[sarex] A SAFER Way To Space Walk

[Arthur Rowe]

* 9-12:A SAFER Way To Space Walk*
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http://www.nasaexplores.com/show2_912a.php?id=03-050&gl=912

space walk using safer and the robotic armIt’s a science fiction movie 
nightmare situation. An astronaut is working outside his spacecraft in 
the vacuum of space when something happens—his tether breaks, a 
satellite hits him, or an evil computer pushes him away from the ship. 
However it happens, the end result is the same. The astronaut ends up 
floating away from his spacecraft into the endless void of space, with 
no way to rescue himself. It’s a horrible fate, but a new device for 
space walking will ensure that it only happens in the movies.

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.

safer is worn on the back of a space suit like a backpackIts relatively 
small size and weight allow for convenient storage on the Station, and 
let EVA crew members put it on in the Station’s 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 walk using a tetherSpace 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.

Astronauts conducted extensive EVAs during the Apollo program. Most of 
that time was spent on the surface of the Moon, rather than in the free 
fall of space. Walking on the Moon meant that astronauts had to be able 
to leave their craft, untethered, in self-contained space suits. It also 
meant that the astronauts did not require microgravity-maneuvering 
units. Such devices were tested inside the Skylab space station. Among 
them were “jet shoes,” a foot-controlled maneuvering unit designed to 
leave the astronaut’s hands free. Backpack and handheld units were also 
tested.

spacewalk using saferThe next extravehicular space walks were not until 
the Space Shuttle era began in the early 1980s. The Shuttle features 
cargo bay doors that allow astronauts easy access to the vacuum of 
space. The Gemini and Skylab research led to the development of a Manned 
Maneuvering Unit (MMU) for Space Shuttle astronauts. The 
nitrogen-jet-powered MMU was essentially a self-contained spacecraft 
that an astronaut wore like a backpack. The MMU was first used on the 
10^th flight of the Space Shuttle. Astronaut Bruce McCandless became the 
first person to fly freely, untethered in space in February 1984. The 
MMU was used a total of nine times on three Space Shuttle missions in 
1984, but has not been used since. Tethers, grips, and other restraints 
proved more than adequate for space walks in a Shuttle cargo bay. The 
Canadarm robot arm proved itself ideal for recovery of satellites and 
other tasks outside the Shuttle. SAFER, which has been described as a 
simplified version of the MMU, has two advantages over the earlier 
system. It is a more convenient size and weight and ideal for an 
astronaut rescue device outside the Space Station.

SAFER is a rare type of technology—the kind that is built hoping that it 
won’t be used. So far, tethers, safety grips, and the robot arm have 
proved adequate to safely keep astronauts where they are supposed to be 
during space walks. But if they ever fail, SAFER will be ready.


Courtesy of NASA's Space Operations Missio