The five Atlantis astronauts completed a practice countdown aboard the space shuttle Monday, a major step toward launching the craft this month on a classified military mission.

The countdown clock stopped as planned at 20 seconds before zero to give the launch team and crew practice in handling a last-moment hold, NASA spokesman Hugh Harris said. The two-day simulation concluded at 11:16 a.m.While aboard, the astronauts tested communications and ran through checklists and other launch-day procedures.

The successful rehearsal cleared the way for shuttle managers to set a firm launch date. The target is Nov. 30.