International Docking System Standard (IDSS)

When two spacecraft built by different countries or companies need to link up in orbit, something has to guarantee that their connection points actually fit together. The International Docking System Standard (IDSS) is the agreed rulebook that makes that possible.

Quick facts

• A docking port is the hatch-and-ring connection where two spacecraft physically join. The IDSS is a publicly released standard that defines what such a port must look like and what loads (forces) it must survive.
• Developed by the ISS Multilateral Coordination Board on behalf of five partner agencies: NASA, Roscosmos (Russia), JAXA (Japan), ESA (Europe) and CSA (Canada).
• First formulated in 2010 and published as the IDSS Interface Definition Document (IDD), and revised periodically since (Revision F was released in July 2022).
• The interface is androgynous, meaning one design can act as either the active (capturing) side or the passive (target) side, so two identical ports can mate.
• Leaves an 800 mm (about 31 inch) diameter passageway for crew and cargo to move through.
• Intended to span visiting vehicles of roughly 5 to 25 tonnes docking to target complexes up to about 375 tonnes.

What it is

The IDSS is a standard, not a single piece of hardware. It specifies the geometry (the shape and dimensions of the interface) and the design loads (the forces the joint must handle), rather than dictating one factory-made part. Any docking mechanism built to the standard is interoperable with any other compliant port. Think of it like the standard for an electrical wall outlet: many manufacturers make outlets and plugs, but because they all follow the same shape, any plug fits any socket. The IDSS does that for spacecraft.

How it works

Docking happens in two phases. First, the Soft Capture System (SCS) on the active vehicle extends forward while the passive port stays retracted. Three guide petals on the ring funnel the two ports into alignment as they touch. A set of six powered legs, arranged like a motorized platform that can move in any direction, absorbs and damps the leftover relative motion and gently pulls the vehicles together. This is the “low impact” approach, replacing the harder closing forces of older systems. The soft-capture mating plane intersects the guide-petal cone at a 1200 mm diameter.

Once the ports are aligned and stabilized, the Hard Capture System (HCS) takes over. Twelve pairs of hooks around the ring close to draw the two interfaces tight, forming a rigid, pressure-sealed structural joint. Guide pins ensure final alignment, and connectors mate to pass power, data, commands, air and communication across the seam. Future provisions allow for water, fuel, oxidizer and pressurant as well. Because the interface is androgynous, the same ring works whether a port is the active “catcher” or the passive “target.” The system supports both fully autonomous and piloted approaches, and includes pyrotechnic cutters (small explosive cutters) as a backup way to separate if normal undocking fails.

Why it matters

For decades, docking systems were largely incompatible. A US system (such as APAS) and a Russian probe-and-drogue could not connect, so a vehicle from one program generally could not link to another’s. By publishing a common, royalty-free standard for shape and loads, the partners ensured that any future compliant spacecraft can dock with any other compliant port. The explicit driving requirement was international crew rescue: a stranded crew could be retrieved by whatever vehicle is available. The standard also enables joint missions, commercial crew and cargo access to the ISS, and future commercial stations and lunar or deep-space craft, because manufacturers can independently build mechanisms that are guaranteed to mate.

Notable examples

• NASA Docking System (NDS): NASA’s IDSS-compliant implementation, an electromechanical low-impact mechanism used as the active hardware on US commercial crew vehicles.
• International Docking Adapter (IDA): converts the ISS’s legacy APAS-95 ports into passive IDSS-standard ports. Two adapters, IDA-2 and IDA-3, are installed and have hosted dozens of dockings.
• SpaceX Crew and Cargo Dragon 2: uses an IDSS-compliant NDS in the active role; first docked to the ISS via an IDA in March 2019 (Demo-1).
• Boeing Starliner (CST-100): uses the NDS and first docked to the ISS in May 2022 (OFT-2).
• ESA’s International Berthing and Docking Mechanism (IBDM) and India’s Bharatiya Docking System (BDS) for Gaganyaan are also being built to the IDSS, with a subscale BDS test in January 2025.