Atlas 2AS rocket sits on the SLC-3E pad at Vandenberg in preparation to launch the Terra satellite in December 1999. Photo: NASA

Launch of a top secret national security payload originally scheduled for this weekend is being delayed to ensure that the Lockheed Martin Atlas 2AS rocket has enough fuel to successfully loft its heaviest cargo to date.

The Atlas-Centaur rocket with four strap-on solid propellant boosters had been slated for blastoff on Saturday from Space Launch Complex 3-East at Vandenberg Air Force Base in California.

However, a special independent review at Lockheed Martin earlier this month uncovered a worry about the lack of a sufficient onboard fuel reserve for the Atlas stage.

The launch, which is governed by the U.S. Air Force, will place into orbit a hush-hush cargo for the U.S. National Reconnaissance Office.

Last Friday, Lockheed Martin recommended to Air Force launch officials that additional RP-1 -- a highly refined kerosene -- be loaded into the vehicle along with changes made to the rocket's propellant utilization plan to ensure there is an adequate fuel margin left over when the sustainer engine cuts off, Lt. Col. Nancy Insprucker, the Program Manager for Medium Launch Vehicles at Los Angeles Air Force Base, said in a written response to questions about the delay.

Illustration of the Atlas 2AS rocket. The sustainer engine is the center engine nozzle on the bottom of the Atlas stage. Photo: Lockheed Martin

"The U.S. Air Force is very pleased with Lockheed Martin's proactive efforts on this crucial MLV-10 mission," Insprucker said. The mission carries the MLV-10 designation since it is one of Atlas rockets the Air Force purchased under the medium-class launch vehicle program.

As a result of the corrective actions, the duration of the daily window in which to launch the rocket will be shortened to optimize the vehicle's lift performance and offset the extra weight from the increased fuel load.

Saturday's published window extended from 1834 to 1859 GMT (2:34 to 2:59 p.m. EDT). The Air Force has not said how much of the window will be cut.

It is also unclear how long the extra work will take to complete. A new launch date had not been booked on the Western Range as of late Tuesday.

"To complete the process, the entire team will need to analyze and assess the system-level impacts of these changes, and ensure we have maximized our chances for mission success," Insprucker's statement went on to say. "After all, this is the first DoD Atlas 2AS mission from Vandenberg AFB, and the heaviest payload that an Atlas 2AS has ever flown."

The current weight record is held by NASA's Terra spacecraft launched by an Atlas 2AS rocket in December 1999 from Vandenberg. The Earth-observing satellite tipped the scales at 10,506 pounds.

It is believed the Atlas' payload limit is about 17,000 pounds for a launch from its West Coast pad.

Meanwhile, Insprucker said this delay will have a side effect of postponing the scheduled Sept. 26 launch of an Atlas 2AS rocket with an NRO payload from Florida's Cape Canaveral until early to mid October.