Bruce McCandless II floating untethered. Image credits: NASA.
On February 7, 1984, Bruce McCandless II slipped out of space shuttle Challenger and became something no human had ever been before: a biological spacecraft.
Astronauts had walked in space before. They had floated beside capsules to perform missions, but they were always physically tethered to the shuttle. McCandless wasn’t. Looking impossibly alone, the astronaut relied on a white pressure suit and a nitrogen-powered backpack. He floated as far as 98 meters (322 feet) from Challenger while the Earth rolled by in blue and white silence behind him.
NASA’s photograph of that moment became one of the defining images of the space age: a lone astronaut, completely untethered. But this daring mission showed two important things: that astronauts could move independently in orbit and that some freedoms are too risky to keep.
Why Even Try Something Like This?
NASA dreamed of astronaut free flight since the early space age, when astronauts first began climbing outside their capsules using tethers.
In 1965, Ed White made the first American EVA using a small hand-held maneuvering gun while remaining tethered to Gemini 4 (Alexei Leonov had performed the first even spacewalk just months earlier). Later Gemini crews tried more ambitious tools, but early spacewalks exposed a brutal truth: moving outside a spacecraft was far harder than it looked.
The most ambitious early system was the Astronaut Maneuvering Unit, a rocket pack planned for Eugene Cernan on Gemini 9A in 1966. The test never happened. Cernan became overheated and exhausted during his spacewalk, his visor fogging as he struggled outside the spacecraft.
That failure left a deep impression. While NASA didn’t give up on its plan, the agency realized it needed more testing.
Skylab helped. Skylab was the United States’ first space station, a precursor to the International Space Station. It was launched by NASA and occupied for about 24 weeks between May 1973 and February 1974. Among the many tests in this orbital workshop, astronauts tested maneuvering devices. McCandless, an electrical engineer and naval aviator selected by NASA in 1966, became deeply involved in that work.
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Gyroscopes in Space
Bruce McCandless II flying untethered in space. Image credits: NASA.
The original Astronaut Maneuvering Unit (AMU) would just not cut it. It got the basics right, but underestimated the difficulty in navigation, especially when it came to astronaut stability.
The Skylab trials showcased the difficulty of controlling raw propulsion. A human flyer in orbit needs stability, and in practice, that means that gyroscopes have to detect unwanted rotation and fire thrusters to adjust it automatically.
After learning several valuable lessons, NASA moved to the 2nd version of the prototype: the Manned Maneuvering Unit (MMU).
The MMU was built by the Martin Marietta company. It looked bulky, almost chair-like. It weighed more than 300 pounds on Earth, which made it difficult to get used to. In orbit, it became weightless, but not massless: every movement still had to respect momentum.
But the MMU fit over the life-support backpack of the shuttle-era spacesuit and used high pressure nitrogen as a propellant. Nitrogen offered a crucial advantage: it was inert, so it would not contaminate delicate satellite instruments or optics.
Most importantly, it seemed to also control the unwanted rotation. Gyroscopic sensors detected it and thrusters corrected it. This meant to that the astronaut himself didn’t have to fight every tiny drift. The machine stabilized the body while the astronaut could focus on work and large-scale navigation.
The MMU was ready to be put to the test.
The Gut-Wrenching Test
NASA’s STS-41B mission launched on Feb. 3, 1984, with Commander Vance Brand, Pilot Robert “Hoot” Gibson, and mission specialists Ronald McNair, Robert Stewart and Bruce McCandless aboard Challenger. It was the shuttle program’s tenth mission and Challenger’s fourth flight.
The mission began with disappointment. Challenger deployed two communications satellites, Westar VI and Palapa B2, but the Payload Assist Module-D rocket motors malfunctioned, leaving both satellites stranded in low, useless orbits instead of in geostationary space.
The failures were costly. They also made the MMU seem suddenly more important. Later that year, on STS-51-A, astronauts used MMUs during the recovery of those stranded satellites. But first, NASA had to prove the backpack worked.
At 8:25 a.m. EST, McCandless opened the hatch.
Bruce McCandless II in 1982.
He moved into the payload bay, backed into the MMU mounted on its support station, locked himself in with latches and a lap belt, then disconnected from shuttle power and communications lines.
He became a free flyer. McCandless joked, “It may have been one small step for Neil [Armstron], but it’s a heck of a big leap for me.”
He eventually moved roughly 300-plus feet from the orbiter. He and Challenger shared essentially the same orbit, but the danger was relative motion. A small mistake could become separation. A failed thruster, a stuck valve, a confused control input, any of these could spell disaster for the mission.
Granted, NASA planned for that. The MMU had redundant systems. The shuttle crew could maneuver the orbiter. The robotic arm could help if geometry allowed. The astronaut was untethered, but there were backup plans in case something went wrong.
But despite the stress and obvious concerns, nothing went wrong. The mission went brilliantly.
The Tragedy That Ended the MMU
The MMU flew only three shuttle missions, all in 1984: STS-41B, STS-41C, and STS-51A. It performed brilliantly. NASA’s history says that on three missions the MMU performed with precision and versatility and showed that humans could maneuver in space free of both spacecraft and tether.
Then came January 28, 1986.
Challenger broke apart 73 seconds after launch on mission 51-L. The Rogers Commission traced the disaster to failure in the right solid rocket booster field joint and condemned a flawed decision-making process. The shuttle shouldn’t have been launched. The report described failures of communication, conflict between engineering data and management judgment, and a management structure that allowed safety problems to bypass key shuttle leaders.
After Challenger, NASA rethought its appetite for risk. Untethered astronaut flight looked less like bold utility and more like an unnecessary single-person hazard. Many MMU jobs could be done with the shuttle’s robotic arm or by tethered astronauts. NASA also moved away from launching commercial satellites on the shuttle, reducing the need for satellite capture and retrieval.
A Striking Legacy
SAFER. Image credits: NASA.
But the MMU didn’t vanish completely. Its legacy lives in SAFER, the Simplified Aid for EVA Rescue.
First tested in 1994 on STS-64, SAFER is a smaller emergency propulsion system worn by astronauts during spacewalks. But this time, the device isn’t meant for routine work or bold solo flights, but rather as a life jacket. If a tether fails, SAFER gives an astronaut a last-resort way back.
McCandless’s own legacy extended beyond that famous photograph. He worked on spacewalking tools and procedures, including a tethering system for managing equipment. He went on to fly again in 1990 on STS-31, the mission that deployed the Hubble Space Telescope. His career also touched the practical side of spacewalking: tools, tethers, repair methods and designs that helped astronauts work without losing equipment to the void.
The MMU belonged to a daring period in shuttle history, when NASA imagined astronauts ranging through orbit to rescue satellites and repair machines by hand. Its retirement reflected on the harder lesson that this approach comes with risks and very real consequences.
But when Bruce McCandless became untethered to all of humanity, he managed to steer himself through space. He became, for a few moments, his very own spacecraft.