NASA Astronauts Step Outside the International Space Station for Repairs

— -- For the second time in a little more than a week, Cmdr. Scott Kelly and his U.S. colleague Dr. Kjell Lindgren stepped outside the International Space Station to conduct repairs.

The pair spent a marathon seven hours and 48 minutes working outside the space station. The agenda for today's spacewalk, which was the second for both men, included work on the space station's cooling system and returned ammonia to desired levels in both the primary and back-up systems.

The duo completed their first spacewalk last Thursday, working for seven hours and 16 minutes outside the space station, which is located about 250 miles in low-Earth orbit.

Lindgren arrived in space in July, marking the flight engineer's first trip.

Kelly is halfway through a year-long stay at the Space Station as part of a study about the long-term effects on humans in space. The results of the study are expected to help NASA better prepare for one day sending humans to Mars.