Space walks, or extravehicular activities (EVAs), are an important part of the assembly of the International Space Station (ISS). By the time the Station is complete, its construction will have required more than twice as many space walks as were previously performed in the entire history of spaceflight. Unlike the Space Shuttle, the Space Station cannot maneuver to rescue a free-floating EVA crew member. NASA is determined to make sure that none of the over 150 ISS assembly space walks ends up like the movie nightmare. One of the ways the agency is doing this is with a device called Simplified Aid For EVA Rescue (SAFER). Essentially a "life jacket" for space walks, SAFER is a self-contained maneuvering unit worn by astronauts like a backpack. The system relies on small nitrogen-jet thrusters to let an astronaut move around in space.
Its relatively small size and weight allow for convenient storage on the Station, and let EVA crew members put it on in the Stations airlock. However, the small size was achieved by limiting the amount of propellant it carries, meaning that it can only be used for a limited time. That is why SAFER is intended primarily for emergency rescue, and not as an alternative to tethers, safety grips, and the Canadarm2 robot arm as a means of getting around the Station. Astronauts control the SAFER device using a hand controller attached to the front of their space suits, and computers assist in its operation. The system has an automatic attitude hold function, in which the onboard computer helps the wearer maintain course. SAFER's propulsion is provided by 24 fixed-position thrusters that expel nitrogen gas and have a thrust of 3.56 Newtons (0.8 pounds) each. SAFER was first tested in 1994 aboard the Space Shuttle Discovery, when astronaut Mark Lee became the first person in 10 years to float freely in space.
Space walking has come a long way since the early days. In June 1965, astronaut Ed White became the first American to conduct a space walk. His space suit was smaller than later EVA suits, since it did not carry its own oxygen supply. Instead, a hose to an oxygen supply on the Gemini capsule connected White. Bundled with the oxygen hose were electrical and communication wires and a safety tether. While outside the spacecraft, White was able to maneuver himself outside the spacecraft using a handheld air-pressure gun. However, it quickly expended its supply of gas. On Gemini 10 and 11, a hose to a nitrogen tank aboard the spacecraft connected a modified version of the handheld device. This allowed the astronauts to use it for a longer period of time. Another maneuvering unit was developed for the Gemini program, but was never used during an EVA. This larger device would have been worn like a backpack.