NASA Docking System (NDS)

When two spacecraft meet in orbit, they need a reliable way to latch together so people and cargo can pass safely between them. The NASA Docking System (NDS) is the modern connector that makes that happen.

Quick facts

• Builder: The Boeing Company, Huntsville, Alabama. Block 1 design qualification testing ran through January 2017.
• Standard it follows: the International Docking System Standard (IDSS) — a publicly published, shared blueprint set by the ISS Multilateral Coordination Board (NASA, Roscosmos, JAXA, ESA, CSA) around 2010.
• Passageway: a transfer tunnel 800 mm (about 31 inches) across for crew and cargo.
• Soft-capture ring: roughly 1.3 m outer diameter, with 3 evenly spaced guide petals for alignment.
• Capacity: designed for vehicles of roughly 5–25 tonnes docking to complexes up to about 375 tonnes.
• Transfers: power, data, and air (later variants are planned to also pass water, fuel, oxidizer, and pressurant).

What it is

NDS is NASA’s version of the IDSS. It is androgynous, meaning every port is built the same way and any port can play either role in a docking — there is no fixed “male” or “female” side. It is also low-impact, meaning it captures an approaching vehicle gently rather than with a hard slam. NDS replaced older docking hardware, growing out of the international Low Impact Docking System (iLIDS) and Boeing’s Soft Impact Mating and Attenuation Concept (SIMAC).

How it works

Docking happens in two phases: soft capture, then hard capture. As the active vehicle (the one driving the connection) nears the target, electric actuators — motorized devices that push parts in and out — extend its soft-capture ring ahead of the main structure. Three guide petals on the ring mesh with the petals on the passive port, funneling the two rings into alignment and correcting small errors in approach angle and offset. When the rings touch, latches engage for “soft capture.”

That ring sits on six servo-controlled legs arranged like a hexapod — a six-legged platform that can move in all six directions an object can travel or turn (a setup engineers call six degrees of freedom). Think of it like a hand catching a thrown egg: instead of stopping it suddenly, the legs flex to absorb and damp the contact energy. That is the “low-impact” feature, and it works even for relatively light visiting vehicles.

Once the connection is steady, the ring retracts, pulling the spacecraft in until the two main interface rings meet face-to-face. Twelve pairs of structural hooks then close to clamp the vehicles rigidly together, guide pins lock in precise final alignment, and a pressure seal plus electrical, data, and air connectors engage — forming the sealed, pressurized tunnel that crew and cargo pass through. Undocking reverses the steps, with pyrotechnic (small explosive) separation available as a backup if the mechanism will not release.

Why it matters

NDS standardizes how spacecraft connect in orbit. Because it follows the publicly released IDSS, any compliant vehicle from any partner nation can dock to any compliant port, no matter who built it. That interoperability is essential for international partnership and for crew-rescue situations, where one nation’s vehicle may need to retrieve another’s crew. Its androgynous, low-impact design lets the same hardware serve as either partner and capture vehicles gently enough to avoid damaging structures. One catch of the role-based approach: each docking still needs one active and one passive port, so two active-only ports — such as two crew capsules — cannot dock directly to each other.

Where it is used

On the International Space Station, NDS ports are not built into the station itself. They are provided by bolt-on International Docking Adapters (IDA-2 and IDA-3), Boeing-built units that convert the station’s older APAS-95/PMA ports to the IDSS standard. IDA-2 launched in July 2016 and IDA-3 in July 2019. (An earlier unit, IDA-1, was lost in the June 2015 Falcon 9 CRS-7 launch failure, which directly limited available commercial-crew ports.)

SpaceX Crew Dragon was the first vehicle to dock to an NDS/IDA port, on the uncrewed Demo-1 mission on 3 March 2019; Crew Dragon and Cargo Dragon 2 use an active IDSS port. Boeing’s Starliner first docked to an IDA during its crewed Crew Flight Test in June 2024. NASA’s Orion spacecraft uses the NDS version of IDSS to dock on Artemis missions. ESA’s International Berthing and Docking Mechanism is built to the same standard, so it too is interoperable with NDS ports — making NDS the universal “handshake” of modern human spaceflight.