NASA began moving Discovery to the launch pad Friday in preparation for only the second liftoff of a space shuttle since the Columbia disaster three years ago.

Discovery's trek of a little more than 4 miles was expected to take about seven hours aboard a crawler-transporter. The shuttle weighs about 6 million pounds.

Its move to the launch pad is a major step toward a liftoff sometime between July 1 and July 19.

After Discovery's mission last summer, the shuttle fleet was grounded because foam insulation was still snapping off the external fuel tank, as it did on Columbia. A chunk of foam knocked a hole in the wing of Columbia, causing the spacecraft to shatter as it returned to Earth, killing all seven astronauts.