Spacewalkers Jeff Wisoff and Mike Lopez-Alegria connect tethers to each other before starting the SAFER tests. Photo: NASA TV/Spaceflight Now

Two high-flying astronauts zoomed about over the shuttle Discovery's cargo bay today, successfully testing emergency jetpacks designed to serve as a lifeline for any space station assembly workers who might someday slip free of his or her safety tether.

"It felt very stable, it was very easy to control and make corrections, just an incredible feeling of being a human satellite moving around the Earth at 25,000 feet per second," said spacewalker Peter "Jeff" Wisoff. "Just an incredible experience."

"That just looks so neat," said Shannon Lucid in mission control, watching the test flight later on videotape.

"It's the closest I've ever felt (to being) like Buzz Lightyear," Wisoff replied.

The dramatic but low-priority test flights were carried out only after Wisoff and Michael Lopez-Alegria completed final work to ready the space station's new Z1 truss for the attachment of a huge set of solar arrays in early December.

One final test, in which the astronauts planned to take turns pretending to be injured to assess the best way to haul an incapacitated spacewalker to safety, was called off when today's excursion ran longer than expected.

But that was considered a minor loss and with the completion of the crew's fourth and final spacewalk, virtually all of the 100th shuttle mission's major objectives have been met.

Wisoff starts his portion of the SAFER demonstration while Lopez-Alegria stands on the shuttle's robot arm. Photo: NASA TV/Spaceflight Now

"We went four for four on this mission as far as EVA objectives were concerned," said spacewalk trainer Daryl Schuck. "A very exciting and challenging day.

"We've been working this for a very long time," he said. "It's very satisfying to be sitting here today to tell you we got everything done we set out to do."

Early Thursday, the astronauts will enter the space station to complete a final bit of outfitting before undocking Friday. In a final milestone, the gyroscopes in the Z1 truss will be spun up briefly to make sure they will work properly when activated early next year.

Today's spacewalk began at 11 a.m.. Wisoff and Lopez-Alegria first tested a claw-like latch atop the Z1 truss to make sure it will be ready to lock down the P6 solar array tower when it arrives in December.

They also moved an unneeded grapple fixture out of the way and tested latches in a manual berthing adapter on the side of the truss that will be used early next year to temporarily hold a shuttle docking port.

The docking port, known as pressurized mating adapter No. 2, or PMA-2, currently is attached to the Unity module's forward hatch. It is where Discovery is docked to the station.

But in late January, PMA-2 will be removed and mounted on the Z1 truss while the U.S. laboratory module, Destiny, is attached to Unity's forward hatch. PMA-2 then will be attached to Destiny's far hatch for subsequent shuttle dockings.

Destiny will get its electrical power from the P6 solar array. Radiators on the array will provide cooling for lab electronics via ammonia coolant lines. During today's spacewalk, Lopez-Alegria and Wisoff opened a tray-like panel on the Z1 truss where those coolant lines eventually will be attached.

Wisoff pilots the SAFER jet backpack with the Russian Zarya module of the international space station as backdrop. Photo: NASA TV/Spaceflight Now

But the jetpack test flights were the clear highlight of the day and even though the shuttle's main TV antenna was on the blink, the crew's steady commentary and quips kept ground controllers - and reporters - up to speed and entertained.

The jetpacks are known by the acronym SAFER, which stands for "Simplified Aid for EVA Rescue." They use pressurized nitrogen gas from 24 small jets to provide thrust in any direction.

The SAFER units clip on to the base of a spacewalker's normal life support backpack. A joystick controls jet firings and a liquid crystal display gives the user data about battery power and the amount of gas left.

In addition - and perhaps most important - a push button can be used to activate a small internal computer that will attempt to fire whichever jets are necessary to stabilize a tumbling astronaut. Normal straight-line velocity is about one mile per hour.

Wisoff and Lopez-Alegria remained safely tethered to the shuttle at all times during today's test flights.

For the first demonstration, Lopez-Alegria was anchored to the end of Discovery's robot arm, holding Wisoff's tether and playing it out to keep the line slack.

After testing his SAFER's basic operation, Wisoff flew from the space station to the shuttle's aft payload bay bulkhead. Japanese robot arm operator Koichi Wakata kept pace, moving Lopez-Alegria along beside Wisoff to keep his tether loose.

The two spacewalkers near the rear of Discovery. Photo: NASA TV/Spaceflight Now

The astronauts then traded places and Lopez-Alegria took his turn.

Both said the jetpacks performed well, handling very much like the computer simulators at NASA's Virtual Reality Laboratory at the Johnson Space Center in Houston where the crew trained before launch.

"How's it compare to VR?" asked Lopez-Alegria after Wisoff had practiced starting and stopping various motions.

"The hovering is harder," Wisoff said. "But i think in terms of the response, it felt very similar."

"I don't think a helicopter pilot would have had a hovering problem," joked Army aviator William McArthur from Discovery's flight deck.

"I thought real men didn't hover," quipped Navy commander Lopez-Alegria.

"Oh contraire." said McArthur.

Before the first flight test started, astronaut Leroy Chiao asked Wisoff "what's it like being a satellite?"

The safety tether can be seen clearly between the two spacewalkers. Photo: NASA TV/Spaceflight Now

"Pretty awesome," Wisoff said. "I certainly feel like Mike is my best friend in the world right at the moment."

He again commented on the similarity to the computer simulator, saying "kudos to those guys for the great (computer) model they did."

After reaching Discovery's cargo bay and completing his flight, a clearly elated Wisoff said "I can't believe what we just did."

"That was fantastic, guys, you made it look easy," said fellow spacewalker Chiao. "You've got two jealous EVA guys in here.

"Thanks for your support," Wisoff said.

After both flight tests were over, Wisoff and Lopez-Alegria paused to marvel at the view as Discovery passed high above Cuba and southern Florida. Live television transmitted directly to the Kennedy Space Center allowed flight controllers to share the moment.

"Oh man, this is so friggin' unbelievable!" Lopez-Alegria exclaimed. "I can see very clearly the runways ... down there in Key West. And Homestead Air Force Base, or whatever used to be Homestead Air Force Base, a big long runway that runs parallel to the coast."

"Yeah, I see it," Wisoff said.

Lopez-Alegria heads toward this camera during his turn in the test flight. Photo: NASA TV/Spaceflight Now

"Do we have to go back to work now?"

A few moments later, the shuttle passed east of North Carolina, prompting comments from native William McArthur on Discovery's flight deck.

"It's pretty cool, we just flew by the Outer Banks of North Carolina," he reported. "And of course, 97 years ago, that's where the Wright brothers first started all this stuff. It's kind of cool, it's almost the 100th anniversary of the Wright brothers' flight and this is the 100th shuttle flight."

"I'm trying to figure out how a West Point graduate became so eloquent," Lopez-Alegria commented.

"Clearly, the education," McArthur replied.

Today's spacewalk officially ended at 5:56 p.m. when the astronauts began repressurizing the shuttle's airlock. This was the 93rd spacewalk in the history of the U.S. space program and the 54th in shuttle history.

During five space station assembly flights, 10 spacewalks have now been conducted with a total time of 69 hours and 34 minutes. Discovery's crew now holds the station assembly record, logging 27 hours and 19 minutes of EVA time during four excursions.