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Astronauts on the International Space Station go outside to fix a stalled component called the mobile transporter, a rail car that moves a robotic arm up and down the length of the structure. The two crew members will spend three and a half hours on the space walk. Dr Lucy Rogers from the Council of the British Interplanetary Society explains what it involves

(PHOTO: NASA astronaut Sunita Williams on a spacewalk in 2012. CREDIT: NASA/Getty Images)