The excursion outside the station was planned in 48 hours, the shortest turnaround time yet for a spacewalk, Space.com reported. The box was believed to be the source of an ammonia leak that could cut the power available on the space station if it persists, the National Aeronautics and Space Administration said.
"It will take some diagnostics, still, over the course of the next several days by the thermal systems specialists to fully determine that we have solved the problem of the ammonia leak," NASA spokesman Rob Navias said in live commentary on the mission. "But so far, so good."
Chris Cassidy, who led the spacewalk, and Tom Marshburn exited the station at 8:44 a.m. EDT and returned just over four hours later.
NASA said the crew of the ISS was not in danger and that the station has been operating as normal, although the rate of ammonia escaping has increased since it was first reported. Ammonia is used to cool the space station's power system.
Mission Commander Chris Had field of Canada, Marshburn and Russian cosmonaut Roman Romanian are scheduled to return to earth Monday, landing in Kazakhstan. That will leave Russians Alexander Musking and Pavel Vinaigrette and Cassidy on the station.