CAPE CANAVERAL, Fla. Its a spacewalkers worst nightmare -- one similar to that which two astronauts could face Sunday night when they take their first forays beyond the airlock of shuttle Atlantis.
Laboring in a life-preserving spacesuit, the astronaut is raising a space station 220 miles (354-kilometers) above Earth when steel-braided safety tethers suddenly snap, casting the construction worker adrift in the airless vacuum of space.
With no space shuttle docked at the outpost, the station itself would be too much of a lumbering giant to chase after the astronaut, who simply tumbles off into the black void.
Within a matter of hours, oxygen supplies within the suit are exhausted, internal caution-and-warning alarms blare, and the astronaut is subjected to tragic but certain death.
NASA intends to avoid this fearsome situation with jet backpacks that would enable spacewalkers to save themselves in an EVA or Extravehicular Activity -- emergency.
"The system has been developed to support what we would consider a worst-case rescue scenario."
Known by the acronym SAFER, the so-called Simplified Aid For EVA Rescue is designed to give astronauts a fighting chance to fly back to the relative safety of the orbiting station.
Said NASA spacewalk engineer Scott Bleisath: "The system has been developed to support what we would consider a worst-case rescue scenario."
Look for Atlantis mission specialists Jeff Williams and Jim Voss to equip themselves with SAFER backpacks during a six-hour spacewalk to service and mount U.S. and Russian construction cranes outside of the outpost.
In contrast with the scenario above, Williams and Voss have the advantage of a shuttle docked to the station during their spacewalk. After an emergency undocking, the shuttle could be used by the remaining crew to chase after a freefloating spacewalker.
Packs powered by nitrogen thrusters
Designed and built by engineers at NASAs Johnson Space Center in Houston, the modern-day "Buck Rogers" jet packs will be attached to the back of the $10 million spacesuits Williams and Voss will don before venturing outside Atlantis.
Weighing in at 85 pounds (38.5 kilograms), the jet backpacks are a smaller, less capable version of the Manned Maneuvering Units tested by astronauts in the early 1980s. Equipped with 24 nitrogen gas thrusters, the SAFER units are controlled with hand-held, joystick-like devices.
In an emergency, the backpacks would enable tumbling astronauts first to right themselves and then scurry back to safety at a rate of about 10 feet (3 meters) per second.
An errant spacewalker, however, would have little time to fool around. The backpacks hold only 13 minutes of propellant.
Prototype jet backpacks performed flawlessly during initial flight tests conducted by astronauts Carl Meade and Mark Lee during a 1994 shuttle mission. But problems cropped up on each of two subsequent trial runs.
During a 1997 shuttle mission to Russias space station Mir, the jet thrusters on a backpack being tested by astronaut Scott Parazynski failed to fire. Then, on a December 1998 International Space Station assembly mission, a jet pack being tested by astronaut Jerry Ross consumed nitrogen gas at a rate much higher than anticipated.
NASA engineers since have been putting the packs through rigorous ground testing to fix problems and ensure that the devices in fact would work properly in an emergency.
Spacewalker Williams isnt worried about the spacewalk he will take starting at 10:31 p.m. EDT on Sunday.
Both he and Voss will use two safety tethers at all times "so that no single failure of a tether will set us free from the station," Williams said.
"In an extreme case," he added, "if we were [cast] off and SAFER couldnt get us back, there is still the option to undock and have the shuttle come pick us up."
The reliability of the backpacks, however, will become increasingly critical once full-time crews take up residency at the station later this year and begin spacewalking work when shuttles arent docked at the outpost.
Consequently, many astronauts particularly those who will be do assembly work outside the station -- think it would be a good idea to squeeze in some more flight-testing in orbit to make certain all the kinks are ironed out of the system.
Said Williams: "Im confident it will work. But I also think we need to continue to look for opportunities to verify its operation in orbit."
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