Canadarm2's Cosmic Catches

Robotic arm grapples free-flying ships to dock them with the International Space Station (ISS)

Canada's expertise in space robotics takes centre stage on the ISS whenever the Canadarm2 captures a visiting space vehicle. For over a decade, the heavy-lifting robotic arm played a critical role in the assembly of the ISS. With the orbiting outpost now completed, Canadarm2's role has changed significantly: the arm has shifted gears from building the ISS to supporting its maintenance. This includes helping to catch free-flying, cargo ships carrying everything from spare parts, to science experiments, to necessities for the crew living aboard the station.

Since the retirement of the Space Shuttle, unpiloted resupply vehicles play a vital part in resupplying the Station. Whereas the Russian Progress ship and the European Space Agency's Jules Verne Automated Transfer Vehicle (ATV) are programmed to dock automatically to the ISS, all other ships are captured and docked by Canadarm2, including Japan's H-II Transfer Vehicle HTV and the two US Commercial Orbital Transportation Services vehicles, (Space X's Dragon and the upcoming Orbital Science's Cygnus).

A high-flying rendezvous with the ISS

e Canadarm2 saisit la palette du véhicule HTV-3

Canadarm2 secures the HTV-3 exposed pallet to the Japanese H-II Transfer Vehicle in August 2012. (Credit: NASA)

Tagging up with the ISS begins soon after a resupply ship reaches orbit. The vehicle slowly increases its orbital height to catch up to the station. Catching a cargo ship calls for both precision flying and deft work from the 17-metre robot arm. Using GPS technology and laser navigation, the ship slowly closes in on the station. Once it moves into position about 500 metres below the ISS, the crew begins to monitor its progress to ensure a safe approach.

The visiting vehicle then gently pulses its steering thrusters to pull itself up from beneath the station. When it reaches a predetermined area just a few metres away, the unpiloted ship's guidance and control system synchronizes its motion with respect to the ISS. At this point, the two spacecraft fly in formation at more than 350 km above Earth.

Cosmic catches are performed by the astronaut crew on board the station. Canadarm2's operator has only a few minutes to safely grapple the vehicle before its flight parameters can be considered too risky. While working at the robotic work station inside the ISS, the astronaut uses Canadarm2 to reach out and grasp the ship.

Once the vehicle is captured, the ISS crew attaches it to one of the station's docking ports. They then open the hatches between the ISS and the cargo vehicle to unload supplies (such as food, clothes and a variety of equipment for science experiments). Once the mission is completed, the ship is then released and follows a controlled path of reentry into the Earth's atmosphere. Like Russia's Progress supply ships, Europe's ATV, Japan's HTV and Orbital Science's Cygnus spacecraft are all designed to burn up on re-entry, and are therefore restocked with waste. SpaceX's Dragon is currently the only reusable space freighter that can return cargo to Earth.