AIR & SPACE MAGAZINE

How Things Work: Soyuz-Station Docking

In orbit, it’s all about connections.

The Soyuz docking assembly's mating adapter is shown in space just feet away from the International Space Station.
The Soyuz docking assembly's mating adapter is shown in space just feet away from the International Space Station. NASA

A near-perfect history of hooking up in space

The glitch that kept a Soyuz spacecraft docked to the International Space Station last fall, when its crew tried to come home, wasn’t the first. Back in March 1966, when Neil Armstrong and Dave Scott linked their Gemini VIII capsule to an Agena target vehicle—the first docking in space—both craft began tumbling out of control, forcing a mission abort. The fault lay not with the docking mechanism, but with a stuck thruster on the crew capsule. Docking was key to the success of the Apollo moon landings, enabling a command module and a lunar module to travel together, disconnect, and reconnect.

The Soviets’ first manned docking attempt failed, but the second, Soyuz 4 and 5, in January 1969, succeeded. Dockings with the Salyut space station followed, as did U.S. dockings with Skylab. In 1975, Americans and Soviets linked up in the Apollo-Soyuz Test Project, and in the late 1990s with space shuttle-Mir dockings. Today, the shuttle and Soyuz bring people to the ISS, while the unmanned Russian Progress, European Automated Transfer Vehicle (ATV), and Japanese H-II Transfer Vehicle (HTV) bring cargo. The shuttle also brings Multi-Purpose Logistics Modules (MPLM), which get berthed to the station by a robotic arm. In 2011, SpaceX hopes to be the first U.S. business to connect a vehicle to the ISS.

How many docking ports are on the station?

Five active ones: four for Russia’s Soyuz and Progress craft, one of which takes the ATV, and a fifth for the shuttle. Several larger common berthing ports accept the HTV, MPLM, and permanent ISS nodes.

What’s the difference between the Soyuz-Progress docking port and the shuttle port?

Soyuz, Progress, and ATVs use the classic “probe-and-drogue” design. This resembles Apollo hardware, and some mid-air refueling systems, where a probe is guided into a cone, then a latch closes. The probe then retracts to pull the two spacecraft together. The shuttle’s system, though, derives from Apollo-Soyuz, designed to prevent either country’s craft from appearing dominant. More “androgynous” and symmetrical, the docking mechanism is an extended ring on the shuttle, with shock absorbers. This gets pressed into the ISS port, and after motion is dampened, pulls the shuttle firmly to the ISS.

Michael Klesius is an associate editor at Air & Space/Smithsonian.

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There are five active docking ports on the International Space Station: four for Russia's Soyuz and Progress craft, one of which takes the European Automated Transfer Vehicle, and a fifth for the shuttle. Several larger common berthing ports accept the Japanese H-II Transfer Vehicle, Multi-Purpose Logistics Modules, and permanent ISS nodes. NASA
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The Soyuz docking assembly's mating adapter is shown in space just feet away from the International Space Station. NASA
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Russian cosmonaut Sergei Krikalev holds the Soyuz docking assembly's mating adapter inside the ISS, where it detaches and stows until needed for undocking. NASA
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FIRST CONTACT: A probe-anddrogue system gives the Soyuz room for error on arrival. The drogue's cone, on the ISS side (at left), is the passive portion of the system. It steers the incoming Soyuz probe, the active part, into the drogue. Anatoly Zak
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SOFT DOCK: A capture latch at the tip of the probe fits into a socket inside the drogue. Shock absorbers in the active docking assembly dampen the motions of the two spacecraft. This results in a "soft dock." Anatoly Zak
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DOCKING START: This cutaway shows how the probe slowly retracts, pulling the Soyuz toward the ISS. Alignment pins on the probe ensure that peripheral connections on the two structural rings meet. Anatoly Zak
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HARD DOCK: Mechanical docking ends 22 minutes after first contact. Sixteen sets of hooks pull the two structures together in a "hard dock." Fluid and electrical links activate. An hour later, the hatch is ready to open. Anatoly Zak

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