The first SAFER flight test in 1994. Photo: NASA

The Discovery astronauts are set for a fourth and final spacewalk today to complete initial outfitting of the international space station and to practice emergency rescue techniques that could mean the difference between life and death for future assembly crews.

Astronaut Peter "Jeff" Wisoff and Michael Lopez-Alegria plan to carry out short test flights using emergency jetpacks intended to help a stranded spacewalker make it back to the shuttle should he or she ever become detached from safety tethers.

"In the old days if somebody were to break free of his tether, which we always have on the shuttle, the shuttle would just go get him," Lopez-Alegria explained in a NASA interview.

"But now we're docked to the station, we can't very well get the whole station over to go get him. So we have to have some means of saving ourselves if that should happen.

"Keep in mind that the odds of that happening are very low," he said. "But if it does, you'd like to be prepared."

Wisoff and Lopez-Alegria also will take turns pretending to be incapacitated to practice techniques for manually hauling an injured spacewalker back to the safety of Discovery's airlock.

For that test, the astronauts will not use the jetpacks. Instead, they will simply tow each other along, pulling hand over hand along handrails and safety tethers to the airlock.

The jetpacks were designed to allow an astronaut to fly back to safety on his or her own in an emergency.

"It's like wearing a parachute when you go flying," said lead flight director Chuck Shaw. "You have it, you know how to use it, but you just hope you never have to."

An illustration of the SAFER backpack. Photo: NASA

The jetpack is called SAFER, which stands for Simplified Aid For EVA Rescue. It is a small fanny pack that attaches to the base of the normal backpack worn by spacewalking astronauts.

Developed by NASA engineers at the Johnson Space Center in Houston, SAFER uses pressurized nitrogen from 24 small jets to provide thrust in three directions: Forward and backward, up and down and side to side.

Using combinations of jets, an astronaut also can control rolls about the body's vertical axis; head-over-heels pitching motions; and cartwheel-like tumbles.

The boxy jet pack clips on to the base of a spacewalker's normal backpack. A joystick is provided to control jet firings and a liquid crystal display gives the user information about battery power and the amount of nitrogen left.

In addition, a push button can be used to activate a small internal computer that will attempt to fire whatever jets are necessary to stabilize a tumbling astronaut.

No more than four jets can fire at the same time to produce straight-line motion. Maximum velocity, which could only be achieved by firing four jets wide open until the unit's nitrogen was exhausted, is about 6 mph. The normal operating velocity, however, will be less than 1 mph.

If an astronaut ever became untethered, he or she could end up slowly tumbling or cartwheeling as they floated away.

"So the scenario is you come off the shuttle with some kind of a rate (velocity) - maybe a rotation rate as well as a translation weight rate - separating you," Lopez-Alegria continued.

In 1994 the first SAFER test included an untethered flight above the payload bay. Photo: NASA

"The first thing you want to do is energize this thing. Turn it on. It will stop any rotations you have automatically.

"Your next task is to find the shuttle, because you may not be looking at it," he said. "So you have to do some maneuvers to point yourself at it. And then, finally, when you do, you want to give some translation pulses to get yourself going back in the right direction."

Wisoff and Lopez-Alegria practiced self-rescue procedures in NASA's Virtual Reality Laboratory in Houston. And in the computer simulations, at least, it sometimes proved difficult to find one's way back to the shuttle.

"You may think you're looking at the shuttle and pointing in the right direction and pulsing to get to the right direction, but, in fact, you may not be," Lopez-Alegria said.

"So, what we want to do is actually try this in space and see how well we can correlate where we think we're pointing to the direction that we're actually going. So, that's the whole purpose of this test."

Both spacewalkers will remain attached to safety tethers at all time.

With one astronaut anchored to the end of the shuttle's robot arm and playing out his partner's safety tether like a fishing line, the other will free fly 50 to 60 feet from the space station down to a camera mounted on the aft bulkhead of Discovery's cargo bay.

Lopez-Alegria will trade places for a second test flight.

The 1994 test was performed by astronauts Mark Lee and Carl Meade on shuttle Discovery's STS-64 mission. Photo: NASA

"Jeff will be the first to try this," said Lopez-Alegria. "First of all, we'll do some tether protocol so Jeff will always remain tethered. The SAFER guy will always remained tethered during the whole operation.

"But the eventual goal is, we're going to start him up on the stack, as we call it, to somewhere around the (Russian Zarya module). And he will aim toward the aft bulkhead, one of the cameras. And, basically, I will be tending him on the arm.

"At one point, I'll be in the arm, basically, in a foot restraint, holding him. And I'll back away. I'll let go of him. And he will orient himself using the hand controller and then try to actually fly as if he were doing a self-rescue."

In reality, the trajectory of an untethered astronaut would play a role in how difficult a self rescue might prove to be.

If the astronaut happened to move away to either side of the station, that is, perpendicular to the plane of its orbit, he or she eventually would return to their starting point due to orbital mechanics.

But if an astronaut moved away below the station toward Earth or above it toward deep space, the level of difficulty could increase dramatically.

Orbital velocity depends only on the radius of an object's orbit. The closer an object is to the body it is orbiting, the faster it moves. The farther away, the slower.

An astronaut moving away toward Earth below the station would move faster as he or she lost altitude, pulling slowly but surely ahead of the station. The situation would be reversed for an astronaut moving away from the shuttle toward deep space.

In either case, aiming themselves correctly to fly back to the station could prove challenging.

SAFER test near the front of the payload bay in 1994 with the Lidar In-space Technology Experiment as backdrop. Photo: NASA

Today's SAFER tests will begin about four stories above Discovery's cargo bay. Each astronaut will fly back down to the rear of the shuttle's cargo bay to test the device's handling characteristics.

They will then take turns hauling each other to Discovery's airlock as if one was injured or otherwise incapacitated. Again, both spacewalkers will be tethered at all times. But for these tests, the SAFERs will not be used.

The SAFER free flights and the incapacitated astronaut scenario are the final items on the agenda - and have the lowest priority - of the objectives for today's spacewalk.

Wisoff and Lopez-Alegria first will reposition a grapple fixture on the newly installed Z1 truss, clearing the way for attachment of a huge set of solar arrays in December.

They will then deploy a utility tray on the truss so a future assembly crew can hook up ammonia coolant lines. The coolant lines will connect to radiators on the P6 solar array scheduled for installation in December to dissipate heat generated by the station's electronics.

The utility tray is a "large, I would say probably 4-foot square metal tray that has cables and also fluid lines connected to it," said Lopez-Alegria. "And it basically is hinged on one side.

"It will open through about 120 degrees or so kind of like a door to allow those ammonia lines to be connected to a future module which is not there yet.

"Jeff will pull some pins ... and then I'll be releasing 32 bolts that hold those four fluid cables, or fluid lines, in place."

A grapple fixture on top of the truss also will be moved to clear the way for attachment of the P6 solar array.