By Paul Rincon
BBC News science reporter, Houston
Discovery astronauts Soichi Noguchi and Steve Robinson are now back inside the space shuttle after completing a six-and-a-half-hour spacewalk.
The two astronauts spent nearly seven hours on the outside
The two shuttle crew members tested repair techniques for heatshield tiles during the first spacewalk of the Discovery's mission.
It was the first spacewalk for either astronaut, and the first for the US since the 2003 Columbia accident.
Nasa is currently considering extending the mission by one day.
Discovery's crew were woken up this morning with a recording of Noguchi's children singing in a choir.
"What a view!" the Japanese Space Agency astronaut said as he stepped outside the airlock at 0946 GMT while the International Space Station (ISS) was over south-east Asia.
Noguchi and Robinson started the Extra-Vehicular Activity (EVA) about an hour late: "Things are always a little bit slow for folks making their way through those procedures.
"We were a little late getting out the hatch but during the EVA, they finished the timeline within the appointed time and picked up a couple of extra activities," said mission operations representative Phil Engelauf.
Using a box containing a sample of heatshield panelling with artificial damage, Steve Robinson tested a gun that dispensed a rubbery sealant called Noax (Non-oxide Adhesive Experimental), which he described as "liquorice-vapour pizza dough".
Robinson forced the sealant into the cracks with a tool like a putty knife.
Thrill of the walk
Noguchi used "a shoe-polish like dauber" to test a so-called emittance wash which appeared to transfer well despite some bubbling.
During the spacewalk, the Japanese Space Agency astronaut made his way over to the Destiny docking module of the International Space Station (ISS) to photograph the port-side cabin of Discovery.
Mission managers have reported that insulation blanket may have come loose in this area just under the commander's window.
"Each spacewalk is filled with complex, highly choreographed procedures," Dave Wolf, an astronaut who has flown on previous shuttle missions, told the BBC News website.
He agreed the EVA would have been a baptism of fire for the two first-time spacewalkers.
"Rarely does an image and a feeling grip one than your first time stepping out of the vehicle on a spacewalk and looking down 250 miles and moving at 18,000mph. But the main task is keeping your mind on the job," Mr Wolf added.
Brains and hard work
Cindy Begley, Nasa's officer for spacewalks, said: "I've had a lot of crews come back and say that the handrail above the [airlock] hatch is dented from people holding on to it so hard."
The EVA lasted approximately six hours and 50 minutes, ending as the shuttle-ISS complex orbited above Australia.
The astronauts restored power to a failed Control Moment Gyroscope on the ISS, which maintains the space station's correct position in space, bypassing a faulty power breaker.
Noguchi also installed a new GPS antenna on the station, which was said to work perfectly.
Experts are now investigating items seen to fall around the orbiter
They also retrieved space station experiments and moved a stowage-platform attachment device.
Meanwhile, mission specialist Charlie Camarda and pilot Jim Kelly have been continuing to inspect the shuttle for damage sustained during the climb to orbit, using the shuttle's 15m-long boom to photograph and scan Discovery's port wing.
The shuttle sustained about 25 impacts during launch on Tuesday, shuttle managers said, compared with an average of roughly 150 for most shuttle launches.
Nevertheless, all shuttle flights are currently grounded while Nasa tries to fix the problem of foam debris peeling off the external tank during lift-off.
Multiple pieces of foam flew off the tank during Tuesday's launch, the largest of which weighed about one pound, slightly smaller than the chunk that doomed space shuttle Columbia in 2003.
The debris came from an area of the tank called the Protuberance Air Load (Pal) ramp, which was not modified following Columbia.
Nasa administrator Dr Mike Griffin said on Friday that he was optimistic Nasa could launch another shuttle this year "by being smart and working hard".
An extra day is expected to be added to the mission at the request of International Space Station managers in order to carry out extra tasks on the station.