Jim Reilly and Danny Olivas have spent the night sleeping in the airlock to accustom themselves to low pressure for today's space walk.
The pair will deploy a 16,183kg solar panel backbone for the ISS but will also examine a tear in the thermal insulation found on the Orbital Manoeuvring System rocket pod.
John Shannon, who heads the mission management team, told reporters that the damage was not considered likely to cause a repeat of the Columbia shuttle disaster, which left seven astronauts dead after the craft's thermal insulation failed.
"We have seen shuttles with much worse damage in this area that have returned safely," he told ABC.
The 6in x 3in gap in the thermal shielding was first picked up during a camera scan of the shuttle once it reached orbit. Such a scan is standard operating procedure after the Columbia flight.
The shuttle captain has already confirmed that he can extend the mission if needed to repair the fault. All shuttle flights now contain tools for orbital repair.
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