听力原文： Two spacewalking astronauts replaced a giant broken flywheel outside the International Space Station on Monday, shoring up an outpost steering system plagued with problems for more than three years. The swap was considered a high priority job for Discovery's seven astronauts, who are in the midst of the first shuttle visit to the station since the February 2003 Columbia accident. The station is equipped with four of the American-made devices, each of which is about the size of a washing machine. At least two must be working properly to steer and steady the station as the 206-ton outpost travels around Earth at a speed of 5 miles per second. Otherwise, ground controllers must fire thrusters on Russian cargo freighters to do the job, exhausting limited outpost propellant reserves. What was the most important job for the astronauts of Discovery?