All but the bottom of the Atlas 2AS rocket is hidden inside the mobile service tower at pad 36A last Friday. Image taken from atop nearby pad 36B. Photo: Justin Ray/Spaceflight Now

A Lockheed Martin Atlas rocket is being disassembled and removed from a Cape Canaveral launch pad this week after its clandestine spy satellite payload was grounded until December due to a classified problem.

The Atlas 2AS rocket had been scheduled for liftoff on March 20 from the Cape's pad 36A. The pad is being cleared so a different Atlas can be prepared for launch in May carrying a U.S. weather satellite.

The U.S. National Reconnaissance Office says its secret payload to fly aboard the Atlas rocket, also known as AC-157, has encountered a problem during final pre-flight preparations.

"We have some issues with the spacecraft at this time," said NRO spokesman Art Haubold.

Details about the problem are not being disclosed, however, due to the classified nature of the satellite program.

"We don't discuss problems with the spacecraft," Haubold said.

On Monday, the U.S. Air Force established December 4 as the new launch date for the mission.

The AC-157 vehicle is comprised of an Atlas booster stage, Centaur upper stage and four strap-on solid rocket motors. Once the rocket is removed from pad 36A, workers will begin erecting a new vehicle -- AC-137 -- starting March 27.

The new rocket, an Atlas 2A with no solid rocket motors, is slated for launch on May 3 to carry the advanced GOES-L weather satellite into orbit for NOAA and NASA.

An artist's concept of an Atlas 3 rocket lifting off from Cape Canaveral. Photo: Lockheed Martin

Meanwhile, preparations remain on track at neighboring pad 36B for the maiden flight of Lockheed Martin's Atlas 3 rocket. Liftoff is planned for April 14 carrying the Eutelsat W4 communications satellite.

"We are rock solid," Adrian Laffitte, Lockheed Martin's Atlas program director, said of the Atlas 3 schedule.

The Atlas launch team plans to conduct a simulated flight test of the Atlas 3 on March 16 and a countdown dress rehearsal in which the rocket is fueled on March 23.

Eutelsat W4 will be transported from its processing facility to pad 36B for attachment to the rocket on April 5.

The launch window on April 14 will extend from approximately 5:35 to 7:48 p.m. EDT (2135-2348 GMT).

The daytime launch should provide a spectacular show for the rocket's two onboard video cameras. One is positioned on the Atlas stage's exterior looking downward; the other is mounted on the Centaur near the single engine to show the Atlas stage falling back to Earth.

Lockheed Martin developed the more powerful Atlas 3 family in the mid-1990s to attract a larger share of the commercial market by launching heavier satellites.

The Atlas 3A to be flown next month will be capable of launching satellites weighing about 9,000 pounds, an increase of 1,000 pounds over the older Atlas 2 series.

The extra boost comes from new Russian-built first stage engines that will power the rockets off the launch pad and toward space. Atlas 3 will be the first American rocket ever to use Russian engine technology.

The Atlas 3B rocket to debut next year will be even more powerful with an enhanced Centaur upper stage.