Historic Untethered EVA with Manned Maneuvering Unit Jetpack & SAFER

 The first historic untethered spacewalk using the Manned Maneuvering Unit (MMU) took place on February 7, 1984, during the STS-41-B mission. Astronaut Bruce McCandless II ventured roughly 320 feet from the Space Shuttle Challenger, becoming the first human to fly freely in orbit without a physical safety tether.

A decade later, on September 16, 1994, during STS-64, astronaut Mark Lee performed the first and only untethered flight test of the Simplified Aid For EVA Rescue (SAFER). Lee intentionally detached his safety tether and maneuvered outside the Space Shuttle Discovery to validate the compact jetpack's emergency self-rescue capabilities. Aside from this historic proof-of-concept test, astronauts wearing SAFER on the International Space Station are always physically tethered; the system remains dormant as a last-resort lifejacket.

The Manned Maneuvering Unit (MMU) and the Simplified Aid For EVA Rescue (SAFER) represent two distinct eras and philosophies in NASA's approach to untethered spacewalks (EVAs). While the MMU was designed as a primary utility vehicle for orbital work, SAFER is strictly an emergency "lifejacket."

The Manned Maneuvering Unit (MMU)

Operated exclusively in 1984 across three Space Shuttle missions, the MMU was a large, 340-pound self-contained backpack that allowed astronauts to fly entirely untethered from the spacecraft.

• Design & Controls: The unit snapped over the astronaut's standard life-support backpack. It featured two armrests extending forward, each housing a hand controller—one for translational movement (up/down, left/right, forward/back) and one for rotational movement (pitch, yaw, roll).
  
  
• Propulsion: It utilized 24 small thrusters powered by pressurized gaseous nitrogen (GN2), providing six degrees of freedom.
  
  
• Mission Profile: It was famously first flown by Bruce McCandless II and was used to capture damaged communications satellites (Solar Max, Palapa B2, and Westar VI) so they could be pulled into the Shuttle's payload bay for repair or return to Earth.
  
  
• Retirement: Following the 1986 Challenger disaster, NASA re-evaluated EVA safety protocols. The MMU was deemed an unnecessary risk, as the Shuttle's robotic arm (Canadarm) proved highly capable of capturing satellites without requiring an astronaut to fly untethered.
  
  

Simplified Aid For EVA Rescue (SAFER)

Introduced in 1994 and worn on every standard ISS spacewalk today, SAFER is a miniaturized, 83-pound propulsion unit designed solely for emergency self-rescue if an astronaut’s primary safety tethers fail and they begin drifting away from the station.

• Design & Controls: SAFER is unobtrusive, clipping to the bottom of the spacesuit's Primary Life Support Subsystem (PLSS) backpack. Instead of bulky armrests, its single hand controller is stowed on the astronaut's chest and manually deployed only in an emergency.
  
  
• Propulsion: Like the MMU, it uses 24 gaseous nitrogen thrusters, but it carries a much smaller propellant supply (roughly 3 pounds of nitrogen), providing a total velocity change (delta-v) of just about 10 feet per second.
  
  
• Automatic Attitude Hold (AAH): Because a snapped tether could impart a violent spin, an astronaut might become too disoriented to fly back. SAFER features an automated stabilization system. When activated, gyroscopes detect the spin and automatically fire the thrusters to stop the tumbling, allowing the astronaut to manually pilot themselves back to the station's structure before the nitrogen runs out.