Animation shows the station's robotic arm preparing to lift the Mobile Base System from Endeavour's payload bay. Photo: NASA TV/Spaceflight Now

To provide the electrical power the station needs for its on-board systems and science hardware, engineers designed a giant truss that attaches to the top of the Destiny laboratory module and extends to either side of the station like huge wings.

At each end of the truss, two giant sets of solar arrays will track the sun and deliver electrical power to the station through the central element of the truss, known as S-zero. The completed truss also would carry ammonia from inside the station to radiators mounted just inboard of the solar arrays to dissipate the heat generated by the station's electronics.

But the truss could not be launched in one piece. Instead, NASA designed a multi-element beam that would be launched in sections and then assembled, one piece at a time, in orbit. The station's robot arm was used to install the first $600 million segment - S-zero - from its current perch on the hull of Destiny.

But the arm is not long enough to attach any outboard truss segments. It was designed from the beginning to move along rails attached to the forward side of each major truss segment, carried by a $190 million U.S.-built motorized cart capable of creeping back and forth to various work sites.

The robot arm, however, cannot attach itself directly to the railcar, or mobile transporter. It needs an interface that can provide power and route data and video back to operators inside the lab.

Animation shows the Mobile Base System being maneuvered for attachment to the Mobile Transporter. Photo: NASA TV/Spaceflight Now

Enter the Mobile Remote Servicer Base System, or MBS, provided by the Canadian Space Agency. The $254 million MBS is equipped with a pair of computers, weighs 3,300 pounds and measures 18.7 feet by 14.7 feet by 9.5 feet.

It provides a latching mechanism to lock down major cargo elements - like truss sections - during movement along the solar array beam; a video system; a latch mechanism for smaller components; and four power, data and grapple fixtures, known as PDGFs, where the robot arm can attach itself. Once attached, the arm can handle components weighing up to 46,000 pounds.

The arm currently is anchored to the Earth-facing side of Destiny's hull. After the MBS is installed, a lengthy series of tests is planned to verify its health. Only then, well after the shuttle's departure, will the Expedition 5 crew command the arm's free end to latch onto one of the MBS' power and data grapple fixtures.

At that point, the other end of the arm will release the PDGF on Destiny's hull to complete the long-awaited "walk off" onto the mobile transporter. And that will clear the way for the August shuttle mission when the arm will be used to attach the first of the outboard truss sections.

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