Two astronauts on the space shuttle Discovery have begun the first of the mission's three planned spacewalks.

Officials at the U.S. space agency say American Steve Robinson and Japan's Soichi Noguchi will spend six hours outside the shuttle inspecting the orbiter for any damage sustained during takeoff.

The astronauts will also test experimental tools and materials developed after the Columbia disaster in early 2003 to see if they can repair damaged heat-shield tiles in space.

Earlier this week, NASA grounded future shuttle flights after learning Discovery's external tank lost four pieces of foam during Tuesday's liftoff.  The flights will not resume until scientists can solve the problem.

Loose foam doomed Columbia when a piece punched a hole in the orbiter's wing during takeoff.  The shuttle overheated and broke apart during re-entry because of a damaged tile, killing all seven astronauts on board.

Some information for this story provided by Reuters and AFP.