Nasa said a back-up computer on the outside of the orbiting lab was not responding to commands. The main computer, called an MDM, is working and the six-man crew is in no danger. But the back-up computer controls some robotic functions needed for this week's planned visit by a supply ship. Mission Control is still deciding whether the computer can be repaired or must be replaced. Nasa plans to launch the SpaceX cargo ship from Cape Canaveral in Florida tomorrow.