NASA Astronauts Make Repairs in Space

NASA astronauts embark on a second spacewalk Wednesday in an effort to repair a vital cooling unit on the International Space Station.      
      
A first attempt by astronauts Douglas Wheelock and Tracy Caldwell Dyson on Saturday failed, despite the fact that it was one of the longest spacewalks in history.  
          
During Saturday's eight hour mission, the astronauts managed to detach the unit's hoses but a toxic ammonia leak disrupted efforts to replace the pump.    
        
This time controllers lowered the pressure in the disabled cooling line in an effort to prevent another leak.             

The station, a $100 billion project of 16 nations, has two ammonia-fed cooling loops to dissipate heat generated by equipment.

One of the systems serving the U.S., European, and Japanese laboratories broke down Saturday after a power spike.             
             
NASA says the three Russian cosmonauts and three NASA astronauts aboard the station are not at any risk.