Finally, in late 1980, just months before the shuttle’s first launch, they got the opportunity they’d been waiting for. NASA’s Solar Maximum Mission satellite, launched earlier that year to study the sun, had been crippled by a malfunction in its attitude control system. Solar Max was designed to be serviced, and in the summer of 1982, NASA made the decision to have astronauts repair the satellite in the shuttle’s cargo bay. For that to happen, one of them would have to capture Solar Max. And for that, he would need the MMU.

With the rescue slated for a shuttle mission less than two years away, the MMU effort kicked into high gear at Martin Marietta. Program manager Bill Bollendonk and his team had to tackle a host of technical issues, including how to safely contain the backpack’s nitrogen supply at a pressure of 3,000 pounds per square inch. They adapted an aluminum alloy tank used on 747 airliners, and wrapped it in Kevlar for extra strength. Bollendonk says that to test the tank’s safety at operating pressures, “we ended up firing a .50-caliber bullet through it. It made a hole, but it didn’t explode.”

To extend battery life, the big power-hungry gyroscopes used to maintain orientation on the Skylab version were removed. The MMU’s control system would use smaller gyros to sense rotation, then immediately fire thrusters to counteract it. Also, the M-509’s pistol-grip hand controllers, which were tiring to operate in pressurized spacesuit gloves, were replaced by small T-handles that needed just a nudge of the fingertips. Adjustable control arms would accommodate the tallest and shortest astronauts.

Calling the MMU a “backpack” was a little deceptive, since, in its final form, it weighed more than 300 pounds. It contained enough nitrogen to accelerate from zero to 45 mph—just once. To conserve that precious supply, astronauts would fly the MMU at a crawl, just a few inches per second. (George Clooney’s zipping around in the movie Gravity was preposterous: If an astronaut acted that recklessly, the MMU’s fuel tank would be empty in no time.)

Practical-minded science fiction writers thought about this for generations before the space program. Take a look at the articles for the broomstick from Arthur C. Clarke's 1952 novel Islands in the Sky. Also, note the Reaction Pistol from Gordon A. Giles 1937 story Diamond Planetoid, the Personal Jet Thrust from Robert Heinlein's 1948 novel Space Cadet.

What is the oldest idea about maneuvering "untethered" in space? Read about the Electrical Tether from Garrett P. Serviss' 1898 story Edison's Conquest of Mars