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Safe harbor: A Soyuz (foreground) and Progress supply vehicle docked to the International Space Station in August 2007. (NASA)

United We Orbit

It's a story of spacecraft meets spacecraft.

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How a space docking feels can depend on which side of the interface you’re facing—whether you’re the docker or the dockee. But when the 100-ton shuttle Atlantis linked up with the 100-ton space station Mir in June 1995, neither crew had any doubt about what was happening.

Just before contact, with the two spacecraft perfectly aligned, the Atlantis crew had pushed a button to fire thrusters that gave them a last nudge into the Russian docking mechanism. For the shuttle astronauts, it was the noise of the thrusters more than anything that signalled their arrival at Mir. “You could hear the booming of the forward jets,” recalls Charlie Precourt, copilot of mission STS-71. The contact itself is “absolutely imperceptible,” says Kevin Chilton, who commanded the third shuttle-Mir docking mission nine months later. You know something’s happening from “all those cannons going off all around you,” but there was no bumping or jostling inside the shuttle cabin.

On the Russian side it was a different story.

The impact felt “like a big hug,” commander Vladimir Dezhurov recalls. “A real man’s hug.” The Mir began quivering, then calmed down. When the station finally stopped shaking, says Dezhurov, “we understood the docking had occurred.”

Over on Atlantis, where shock absorbers in the docking system dampened the force of impact, the mechanism “bounced like a baby carriage,” Precourt says, but the back-and-forth motion was too subtle to be sensed directly. “The only way we could tell there was any rebound at all was to look in the camera.”

The first orbital hook-up of U.S. and Russian spacecraft in two decades had come off without a hitch.

Docking has been part of the spaceflight repertoire for more than 30 years, and as often happens, NASA has made a complex and challenging operation look boring and routine. In practice it is anything but. Robert “Hoot” Gibson, who commanded Atlantis during the first shuttle-Mir mission, calls space docking “a cross between air-to-air refueling and a carrier landing.” When the two spacecraft are still at a distance, it seems easy. “But the closer you get, the tighter you control, and the smaller the allowable errors can be,” he says. With an unlucky combination of equipment problems and human error, things can go spectacularly wrong, and that’s reason enough to regard each space docking with apprehension and respect.

When Neil Armstrong completed the world’s first orbital docking, connecting his Gemini VIII capsule to an Agena target vehicle in 1966, his joy was soon overshadowed by a life-threatening out-of-control tumble that led to an emergency splashdown in the Pacific (the fault lay in a stuck thruster on the Gemini, not in the docking technique). On the very next flight, a shroud covering the target’s docking port failed to open fully, making docking impossible.

Docking problems frustrated Russia’s first space station mission in 1971 and nearly aborted NASA’s first Skylab mission two years later. When the Russians added the Kvant science module to the Mir station in 1987, an errant trash bag got stuck in the docking interface, preventing an airtight seal until spacewalking cosmonauts removed it. Other failures and close calls convinced both U.S. and Russian space engineers that nothing about space docking would ever become routine.

Bumping two large masses together in orbit without damaging or breaking anything makes for a tricky physics problem. Vehicles docking on Earth have at least some of their motion already constrained at the time of contact. Freight cars move along rails, ships float on water, even aircraft have aerodynamic stability. But in space, position and orientation can vary in all three dimensions, and can change at different rates. All these variables—Precourt calls orbital docking an “eight-degrees-of-freedom problem”—have to be controlled simultaneously to make sure the final contact happens within the mechanical limits of the docking hardware. On Earth we also encounter natural damping forces—friction, air and water resistance, the restraining forces of rails or cables. In space, all the energy has to be absorbed and damped out within the vehicles themselves.

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