The MMU may have been the coolest space vehicle ever. So why did its career end as soon as it began?

Bob Stewart tests the MMU in 1984. No astronauts since have ranged farther from their ship. (NASA)
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Still, even some insiders couldn’t help feeling nervous about what McCandless and Stewart were about to do. NASA’s Paul Bailey, an orbital mechanics specialist who helped train astronauts to fly the jetpack, felt confident in both Martin and in Ed Whitsett, but as the STS-41B launch approached, he had a recurring nightmare: He would be sitting on the shuttle’s tail section, with Earth filling the sky, just as McCandless and Stewart floated out of the airlock. “I was looking down the cargo bay over at the MMUs. And I thought, Holy mackerel, these people think I know what I’m talking about! And they’re going to die!” Waking up in a cold sweat, Bailey would grab his MMU reference book, go through it one more time, and reassure himself he hadn’t forgotten anything.

On February 7, 1984, Bailey and his colleagues were at their consoles in Houston as McCandless prepared to test the jetpack for real. After a few minutes working the controls in the confines of Challenger’s payload bay, the astronaut was satisfied that the MMU flew just as expected in nearly every way; the only surprise was a noticeable chugging sensation when the forward or backward translation thrusters were firing. The vibration turned out to be harmless and had a straightforward explanation: The thrusters were designed to fire through the jetpack’s center of gravity, which was offset slightly from that of the suited astronaut. As a result, each firing produced a small rotational force, which the MMU’s sensitive attitude-control system had to counteract.

Finally, with the world watching on live television, McCandless backed away from Challenger and into space. While his crewmates tracked him with the shuttle’s radar, McCandless measured distance with his own low-tech device: a metal rod notched to show the apparent size of the payload bay from different distances. After flying 150 feet away and returning, he ventured out to 320 feet, an orbital spacewalk record that still stands. He’d planned to take a moment during the journey to turn away from the shuttle and look out at the universe, savoring the experience of being a separate satellite, but he was so focused on reporting to his crewmates and mission control that he forgot.

The image of McCandless, small and alone against the black sky, struck an emotional chord with the public, and for the last 30 years McCandless has had to explain why, contrary to many people’s expectations, he wasn’t scared. “I still have a memory of comfort,” he says, based on his intimate familiarity with the device he’d helped to create.

McCandless remembers another feeling, though: cold. The shuttle suit’s cooling system was designed to keep astronauts comfortable even when they worked up a sweat. But flying the MMU was nearly effortless, and as McCandless glided through the void, away from the warm cocoon of the payload bay, “at one point I was shivering and my teeth were chattering,” he says.

The chill did nothing to dilute the satisfaction he felt at seeing the MMU perform as well as anyone had hoped. Bob Stewart offered his own assessment: The only way the jetpack could have been easier to fly, he says, would have been to “wire it directly to your brain.” And for Paul Bailey, the sight of McCandless and Stewart flying as no one had flown before was enough to end his recurring nightmare.

Two months later, on shuttle mission STS-41C, scientist-astronaut Pinky Nelson was at the controls of an MMU heading for the crippled Solar Max. In Denver, where Martin Marietta had built a huge simulator that moved on rails, Nelson had trained to fly between the solar panels of the slowly spinning satellite, then use a special capture device to grab onto a trunnion pin on the satellite’s side. Once firmly attached, Nelson was supposed to trigger the MMU’s attitude-hold feature to stabilize the satellite so it could be captured with Challenger’s robotic arm.

Things didn’t work out as planned, though—not because of any problem with the MMU, but because a small plastic nub near the trunnion pin, something that wasn’t in any reference drawing, confounded the capture device. After three unsuccessful tries, Nelson’s collisions with the satellite set it slowly tumbling. In a last-ditch effort, he grabbed the end of a solar panel and put the MMU into attitude hold, its thrusters firing vigorously to steady itself and the satellite. For a moment, Solar Max seemed to have slowed enough to be captured by the robotic arm. “I thought, We did it,” remembers Nelson. “And then I made a stupid mistake…which was to let go.”

Without the 830-pound mass of Nelson, the MMU, and the capture device attached, the satellite again began tumbling. Low on fuel, a frustrated Nelson returned to his crewmates, who all thought they’d lost their mission. But in a heroic effort, controllers at NASA’s Goddard Space Flight Center in Maryland were able to stabilize Solar Max remotely, and a couple of days later it was steady enough for Challenger’s arm to grab. Although the repair was carried out without any help from the MMU, Nelson had made a solo flight he’d never forget.

The jetpack did not get a chance to do what it was designed for until the following November, when STS-51A spacewalkers Joe Allen and Dale Gardner used it to retrieve a pair of errant communications satellites for return to Earth. To MMU insiders like NASA engineer Cliff Hess, it seemed as if the jetpack had finally established itself as an essential part of the shuttle’s toolkit. “We designed the MMU to be used on one flight every three years,” observes Hess, “and it wound up being used three times in one year, 1984. So we thought, Wow, this is great, we’re really moving.”

It didn’t work out that way. The MMU would never fly again—in large part because it was upstaged by the shuttle itself. The vehicle was so maneuverable that it proved simpler to just fly the orbiter over to an object or person and grab it, either with the robot arm or a gloved hand. This became clear even during the MMU’s inaugural mission. During one spacewalk when McCandless was not wearing the jetpack, he accidentally let a foot restraint float out of the cargo bay. Commander Brand told him and Bob Stewart to hang on, then steered the orbiter to within a couple of feet of the object so McCandless could retrieve it. After Pinky Nelson’s failed attempts to grapple Solar Max on the next flight, STS-41C commander Bob Crippen, using the shuttle’s thrusters in a special low-impact mode, was able to fly Challenger and its robotic arm right up to the satellite without disturbing it.

Following the 1986 Challenger disaster, safety regulations were instated that would have required the jetpack to undergo an expensive re-qualification; program managers, grappling with the cost of returning the shuttle to flight, were unwilling to spend the money.

When NASA was in the early stages of planning what would become the International Space Station, the question of the MMU’s utility came up again. Stewart was among its fans. “To be able to go out and fly around and repair the space station—gosh, if they had the MMU, this would be a piece of cake.” There was one drawback: If the jetpack failed, there would be no shuttle coming to the rescue. Stewart saw that as a low risk, but it worried others, and support for using the MMU on the station flagged.

Ed Whitsett had always hoped a fleet of astronauts wearing MMUs would assemble the space station. At the time of his death he had been working on a small maneuvering pack designed to attach to a spacesuit backpack for emergency use. In September 1994, testing the Simplified Aid For EVA Rescue (SAFER), STS-64 spacewalker Mark Lee became the last human to fly untethered through space.

Today, some EVA specialists advocate a next-generation MMU to help astronauts explore low-gravity worlds like asteroids or the moons of Mars, but so far NASA hasn’t approved development. The astronaut jetpack remains a capable—some would even say compelling—technology in search of a mission.


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