Cape Canaveral - Two space station astronauts have floated outside on Christmas Eve in hopes of finishing urgent cooling system repairs.It's only the second Christmas Eve spacewalk in Nasa history. The first was 14 years ago. And it's the second space walk in four days for US astronauts Rick Mastracchio and Michael Hopkins."It's like Christmas morning, opening up a little present here," Mastracchio said as he checked his tool kit.Mastracchio and Hopkins removed a faulty ammonia pump at the International Space Station during Saturday's outing. On Tuesday, they'll work to install a new pump 418km above the planet.The external cooling line - one of two - shut down on 11 December. The six-man crew had to turn off all non-essential equipment, including experiments.Nasa's only previous Christmas Eve space walk occurred in 1999 during a Hubble Space Telescope repair mission.But Nasa's most memorable Christmas Eve was on 24 December 1968. Apollo 8 astronauts read from Genesis, the first book of the Bible, as they orbited the moon on mankind's first lunar flight.A Moscow-led space walk, meanwhile, is set for Friday. Two Russian crew members will install new cameras and fresh experiments outside.