US astronauts on risky spacewalk to fix cooling system

Two US astronauts stepped out on a risky spacewalk Friday to complete the repair of an ammonia cooling system at the International Space Station. Highly toxic ammonia is used to cool electronics at the orbiting outpost, and the thermal system has been plagued by problems. Scott Kelly and Kjell Lindgren switched their spacesuits to battery power at 6:22 am (1122 GMT) and floated outside the orbiting outpost moments later, according to a live broadcast on NASA television.