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.

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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.

Shape Created with Sketch. Nasa spacewalks in pictures Show all 11 left Created with Sketch. right Created with Sketch. Shape Created with Sketch. Nasa spacewalks in pictures 1/11 Italian astronaut Luca Parmitano participates in a spacewalk on 9 July. In official jargon spacewalks are known as extravehicular activity (EVA). Parmitano is seen here anchored to the Canadarm2 mobile foot restraint. 2/11 Nasa astronaut Chris Cassidy exits the Quest airlock during the six-hour spacewalk alongside Parmitano on the 9th. 3/11 Always time for a quick selfie. Parmitano grabs one for Facebook on the 9th. 4/11 Cassidy is seen dwarfed by some of the complex structures of the ISS during a later spacewalk on 16 July. 5/11 Parmitanto uses a digital camera on the 16th. A little more than an hour into the mission Parmitano reported water floating inside his helmet 6/11 Cassidy is seen outside during the 16 July spacewalk. Although the water in Parmitano's helmet posed no threat, Nasa decided to end the spacewalk early. 7/11 The view across the way. An image captured from 26 June of the waning gibbous moon. The picture was taken from a position 225 miles about the Eqautor near the northern coast of Africa. 8/11 Luca Parmitano (left) and Chris Cassidy (right) are seen out of their suits on board the ISS later in July. Juggling seems decidedly easier in zero-gravity conditions. 9/11 Parmitano is seen installing a new Life Support Module on the Biolab of the ISS. This will be used to study the effects of microgravity on different sorts of organisms. Growing plants in space will be one of the key steps towards permanent habitation. 10/11 It's not all astronauts that hang about outside the ISS. This pictures shows the Japanese HTV-4 unmanned cargo spacecraft. It delivered equipment to the space station and was jettisoned in August, burning up in the Earth's atmosphere during reentry. 11/11 Russian cosmonaut Alexander Misurkin is seen outside the iSS in August 2013. Misurkin is wearing the Russian-made Orlan spacesuit rather than the standard Nasa issue attire. 1/11 Italian astronaut Luca Parmitano participates in a spacewalk on 9 July. In official jargon spacewalks are known as extravehicular activity (EVA). Parmitano is seen here anchored to the Canadarm2 mobile foot restraint. 2/11 Nasa astronaut Chris Cassidy exits the Quest airlock during the six-hour spacewalk alongside Parmitano on the 9th. 3/11 Always time for a quick selfie. Parmitano grabs one for Facebook on the 9th. 4/11 Cassidy is seen dwarfed by some of the complex structures of the ISS during a later spacewalk on 16 July. 5/11 Parmitanto uses a digital camera on the 16th. A little more than an hour into the mission Parmitano reported water floating inside his helmet 6/11 Cassidy is seen outside during the 16 July spacewalk. Although the water in Parmitano's helmet posed no threat, Nasa decided to end the spacewalk early. 7/11 The view across the way. An image captured from 26 June of the waning gibbous moon. The picture was taken from a position 225 miles about the Eqautor near the northern coast of Africa. 8/11 Luca Parmitano (left) and Chris Cassidy (right) are seen out of their suits on board the ISS later in July. Juggling seems decidedly easier in zero-gravity conditions. 9/11 Parmitano is seen installing a new Life Support Module on the Biolab of the ISS. This will be used to study the effects of microgravity on different sorts of organisms. Growing plants in space will be one of the key steps towards permanent habitation. 10/11 It's not all astronauts that hang about outside the ISS. This pictures shows the Japanese HTV-4 unmanned cargo spacecraft. It delivered equipment to the space station and was jettisoned in August, burning up in the Earth's atmosphere during reentry. 11/11 Russian cosmonaut Alexander Misurkin is seen outside the iSS in August 2013. Misurkin is wearing the Russian-made Orlan spacesuit rather than the standard Nasa issue attire.

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.