Spacewalk may be needed to fix computer breakdown on International Space Station

Nasa has confirmed a back-up computer on the exterior of the ISS is not responding to commands

A computer breakdown on the International Space Station could require astronauts to undertake a spacewalk.

Nasa confirmed on Friday that a backup computer on the exterior of the ISS called a Multiplexer-Demultiplexer (MDM) is not responding to commands.

MDM computers control some systems associated with robotics aboard the space station. The primary MDM is functioning normally, Nasa said, and Mission Control is now trying to determine whether the back-up computer can be brought back online or must be replaced.

A replacement would involve astronauts spacewalking.

It said the computer outage does not pose an immediate risk to the six Expedition 38/39 astronauts aboard the space station, who include Mikhail Tyurin, Koichi Wakata and Rick Mastracchio. They are due to return to Earth on 14 May.

But the computers control some robotic functions that would be needed for next week's planned visit by a private SpaceX supply ship.

The ship contains nearly 5,000 pounds (2267.99 kilograms)of supplies and will return to earth with 3,500 pounds of cargo, including science results for analysis.

SpaceX is supposed to launch the unmanned Dragon capsule on Monday. Astronauts use the space station's big robot arm to grab onto the Dragon capsule and attach it to the outpost.

When Dragon is captured and berthed to the Harmony node, there will be five space vehicles parked to the international space laboratory.

NASA is still aiming for a Monday launch by SpaceX, but that could change, depending on the status of the computer.