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.
A back-up computer would need to be operating for redundancy of those robotic systems.
Mission Control is deciding whether the computer can be repaired or must be replaced. Nasa is still aiming for a Monday launch from Cape Canaveral of the SpaceX cargo ship. But that could change, depending on the faulty computer.
Astronauts use the space station's big robot arm to grab on to the Dragon capsule and attach it to the outpost.
The space station is currently home to two Americans, one Japanese and three Russians.
Nasa is paying Space Exploration Technologies - or SpaceX - and Virginia-based Orbital Sciences to make space station deliveries. Russia, Japan and Europe also conduct occasional supply runs.