At Vandenberg Air Force Base in California, launch of NASA's AIM spacecraft aboard an Orbital Sciences Pegasus XL rocket remains on schedule for 1:26 p.m. PDT on Wednesday, April 25. According to the launch forecast, there is no chance of weather violating launch criteria.

AIM Launch Day Events

Unlike other rocket launches, the Pegasus XL launch vehicle is carried aloft by an L-1011 carrier aircraft and released from the plane's underbelly at a designated "drop" point and time. For Pegasus launches, launch time is considered the "drop" time, or the time when the Pegasus is released from the aircraft.

Several key countdown events will occur during the launch countdown.

Approximately two hours before takeoff of the L-1011 carrier aircraft and its payload, the ground communications and power are disconnected between the aircraft and the "hot pad" portion of the runway.

About half an hour later, if tower clearance has been granted, the L-1011 rolls to the end of the runway and the chase planes are notified.

At L-1:03 hours, the Ground Launch Team is polled to determine if the L-1011 is ready for takeoff. If all team members report a "go," the aircraft takes off minutes later.

Thirty-six minutes before the drop, the carrier aircraft begins a turn, flying under the drop point. About ten minutes later, the L-1011 pilot reports any turbulence, winds or clouds.

The final weather briefing takes place about eight minutes before the drop. Three minutes later, the launch team is polled to determine final launch readiness. The terminal count begins one minute later, at L-4 minutes.

In the final 90 seconds before Pegasus performs the drop, the pilot acquires the necessary launch heading and the Pegasus' fin battery is activated at L-45 seconds.

Ten seconds before the drop, the launch conductor gives the official "go" for launch. When the clock ticks down to T-0, the Pegasus is released from the L-1011 aircraft.