A computer breakdown at the International Space Station may require a spacewalk by astronauts.
Nasa said a back-up computer on the outside of the orbiting lab was not responding to commands.
The main computer, called an MDM or multiplexer-demultiplexer, is working and the six-man crew is in no danger. But the computers control some robotic functions that would be needed for next week's planned visit by a private SpaceX supply ship.
Mission Control will decide shortly as to whether the computer can be repaired or must be replaced.