It is 40 years since the iconic image of Bruce McCandless II, floating free and untethered above the Earth while testing out the Manned Maneuvering Unit (MMU), was snapped from the Space Shuttle Challenger.

The photograph, taken during the STS-41B mission, shows McCandless testing the astronaut propulsion unit, which allowed 'nauts to perform untethered spacewalks, before it was used on subsequent missions.

The MMU had been a long time coming. The US Air Force developed something similar in the form of the Astronaut Maneuvering Unit (AMU), which was to be tested during Project Gemini but was abandoned after astronaut Eugene Cernan struggled to reach the device during Gemini 9A in 1966. The harrowing story of Cernan's near-catastrophic attempt to test out the AMU is well documented in his book The Last Man on the Moon.

McCandless II was selected as part of the NASA Astronaut Group 5 in 1966. While nine of the group flew to the Moon and others made it to Skylab, McCandless's first flight to space was STS-41B, which launched on February 3, 1984.

There was little need for untethered EVAs during the Skylab and Apollo programs other than walking around on the Moon in the case of the latter. Skylab's interior was large enough to check out some designs for the device, but any venturing outside the orbiting laboratory was strictly tethered.

McCandless was the backup pilot for the first Skylab mission and was a co-investigator on the M-509 astronaut maneuvering experiment for the lab. He had also collaborated on the development of the MMU itself, and so was a logical choice for that first EVA.

The astronaut ventured as far as almost 100 meters away from Space Shuttle Challenger during the test. Should there have been a malfunction, the orbiter would have been used to rescue the astronaut.

McCandless used his helmet's visor to protect his eyes from the sun, making it difficult to identify him. NASA reported the astronaut as saying: "My anonymity means people can imagine themselves doing the same thing."

The untethered EVA occurred during what some have since dubbed "The Golden Age" of the Space Shuttle program before the Challenger disaster of 1986. In his book Riding Rockets, former astronaut Mike Mullane recalled: "With its fifty-foot-long robot arm and spacewalking astronauts, the shuttle repeatedly demonstrated its unique ability to put man to work in space in ways never before possible."

McCandless's feat occurred on only the tenth orbital flight of the Space Shuttle.

The MMU itself used gaseous nitrogen as a propellant, with thrusters controlled by hand controllers at the end of the two arms that folded into place after an astronaut backed into the device. When fuelled, its mass was just under 150 kilograms.

The vision for the MMU was to permit astronauts to maneuver around structures without requiring a cumbersome tether, and it was used on three shuttle missions in 1984, including STS-41B. However, after the Space Shuttle Challenger disaster, a safety review judged the MMU as too risky, certainly when compared to what could be achieved with robotics, and the MMU was consigned to museums.

A much smaller successor, the Simplified Aid for EVA Rescue (SAFER), has since been used on EVAs but is only for emergencies.

As for McCandless, this writer was present at a talk he gave in the latter part of 2017 in which the former astronaut lamented the growth of a risk-averse culture at some agencies, including NASA. McCandless died on December 21, 2017. ®

https://www.theregister.com//2024/02/05/mccandless_untethered_spacewalk/