On Tuesday, a pair of Discovery astronauts installed a two-tonne truss on the ISS during the first of three space walks in the 12-day mission that began Saturday.

Astronauts on Wednesday were trying to retract a 115-foot (35-meter) solar array on the ISS that had been unfurled for more than six years, when they encountered a kink, NASA said.

Some 20 minutes after starting to refold the array, the system apparently jammed. Mission Control Center at Houston, Texas, was examining the options, a commentator on NASA television said.

"A map never goes back to the way you bought it, it just doesn't," said John Curry, flight director for the space station.

A partial refolding of the array is necessary to allow a new solar array, delivered and installed on the ISS in September, to rotate to track the sun.

The new array will double the space station's electricity supply and is expected to be activated Wednesday once another array is retracted.

At that point the extension and rewiring of the space station can begin in two highly complex space walks, scheduled Thursday and Saturday.

Discovery mission specialist Robert Curbeam and Sweden's first astronaut Christer Fuglesang, of the European Space Agency, performed the first space walk and were tapped for Thursday's task.

The astronauts are tasked with reconfiguring and rewiring the electricity and climate control of the US-made portion of the ISS from its present, temporary set-up.

During the work, power to half of the ISS will be switched off.

On Saturday Curbeam and mission specialist Sunita Williams will install cameras outside the ISS expected to greatly facilitate future construction work.

A minor hiccup in the Discovery construction mission, apparently without serious consequences, happened during the first space walk, when Fuglesang lost an extender on a tool and it floated off in space, Tricia Mack, director of the space walk, told a news conference late Tuesday.

The girder-like ISS is being assembled piece by piece. Construction resumed in September with the Atlantis mission, after a three-year hiatus following the 2003 Columbia disaster.

The Discovery mission is part of 14 shuttle flights NASA has planned over the next four years to finish the ISS by 2010, when the shuttle fleet, down to three vehicles, is to be retired.

Discovery blasted off late Saturday from Cape Canaveral, Florida -- the first nighttime liftoff in four years. It docked on the station Monday and is to remain there eight days.

National Aeronautics and Space Administration engineers said two preliminary inspections carried out after takeoff and shortly before Discovery docked with the ISS found no damage to the Discovery's heat shield during launch.

Such inspections on the shuttles have become routine since the Columbia tragedy.

Columbia's heat shield was pierced by foam insulation that peeled off its fuel tank during liftoff, causing the shuttle to disintegrate during its return to Earth in February 1, 2003, killing all seven astronauts on board.

All rights reserved. copyright 2018 Agence France-Presse. Sections of the information displayed on this page (dispatches, photographs, logos) are protected by intellectual property rights owned by Agence France-Presse. As a consequence, you may not copy, reproduce, modify, transmit, publish, display or in any way commercially exploit any of the content of this section without the prior written consent of Agence France-Presse.