CAPE CANAVERAL -- A top-secret payload is scheduled to blast off aboard an Atlas rocket early Thursday, marking the end of an era at Launch Complex 36 at Cape Canaveral Air Force Station.

The 17-story rocket -- which is slated to take off at 2:41 a.m. Thursday -- will be the last Atlas to fly from the dual-pad complex -- home to the venerable launch vehicles since 1961.

"It's going to be sad but celebratory," said Julie Andrews, a spokeswoman for Atlas manufacturer Lockheed Martin. "There's a lot tied up in that real estate."

Just north of the tip of the Cape, the complex was built in 1961 and 1962 for NASA's Atlas-Centaur program. A total of 144 missions -- including historic NASA Surveyor, Mariner and Pioneer flights -- have been launched from the two pads there.

Thursday's scheduled flight will be the last launch of a Lockheed Martin Atlas 3 rocket, and if all goes well, the 75th consecutive success for the company's Atlas family of rockets.

Future missions will be launched on Atlas 5 rockets at Launch Complex 41.

A new tenant -- SpaceX of El Segundo, Calif. -- intends to begin launching Falcon 1 and Falcon 5 rockets from Complex 36 in late 2006.

The last Atlas 3 will carry a classified payload for the National Reconnaissance Office. But the weather forecast for the flight is not good.

A low-pressure system is expected to sweep across northern Florida from the Gulf of Mexico, bringing with it thick clouds and isolated showers. Meteorologists say there is an 80 percent chance that conditions will prohibit a launch Thursday.

The weather is expected to improve a bit on Friday. There's a 60 percent chance that thick clouds and gusty winds would prohibit a launch that day.

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