Astronaut Piers Sellers, his feet anchored to the end of a 50-foot-long boom attached to the shuttle Discovery's robot arm, bobbed about and simulated repair work today in a dramatic test of the space crane's stability for possible use in future shuttle repairs.

With fellow spacewalker Mike Fossum looking on from the shuttle's cargo bay, Sellers went through a scripted sequence of movements - lay backs, rotations, simulated tool retrievals and foot platform relocations - and the long, spindly looking arm/inspection boom combination appeared relatively stable.

It swayed in gentle, slow-motion fashion as Sellers moved about, causing the brakes on the arm's wrist joint to slip at one point. But to the untrained eye, it appeared stable enough to use as a work platform. Whether NASA managers will deem it safe for actual repair work, when an astronaut would be much closer to the shuttle's fragile heat shield, remains to be seen.

"OK, Piers, first test point," shuttle pilot Mark Kelly called from Discovery's flight deck. "You're going to retrieve the digital camera, pick a target, take two photos and put the camera back."

"I've got the camera in my hand," Sellers replied a moment later, floating 14 feet or so above the open payload bay. "Taking a photo ... one, two. Putting the lens cover back on."

"No detectable motion against the moving background," someone observed.

"Camera stowed. Complete," Sellers reported. "Movement stopped."

"I saw almost no motion," Fossum said. "Just a tiny bit."

"Yeah, we didn't see any here," Kelly agreed.

Next, Sellers did a slow "lay back," bending at the knees and reclining for a few seconds before pulling upright again.

"There's a pretty view," Sellers observed as the shuttle-space station sailed high above the South Pacific Ocean. "And coming forward again. Stopped. Got a very slow gentle sway in and out of the bay. Still continuing. Period about six or seven seconds, it looks like. Motion stopped."

"Roger. Next test point is a medium to normal layback," Kelly said.

"The sway this time is a kind of circular, fore and aft and in and out of the bay," Sellers reported. "Period is about six seconds again. Motion stopped. Wow. Damped out quick."

As the testing continued, Sellers got smoother in his motions.

"Just a general comment. It gets easier as you go along, doing all these tasks, you know, at the end of a skinny little pole. A little practice makes perfect. Or adequate."

After working through the entire sequence, Lisa Nowak, operating the robot arm, brought Sellers back down into the cargo bay. Fossum was supposed to join Sellers on the boom, but his 85-foot-long safety tether refused to retract properly. The spacewalkers attempted to manually wind the reel, but it wouldn't work.

Flight rules forbid anyone from riding on the boom without a safety tether all the way back to the shuttle's robot arm just in case the boom somehow separated from the arm. Flight controllers eventually told the spacewalkers to retrieve a spare tether before proceeding.

Then, almost as an afterthought, mission control asked the astronauts to make sure the reel's winding mechanism was unlocked.

"Discovery, Houston, for EVA, with one more question about the damaged safety tether. We want to just make sure it's in the unlocked position."

"OK, we'll check," Kelly said.

"It's in the locked position," Sellers said.

"No," Fossum said.

"Look!"

"Oh, no!"

"Houston, Discovery, thanks for that recommendation," Kelly radioed.

"Oh, that's pretty embarrassing," Fossum said.

"OK, we'll be good to go in just a moment," Kelly said.

Television views from space have been spectacular this morning, with Sellers silhouetted against the limb of the Earth, with blue seas and white clouds on one side and the stark black of deep space on the other. Both astronauts clearly enjoyed the view.

"Hey Piers, take a second and look at the Earth here," Kelly called from Discovery's flight deck before the crane tests began. "I think you've got Ireland and England coming up there."

"My left? Oh!" exclaimed Sellers, who was born in Crowborough, Sussex.

"Wow," Fossum marveled.

"Oh my goodness. It's a beautiful day in Ireland!"

"Indeed it is."

"So to everybody in Ireland, hello!" Sellers said. "You look beautiful today."

A few moments later, Fossum laughed aloud as he floated above central Asia at five miles per second, saying "This is a cool view."

"That's the Caspian sea down there, Mike," Kelly replied.

"OK, I was going to ask," Fossum said, a few minutes later adding, "I'm in a dream. Nobody wake me up."

Additional coverage for subscribers:
VIDEO: ROBOT ARM MOUNTS CARGO MODULE TO THE STATION PLAY
VIDEO: LEONARDO CARGO MODULE LIFTED OUT OF PAYLOAD BAY PLAY
VIDEO: FLIGHT DIRECTOR EXPLAINS MODULE INSTALLATION PLAY
VIDEO: OVERVIEW OF CARGO CARRIED IN LEONARDO PLAY
VIDEO: LIVE RADIO INTERVIEWS WITH CREW CBS | FOX | ABC | NPR

VIDEO: DAY 3 MANAGEMENT TEAM BRIEFING DIAL-UP | BROADBAND
VIDEO: POST-DOCKING MISSION STATUS DIAL-UP | BROADBAND
VIDEO: SHUTTLE CREW FLOATS INTO THE STATION PLAY
VIDEO: DISCOVERY DOCKS TO THE SPACE STATION PLAY
VIDEO: WATCH THE ENTIRE PIROUETTE BACKFLIP MANEUVER PLAY
VIDEO: FLIGHT DIRECTOR EXPLAINS RENDEZVOUS AND DOCKING PLAY

VIDEO: INITIAL TANK ASSESSMENT DIAL-UP | BROADBAND
VIDEO: DAY 2 MISSION STATUS BRIEFING DIAL-UP | BROADBAND
VIDEO: FLIGHT DIRECTOR EXPLAINS INSPECTIONS WITH BOOM PLAY
VIDEO: FLIGHT DIRECTOR EXPLAINS ROBOT ARM INSPECTIONS PLAY

VIDEO: THE FULL LAUNCH EXPERIENCE PLAY
VIDEO: RIDE ALONG DURING LAUNCH VIA CREW MODULE CAMERA PLAY
VIDEO: AMAZING FOOTAGE FROM WB-57 HIGH-ALTITUDE AIRCRAFT PLAY

VIDEO: SHUTTLE LANDING FACILITY TOWER PLAY
VIDEO: PLAYALINDA BEACH TRACKER PLAY
VIDEO: BEACH MOUND TRACKER PLAY
VIDEO: PATRICK AFB TRACKER PLAY
VIDEO: UCS 23 TRACKER PLAY
VIDEO: UCS 11 TRACKER PLAY
VIDEO: CS 6 TRACKER PLAY
VIDEO: CS 2 TRACKER PLAY
VIDEO: CS 1 TRACKER PLAY
VIDEO: VEHICLE ASSEMBLY BUILDING ROOF PLAY
VIDEO: LAUNCH PAD FRONT CAMERA PLAY
VIDEO: COMPLEX 39 PRESS SITE PLAY
VIDEO: LAUNCH PAD SIDE VIEW PLAY
MORE: STS-121 VIDEO COVERAGE
SUBSCRIBE NOW