Florida ? Instead of enjoying a relaxing day at the international space station, the space shuttle Discovery’s seven astronauts prepared Sunday for an unplanned, fourth spacewalk to get a stubborn, half-retracted solar array to fold up.

Discovery’s crew prepared spacesuits, relocated the station’s robotic arm and mobile platform so they can be used during the spacewalk, and moved cargo from the station to the shuttle for the trip home.

The spacewalk by U.S. astronaut Robert Curbeam and Swedish astronaut Christer Fuglesang of the European Space Agency was to start at 12:47 p.m. CST today. NASA managers hope fixing the array problem will not take more than 4 1/2 hours, but the spacewalkers will be equipped to stay longer than that.

Discovery’s return to Earth was pushed back a day to Friday because of the extra spacewalk. Because of supply limits, the astronauts need to be on the ground no later than Saturday.