CAPE CANAVERAL, Fla. — Two space station astronauts ventured out on a rare Christmas Eve spacewalk today, hoping to wrap up urgent repairs to a cooling system.

It was the second spacewalk in four days for U.S. astronauts Rick Mastracchio and Michael Hopkins, and only the second Christmas Eve spacewalk in NASA history.

NASA ordered the spacewalks to revive a critical cooling loop at the International Space Station. All nonessential equipment had to be turned off when the line conked out Dec. 11, and many science experiments halted.

Mastracchio and Hopkins removed a faulty ammonia pump during Saturday’s outing. Today, they worked to install a new pump 260 miles above the planet.

“It’s like Christmas morning opening up a little present here,” Mastracchio said as he checked his toolkit.

NASA’s only previous Christmas Eve spacewalk occurred in 1999 during a Hubble Space Telescope repair mission.

But NASA’s most memorable Christmas Eve was on Dec. 24, 1968. Apollo 8 astronauts read from Genesis, the first book of the Bible, as they orbited the moon on mankind’s first lunar flight.