Shuttle astronauts make final spacewalk

Discovery astronaut Michael Lopez-Alegria rides the shuttle arm

Astronauts from the space shuttle Discovery are preparing to simulate two space accidents for their fourth and final spacewalk.

Using a small jet pack, Jeff Wisoff and Michael Lopez-Alegria will test whether an astronaut can turn himself well enough to return to the shuttle after floating away.

"They want to do it, and we want them to do it," said Daryl Schuck, the lead spacewalker in mission control.

The astronauts are preparing the International Space Station for a permanent crew.

On Wednesday, the alternate pair of spacewalkers, Leroy Chiao and Bill McArthur, re-routed cables and installed power converters ahead of the arrival of large solar-panelled "wings".

The astronauts were out of Discovery for almost seven hours.

Discovery's crew has already added two segments to the ISS

"It's just amazing to look at your feet and see clouds going by," said Bill McArthur some 370 kilometres (230 miles) above the Earth.

The pair were working on the Z-1 truss, a metallic structure which houses gyroscopes and radio communications equipment.

Solar energy

It will be the base for solar panels, each more than 30 metres (100 feet) long, longer than the space station itself.

Converters will regulate the electricity generated by its photoelectric cells to a steady 125 volts for use by the station.

When eight panels are in place, the space station will be large enough to cover a football field. The panels will supply energy equivalent to the electricity needed to power 55 homes.

The ability to do energy-intensive science sets the ISS apart from stations like the US Skylab and Russia's Mir.

Discovery's 11-day mission is the 100th for the US shuttle programme and the last before the International Space Station becomes permanently occupied.

The Expedition One crew of two Russians and their American commander is set to arrive in early November.

The station is a joint effort by 16 countries including the United States, Russia, Japan, Europe and Canada.