Canadarm2

Canadarm2 is a giant robotic arm bolted to the outside of the International Space Station. It built much of the Station, repairs it, and reaches out to catch cargo ships as they float nearby.

Quick facts

• Official name: Space Station Remote Manipulator System (SSRMS) — “remote manipulator” simply means a robotic arm operated from a distance.
• Built by: MacDonald, Dettwiler and Associates (MDA) of Brampton, Ontario; owned and operated by the Canadian Space Agency (CSA).
• Launched: April 19, 2001, on mission STS-100 aboard the Space Shuttle Endeavour.
• Length: about 17–17.6 metres (roughly 58 feet) when fully extended.
• Mass: about 1,500–1,800 kilograms (roughly 3,300–4,000 pounds); sources vary.
• Diameter: 35 centimetres (14 inches).
• Joints: 7 motorized joints — three at the shoulder, one elbow, and three at the wrist, much like a human arm.
• Payload capacity: up to about 116,000 kilograms (256,000 pounds).

What it is and how it works

Canadarm2 is a symmetrical arm: two long booms (the straight “limb” sections) joined by an elbow joint, with three rotating joints clustered at each end to act as a shoulder and a wrist. Together those seven motorized joints give it seven degrees of freedom — meaning seven independent ways it can swivel and bend — for a wide, human-like range of motion.

Both ends finish in identical “hands” called Latching End Effectors (LEEs). Inside each one, three snare wires close around the shaft of a grapple fixture — a peg-like grab point — to lock on with a firm, powered grip. Think of it as a hand that snares a handle and won’t let go.

Because both ends are the same, the arm has no fixed base. To relocate, it “walks” hand-over-hand, like an inchworm: it grips one Power Data Grapple Fixture (a special grab point that supplies electricity, data, and video) with one hand, lets go with the other, swings around, and grabs the next fixture. Whichever end is anchored becomes the temporary base. The network of these fixtures around the Station — plus a rail-mounted cart that carries the arm along the Station’s backbone — sets how far it can reach.

To catch a spacecraft, an operator extends the arm and snags a grapple fixture on a vehicle holding steady nearby; controllers then guide the arm to “berth” the ship to a berthing port. Astronauts fly it from two Robotic Work Stations inside the Station using joysticks and screens, or ground controllers at CSA and NASA fly it remotely.

Why it matters

Canadarm2 made the assembly of the International Space Station possible. It installed modules and large structural truss segments — the Station’s backbone — that the older Space Shuttle arm could not handle. It also performs maintenance, moves replacement parts and astronauts during spacewalks, and lets crews do robotically what would otherwise require risky time outside the Station.

It was never originally designed to catch spacecraft, yet that became one of its defining jobs. Cargo ships fly up, hold a stable position nearby, and are grappled by Canadarm2 instead of docking on their own. This matters because the Station and an incoming ship both orbit at roughly 400 kilometres up and about 28,000 kilometres per hour, so the vehicle must hold steady within the arm’s reach before it can be grabbed — a delicate, time-critical move.

Notable examples

• STS-100 (April 2001): Endeavour delivered and installed Canadarm2, starting its assembly role.
• HTV-1 (Sept. 17, 2009): the first free-flying capture — Canadarm2 grappled Japan’s H-II Transfer Vehicle, pioneering the catch-and-berth method.
• 50th cosmic catch (August 2024): Canadarm2 grappled a Northrop Grumman Cygnus cargo ship for its 50th catch of a free-flying spacecraft.
• SpaceX Cargo Dragon and Northrop Grumman Cygnus: routinely captured and berthed for resupply, including the Cygnus XL “S.S. Steven R. Nagel” in April 2026.
• Mobile Servicing System teammates: Canadarm2 is the central piece of this three-part system, working with the Mobile Base System (a rail cart) and Dextre, a two-armed “robotic hand” it can hold for delicate parts swaps. That heritage now leads to Canadarm3, Canada’s contribution to NASA’s lunar Gateway.