Mission Specialists Scott Parazynski and Doug Wheelock are scheduled to leave the station in order to conduct a fourth spacewalk and to repair a torn solar array.

This mission is one of the riskiest spacewalks ever and Scott Parazynski, the US physician-astronaut who grew up in Senegal, Lebanon, Iran and Greece, will brave dangers including electric shock and cuts after he leaves the orbiting International Space Station.

The 6.5-hour spacewalk begins with Parazynski riding the station’s robotic arm up to the damaged area of the array. During the mission he will be 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 is still generating electricity, so the spacewalker will be equipped with insulated tools.

US astronaut Doug Wheelock will participate in the spacewalk but remain at the base of a different solar array to help direct the movement of two robot arms being operated by other astronauts from inside the space station and the docked space shuttle Discovery.

The distance from the station’s center is about 165 feet out on the truss and approximately 90 feet up to the damaged site.

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.

The repairs are needed to boost the space station's power supply, which must be increased to support more than the three-member ISS crew and to run the new European and Japanese modules scheduled to be installed in future shuttle missions to expand the space station.

Though this will be the first operational use of the OBSS to reach a worksite, the task was demonstrated during a spacewalk on the STS-121 mission in July 2006 to prove the boom could provide a stable environment for this type of work.