HOUSTON - Two shuttle astronauts went through the motions of an orbiter heat shield repair while perched at the end of an extra-long robotic arm during a Saturday spacewalk outside the International Space Station (ISS).

NASA astronauts Piers Sellers and Michael Fossum spent most of their mission's first spacewalk at the tip of their Discovery shuttle's 100-foot (30-meter) robotic appendage to determine its stability for the delicate work that would be required to fix a damaged orbiter heat shield.

"My first impression is that it damped out a lot faster than I thought it would," Sellers said of the extended arm's movements after the tests were completed.

Sellers and Fossum each spent time bouncing on the end of Discovery's 50-foot (15-meter) inspection boom, which itself was perched at the tip of the orbiter's 50-foot (15-meter) robotic arm. They leaned back and forth, performed typical spacewalk activities - such as grabbing a camera or reaching for tools - then moved close the main truss of the ISS for some mock shuttle tile and wing panel repairs.

"It's like being at the end of a fishing rod, isn't it," Sellers told Fossum during the tests.

"Ain't that the truth," Fossum replied.

Fossum performed much of the mock shuttle repair work and used a special strain gauge to measure the force he applied to the station's surface. Some movements went surprising easy, while others required extra effort and concentration to perform.

"Okay, moving in...you've got to let your body go out...this would take some practice," Fossum said while performing a mock shuttle wing leading edge repair, adding that he had to lift his toes inside his spacesuit just to compensate for the orbital boom's motion.

Discovery's STS-121 mission is NASA's second shuttle test flight since the 2003 Columbia accident. Today's seven-hour spacewalk, which began at 9:17 a.m. EDT (1317 GMT) as the space station flew over Asia, is the first of two extravehicular activities (EVAs) directly aimed at testing orbiter heat shield repair methods. Additional repair techniques will be tested during the flight's third spacewalk currently set for Wednesday.

NASA developed Discovery's orbital inspection boom in direct response to the Columbia tragedy, in which heat shield damage led to the loss of one orbiter and seven astronauts. The boom has been used on two missions - Discovery's STS-114 flight in 2005 and the current STS-121 spaceflight - to scan the shuttle's heat shield for damage.

Engineers on Earth will pore over the results of today's tests to determine just how effective Discovery's orbital inspection boom can be as a work station.

Tony Ceccacci, lead shuttle flight director for Discovery's STS-121 spaceflight, said Friday that today's tests would help engineers decide whether repairs can be staged from the boom as is, or whether a specialized workstation would have to be developed to attach to its tip.

Railcar Repair

Sellers and Fossum appear to have breezed through their first task in today's spacewalk: putting the mobility back into the space station's railcar-like Mobile Transporter.

Less than one hour into their spacewalk, the astronauts installed a blade blocker into a cable cutter system on the top - or zenith - side of the Mobile Transporter. The fix will prevent a guillotine-like blade from inadvertently slicing a power, video and data cable as the Mobile Transporter moves along the station's main truss.

A similar cutter system on the bottom - or Earth-facing, nadir - side of the Mobile Transporter inexplicably fired on Dec. 16, 2005, and severed a backup cable.

An attempt by the space station's previous crew to safeguard the remaining cable from the same glitch failed when Expedition 12 commander Bill McArthur was unable to drive a safing bolt into the mechanism during a February 2006 spacewalk. He and then-station flight engineer Valery Tokarev removed the cable from the cutting system entirely to be safe, but the fix effectively immobilized the Mobile Transporter.

The Mobile Transporter is a vital piece of ISS hardware because it serves as moving base for the station's robotic arm and a carrier for large pieces of hardware, such as a new solar array tower slated to be delivered to the orbital laboratory in late August.

With Sellers and Fossum's Saturday repair, the Mobile Transporter can now be moved into position for the next STS-121 spacewalk - set for Monday - when the two astronauts will replace the railcar's baby grand-piano-sized Trailing Umbilical System (TUS). It was the TUS system's cable that was severed in the Dec. 16 glitch. Replacing it will add full redundancy to the Mobile Transporter and pave the way for later space station construction.

"Hey there's the TUS," Fossum said as the initial Mobile Transporter repair was completed. "We'll see you again another day."

Today's spacewalk was the fourth EVA for Sellers and the first for Fossum, who is also making his first spaceflight with Discovery's STS-121 mission.

"There's a large planet below me, so that's good," said Sellers, who later saw his homeland, England, and Ireland pass beneath his spacesuit-clad boots. "Oh my goodness, it's a beautiful day in Ireland."

By the numbers, today's spacewalk marked the 66^th EVA in support of the space station and the 19^th staged from its Quest airlock.

"God, this is a mind-blowing trip," Fossum said of the spacewalk.

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