The Discovery astronauts Scott Parazynski and Doug Wheelock began working outside the International Space Station (ISS) Saturday to repair a torn solar array in one of the riskiest spacewalks ever, NASA reported.

Parazynski rode the station’s robotic arm up to the damaged area of the array. He is secured in a foot restraint on the end of the Orbiter Boom Sensor System, or OBSS – the extension to the shuttle robot arm used for inspection of the orbiter’s thermal protection system.

The solar panel - the oldest on the ISS - was damaged while astronauts were trying to move and deploy it earlier this week. The mishap prompted NASA ground control to cancel the remaining spacewalks, one of which was intended to repair a joint problem on yet another solar panel.

As Parazynski installs homemade stabilizers and releases the snag suspected of causing the tear in the array panel, Wheelock will assist from the base of the solar array.

Parazynski will have to avoid touching the electrified solar panels and rely entirely on tools that were rounded up on the shuttle.

NASA fears that the damage could get worse if it is not repaired, possibly making the solar array unusable.

The rips could cause serious problems because it places a lot of stress on the area and could tear further. If that occurs it could destabilize the array and even force it to be jettisoned, ISS programme manager Mike Suffredini said earlier this week.

NASA left open the possibility of undertaking the fifth spacewalk on Sunday if the rips were not completely repaired Saturday.