NASA TV

The International Space Station's robotic arm unloads grapple-bar assemblies from the unpressurized "trunk" of SpaceX's Dragon cargo capsule on Wednesday.

By Miriam Kramer Space.com

NASA engineers used a robotic arm on Wednesday to unpack the first exterior cargo ever delivered to the International Space Station by an American-built commercial supply ship.

A robotics team at NASA Mission Control in Houston remotely controlled the space station's 58-foot (17-meter) Canadarm2 robotic arm to unload two so-called grapple bars from the unpressurized "trunk" of the privately built unmanned Dragon space capsule. The Dragon's trunk is a cylindrical cargo section beneath the spacecraft's re-entry module.

The Dragon spacecraft, built by California-basedSpaceX, was launched to the space station on Friday and arrived two days later, delivering about 1,200 pounds (544 kilograms) of supplies to the orbiting lab. It's the second of 12 scheduled SpaceX cargo deliveries for NASA under a $1.6 billion agreement.

SpaceX launched a demonstration flight to the space station last May and made its first cargo delivery inOctober. But both of those missions only carried items inside the Dragon's pressurized capsule, which is accessible to astronauts on the station through a docking hatch. [See photos of Dragon's space station arrival]

The current mission marks the first time SpaceX has ever delivered gear meant for the outside of the space station using the Dragon's trunk. SpaceX built the support hardware holding the grapple bars in place on the Dragon capsule, company officials said.

The six astronauts living aboard the space station finished unloading the pressurized cargo section on Monday, leaving only the grapple bars to be retrieved. "These bars, which together weigh about 600 pounds [272 kilograms], can be used to remove failed radiators on the stations S1 and P1 truss segments, should that ever be deemed necessary," NASA officials said in a statement.

The grapple bars will be stored in a temporary spot on the International Space Station exterior for now, but will eventually be mounted to a permanent storage point, NASA officials said.

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NASA unpacks 'trunk' of SpaceX's cargo craft