Astronauts ready for walk

Two astronauts are preparing to venture outside the International Space Station for an emergency spacewalk to replace a failed computer, one of two which control major American systems aboard the orbiting outpost.

The primary device failed on Saturday, leaving the orbiting laboratory to depend on a backup system to route commands to its solar power system, radiators, cooling loops and other equipment.

The station flies about 400 km above Earth and orbits the planet about every 90 minutes.