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From the National Air and Space Museum / Udvar-Hazy Center. (Dan Winters)

The Story of NASA’s Jet-Propulsion Backpack

Thirty years ago, astronauts set out on the first untethered space odyssey

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It’s an astronaut’s wildest dream: to fly effortlessly through space, like a modern-day Buck Rogers. Thirty years ago, for a brief moment, a handful of space shuttle astronauts got to live the dream, thanks to a jet-powered backpack called the manned maneuvering unit, or MMU.

In February 1984, Bruce McCandless and Bob Stewart were the first to test-fly the MMU in space when they each ventured more than 300 feet from Challenger. (A photograph of the free-flying McCandless instantly became one of NASA’s most sought-after images.) Despite the risks of that untethered moment, McCandless took the test in stride. “I knew the laws of physics hadn’t been repealed recently,” he later said of his confidence in the MMU.

His fellow astronaut was equally unfazed. “I decided that this was the easiest thing I had ever flown,” says Stewart, a former test pilot. “The only way you could make it easier would be to wire it directly to your brain.”

McCandless’ faith in the MMU was the result of long experience: He had played a major role in developing it. The apparatus was the brainchild of engineer Charles “Ed” Whitsett, who researched the idea for his master’s thesis in 1960 as a young Air Force officer. By the late 1960s he had joined forces with McCandless to produce a test version tried out by astronauts inside the roomy Skylab space station in 1973. In 1977, Whitsett arrived at NASA, where he and McCandless used the Skylab results to improve their design.

Powered by 24 small gaseous nitrogen thrusters, the manned maneuvering unit let astronauts move untethered in space. (Dan Winters)
Using joysticks to control the MMU, astronaut Bruce McCandless flew 320 feet—the farthest an astronaut had ever ventured from the safety of his ship. (NASA)
McCandless tested this MMU, marked with the serial number 3, on a February 7, 1984, spacewalk from the Challenger. (Dan Winters)

In its final form, the MMU, produced by Martin Marietta Aerospace, weighed 300 pounds—more like a refrigerator than a backpack—and was outfitted with 24 small thrusters powered by compressed nitrogen. Two motion-control handles were mounted on armrests. The push of a button triggered the MMU’s attitude-hold mode, in which data from motion-sensing gyroscopes directed the firing of thrusters to maintain a desired orientation in space.

The MMU was designed to be so simple to operate that almost anyone could fly it with minimum training. “It’s the rent-a-car concept,” Whitsett said of an astronaut’s need for simplicity. “He just gets on and goes.” For safety, and to conserve fuel, the MMU was never flown faster than a crawl. (That, by the way, is the reason that the MMU’s recent appearance in the film Gravity was completely unrealistic: Even if an astronaut aspired to the recklessness of George Clooney’s on-screen hot rodding, the MMU had far too little fuel to allow for that.)

A couple of months after McCandless and Stewart took the MMU for its first spin, astronauts put the invention to work. A satellite called Solar Max had suffered a malfunction. Whitsett and McCandless helped convince NASA to mount a rescue mission, launched in April 1984.

The astronaut assigned to snare Solar Max was George “Pinky” Nelson. At Martin Marietta he trained in a simulator to fly to the slowly spinning satellite, match its rotation, then close in before using a capture device to “dock” with a trunnion pin projecting from the satellite. Once attached, Nelson would put the MMU into attitude-hold mode, letting its thrusters halt the spin. His crew mates inside Challenger would grab Solar Max with the shuttle’s robotic arm and place it in the cargo bay, where it would be repaired on a later spacewalk.

In space, however, it didn’t turn out that way. The MMU flew perfectly, but the capture device would not grab on. (The failure was later traced to a small protrusion next to the trunnion pin that wasn’t in the blueprints.) Running low on fuel, a frustrated Nelson returned to Challenger.

Ultimately, the repair mission succeeded without the MMU—ground controllers were able to slow the satellite remotely and astronauts used the shuttle’s robotic arm to grab Solar Max. But today Nelson has nothing but praise for the jet-powered backpack. “It’s a beautiful example of aerospace engineering,” he says.   (The MMU piloted by McCandless is now displayed at the National Air and Space Museum/Udvar-Hazy Center.)

The MMU did prove itself as a satellite-rescue tool in November 1985, when astronauts Joe Allen and Dale Gardner piloted it to retrieve a pair of errant communications satellites.

But after the Challenger disaster in 1986, NASA re-evaluated shuttle missions, including spacewalks, and the MMU was deemed unnecessary. “It became pretty obvious that you didn’t need it,” explains Nelson. “The shuttle had such an amazing capability to fly right up to something, and it made more sense to just reach out and grab it, either with the [robotic] arm or just with a person, that the MMU became a really cool piece of technology that didn’t quite have a purpose.”

“Too bad,” Nelson adds, “because it’s a very cool machine.”

He remembers a moment on the way to Solar Max. “I relaxed and looked around, and saw the shuttle coming up behind me, and the satellite in front of me, and the Earth going by underneath, and I thought, ‘Jeez, I can’t believe they let me do this!’”

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