Nasa astronauts Reid Wiseman and�Barry Wilmore preparing for their spacewalk inside the ISS

Spacewalkers start preparations for commercial space taxi arrival

Two US astronauts performed a six and a half hour spacewalk on Wednesday to replace a failed voltage regulator and start a year-long reconfiguration work to prepare the space station for the arrival of private astronaut carriers.

The International Space Station (ISS) flight engineers Reid Wiseman and Barry 'Butch' Wilmore performed the spacewalk on an extremely tight deadline during a night-time pass of the station when its solar panels are not generating electricity.

Racing against time during a 35-minute time slot, the astronauts managed to remove the failed unit and install the replacement with only two minutes left before the station entered daylight.

Along the way they struggled with the bolts holding the voltage regulator in place.

"The PGT (pistol grip tool) doesn't have enough power to turn it right now," Wiseman radioed to Mission Control in Houston. "I can feel it binding up."

The voltage regulator, officially called a sequential shunt unit or SSU, controls the amount of power transmitted from the solar panels to the space station’s electricity system.  Each of the eight power channels of the station is regulated by one such unit, limiting the power going through to the system to 160 volts thus preventing an overload.

The component malfunctioned in May this year, forcing Nasa to transfer space station systems dependent on the failed power channel to a backup. No operations were hampered, but Nasa wanted that channel operating again for full capability and redundancy.

After the flight controllers confirmed the new unit was working properly, the two astronauts started preparations for the arrival of new private space taxis expected to start ferrying astronauts to the ISS on behalf of Nasa in 2017.

Overall ten spacewalks are expected to be required to reconfigure the $100bn research complex, preparing docking ports and other amenities for the arrival of the new vehicles.

During the Wednesday spacewalk Wiseman and Wilmore relocated a camera support mast, clearing a path for the station's robotic crane to move a storage module from the Unity to the Tranquility connection nodes next summer.

The astronauts also moved a wireless transmitter and installed a new camera.

The start of the reconfiguration follows signing of contracts between Nasa and Boeing and SpaceX to build space capsules capable of carrying human crew and start operating them in three years.

The repair spacewalk was delayed until mid-October due to problems with Nasa spacesuits. In 2013 European astronaut Luca Parmitano nearly drowned after his helmet flooded due to a black valve, forcing Nasa to suspend all but emergency activity outside the station.

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