Bruce McCandless II, 320 feet from Challenger, February 7, 1984. Robert Gibson / NASA.

On February 7, 1984, Bruce McCandless detached from the space shuttle Challenger and floated alone, untethered, three hundred feet from any other human structure. He was moving at 17,500 miles an hour. Nothing connected him to the shuttle but a nitrogen-thruster backpack he had helped design over eighteen years on the ground. He was forty-six years old. The crewmate's photograph — a small white figure against the black — became one of the most reproduced images of the twentieth century.

McCandless was selected as a NASA astronaut in 1966. He did not fly. While Armstrong walked on the Moon, McCandless ran simulations. While Apollo 13 fought to come home, McCandless was on the ground, talking the crew through procedures. The Apollo program ended. Skylab came and went. He did not fly.

What he did instead was build the Manned Maneuvering Unit. The MMU was a nitrogen-jet backpack that would let an astronaut detach from a spacecraft and fly free. McCandless was the project's principal designer and human test subject. For eighteen years, he was the man who knew the machine.

His first spaceflight assignment came in 1984. STS-41B. He was the test pilot for his own invention.

The MMU had never been used by a human in vacuum. Simulations only get you so far. If a thruster stuck open, or the regulator failed, or the suit punctured, there was no recovery. The shuttle could not maneuver fast enough to chase a drifting astronaut. He understood this.

He fired the thrusters and drifted backward away from the cargo bay. At 320 feet from Challenger, he stopped. The shuttle hung in space behind him. The Earth turned below. Crewmate Robert Gibson photographed him through a window. The image showed a man-shaped white figure against absolute black, no tether, no line. The photograph ran on the cover of every major newspaper.

McCandless went out a second time on the same mission. He flew the MMU again. It worked.

Six years later, on STS-31, he helped deploy the Hubble Space Telescope. Hubble outlived him. It is still working.

The MMU was retired after the Challenger explosion in 1986. It was never used again. McCandless is the only person to have flown one operationally.

He died in 2017 at the age of eighty. In his last years, the photograph from 1984 hung in elementary-school classrooms in countries he had never visited. It hangs in the National Air and Space Museum still. The figure in the suit has no face. The viewer cannot see who it is. He liked that. He said the photograph was about the moment, not the man.

Source verification: NASA mission report STS-41B; McCandless obituary, New York Times, December 22, 2017; "Extravehicular Activity: The History of the Manned Maneuvering Unit," NASA SP-2007-4232.