CAPE CANAVERAL, Fla. — Spacewalking astronauts completed urgent repairs at the International Space Station on Tuesday, replacing equipment that failed three days earlier and restoring a backup for a vital data-relay system. It took commander Peggy Whitson much longer than expected to install the spare unit. Success finally came after her spacewalking partner, Jack Fischer, blasted the area with nitrogen gas to clear away flecks of metal. Mission Control noted that the failure occurred only 2 days, 21 hours, 38 minutes earlier, “and we are already back in a good position, so excellent work.