CAPE CANAVERAL, Fla. - Two shuttle Endeavour astronauts floated outside the International Space Station on Friday for the first of four spacewalks to ready the outpost for operations after the US shuttle fleet is retired this summer.Wearing bulky pressurized spacesuits, Andrew Feustel and Gregory Chamitoff ventured through the station's airlock for a spacewalk that lasted about six hours.Ground controllers cut the spacewalk short after a carbon dioxide sensor failed in Chamitoff's space suit and asked him to keep a close lookout for signs of oxygen deprivation."With all your station experience you know your symptoms well," Endeavour crewman Mike Fincke told Chamitoff, who was out for his first spacewalk.There was no sign that Chamitoff's carbon dioxide levels would rise, NASA said.NASA has ordered an extra inspection of Endeavour after inflight inspections showed damage to its heat-resistant belly tiles due to debris impacts during launch.On Saturday the crew will use the shuttle's laser and camera scanners to examine a damaged area about 3 inches (7.6 cm) long near the shuttle's right-side landing gear door."There's nothing alarming here and we're really not concerned," Leroy Cain, the mission management chairman, told reporters. Cain said NASA engineers would likely clear the shuttle for its landing in Florida, now scheduled for June 1, after the inspection.NASA added heat shield inspections and repair kits after losing shuttle Columbia in 2003 due to heat shield damage. Columbia's seven-member crew perished in the disaster.In a worst-case scenario, spacewalking astronauts would patch or repair the damaged tile before Endeavour was cleared for landing. The tiles are part of the shield that protects the shuttle from the searing heat of atmospheric re-entry.On Friday's spacewalk, the astronauts retrieved a materials science experiment, installed antennas for an external wireless communication system and made preparations for later spacewalks. They aborted a plan to make connections for the wireless system due to Chamitoff's suit troubles."It's been really great watching you guys," Fincke told Chamitoff and Feustel as they returned to the station. "I wish I was there."Endeavour, which is scheduled to spend 12 days at the station, arrived there on Wednesday on NASA's next-to-last shuttle mission to deliver the $2 billion Alpha Magnetic Spectrometer and a pallet of spare parts.Both were installed with robotic cranes to the outside of the station's metal truss.