An astronaut tests the SAFER system for the first time during a spacewalk in 1994. Photo: NASA

Wisoff and Lopez-Alegria will spend the second half of the final spacewalk testing their SAFER jetpacks.

"The SAFER is a cold gas, sort of Buck Rogers kind of backpack that we use to, in the event that we should become untethered from the shuttle, we can actually use it to rescue ourselves and fly ourselves back," said Lopez-Alegria.

"So actually we're going to do some flying, kind of like a test program, of this SAFER," he said. "Each of us will perform one of those flights. At the very end of the EVA, we're going to take the other, who will sort of pretend that they're incapacitated, and translate with them through the payload bay and actually put them in the airlock."

The day after the final spacewalk, the astronauts will re-enter the space station one more time to wrap up the delivery of supplies and other equipment needed by the station's first full-time crew. They also will do a bit more work to complete the electrical hook up of the Z1 truss.

If all goes well, the astronauts will re-seal the station Oct. 13, undock the following day and land back at the Kennedy Space Center the afternoon of Oct. 16.

Exactly two weeks later, Shepherd and his crewmates are scheduled to blast off from Kazakhstan to open a new era of around-the-clock station activity. The P6 solar array will be launched a month later and the U.S. lab module, Destiny, is scheduled to arrive in late January.

"It'll become a home for our astronauts and cosmonauts and international crews as well as a world-class research facility," said station program manager Tommy Holloway.

"This is an exciting time in the history of human space flight," he said. "There are going to be a lot of challenges in front of us, a lot of difficult problems to solve and it's a good time to be in this business. I am excited about where we are and what we have in front of us."