Astronauts are rewiring the ISS

Astronauts Robert Curbeam and Christer Fuglesang began the five-hour excursion at 1441 EST (1941 GMT) on Thursday.

To prepare for the work, flight controllers on the ground powered down half of the station's systems.

A third spacewalk is planned for Saturday, but astronauts may also perform a fourth, unscheduled outing to fix a stuck solar panel on the ISS.

The array, which needs to be folded away to make room for new solar panels, jammed as the astronauts attempted to retract it by remote control.

It has been six years since the 35m (115ft) wing, which has served as the space station's primary power source, was deployed and it has not been retracted during that period.

New solar panels, fitted in September, will take over the job of powering the ISS, boosting its power by about 50%.

The team spent hours grappling with the panel, comparing the process to folding up a road map, before finally admitting defeat after the system stuck.

A solar panel on the space station failed to retract

ISS resident Michael Lopez-Alegria told Houston: "We all tried as much up here as you guys did on the ground and it just wasn't going to work for us today.

"But that stuff happens and I'm sure we'll get through it."

On Friday, the team will attempt to wiggle the panel free from inside the ISS.

However if this fails, Nasa officials are considering the possibility of an extra spacewalk to carry out a hands-on repair, which would take place on Sunday or Monday.

Night flight

The mission is taking place to finish the $100bn (£50bn) station. At least 14 more missions are needed.

Discovery blasted off from Florida at the weekend in the first night launch of a space shuttle for four years, after being delayed for two days because of bad weather.

Discovery's launch left a fiery trail in the Florida sky

The landing date is set for 21 December back at the Kennedy Space Center.

It is as yet unclear whether an additional spacewalk would prolong the trip which is scheduled to last 12 days.

Discovery's crew consists of Commander Mark Polansky, pilot William Oefelein and mission specialists Robert Curbeam, Joan Higginbotham, Nicholas Patrick, Sunita Williams and the European Space Agency astronaut Christer Fuglesang.

A British-born astronaut, Nicholas Patrick, is among Discovery's crew. For five of the seven astronauts, this is their first shuttle flight.

One of crew, US astronaut Sunita Williams, will stay on at the ISS when Discovery heads home, taking German Thomas Reiter back to Earth.