 2005, an astronaut for the first time performed a spacewalk to remove such protuberances.
Deputy shuttle program John Shannon indicated that a spacewalk could take place if necessary to take out gap filler deemed potentially dangerous.
"We don't rule out anything until the analysts come in and say we don't have a problem," Shannon told reporters. "Right now the options are on the table."
NASA added an extra day to the mission on Friday in order to have time to conduct a third spacewalk.