STORY WRITTEN FOR & USED WITH PERMISSIONPosted: April 10, 2010Astronauts Rick Mastracchio and Clay Anderson are gearing up for a planned 6.5-hour spacewalk early Sunday, the second of three planned by the shuttle Discovery's crew. The goals of this excursion are to install a new ammonia coolant tank on the station's power truss and to move no-longer-needed debris shielding into the station for return to Earth.Credit: NASAThis will be the 142nd spacewalk devoted to station assembly and maintenance sinceconstruction began in 1998, the sixth so far this year and the fifth overall forboth Mastracchio and Anderson.The spacewalk is scheduled to begin at 2:16 a.m. ED Sunday, when the astronautsswitch their spacesuits to battery power. For identification, Mastracchio, call sign EV-1, will be wearing a suit with red stripes around the legs. Anderson, EV-2, will be wearing an unmarked spacesuit.The International Space Station features two independent coolant loops thatcirculate ammonia through huge radiator panels to dissipate the heat generated by the lab's electrical systems. Each loop features a nitrogen pressurization system,an ammonia reservoir and internal bellows, or accumulators, that allow for theexpansion and contraction of the coolant as the station moves through sunlight and orbital darkness."The coolant that we use is ammonia, so it gets circulated by a pump, it picks upheat from all our avionics and through a heat exchanger, picks up the heat from our internal(equipment) inside all the laboratories," said Flight Director Ed Van Cise."That ammonia gets circulated out to our radiators and the radiators allow us toreject that heat from the ammonia out to the much colder space environment. That's how we keep everything cool."When the truss segments housing the two ammonia tanks were launched, "the vast majority of the system was launched dry," Van Cise said. "It had nitrogen in it, we vented the nitrogen and then we had to fill the system with ammonia. So the originaltanks that flew up full were depleted. If we
