Atlas 2AS rocket blasts off from the SLC-3E pad at Vandenberg with the classified NRO payload. Photo: Karl Ronstrom for International Launch Services SEE LARGER IMAGE

A Lockheed Martin Atlas 2AS rocket lumbered off its foggy launch pad Saturday morning carrying its heaviest cargo ever, a hush-hush spy satellite payload that observers suspect will eavesdrop on enemy ships sailing across the vast oceans of planet Earth.

Liftoff occurred at 1525:05 GMT (11:25:05 a.m. EDT; 8:25:05 a.m. PDT) from Space Launch Complex-3 East at Vandenberg Air Force Base in California, one minute later than originally announced by officials.

After a relatively smooth countdown, the Atlas' liquid-fueled main engines roared to life and two of its strap-on solid rocket motors ignited to propel the vehicle skyward. Vandenberg's trademark fog and low clouds hung over the launch pad, swallowing the rocket as it cleared the tower.

Remarkably, this was Vandenberg's first space launch of 2001. Several other rocket flights earlier in the year were delayed due to technical troubles.

The Atlas followed its planned trajectory, heading south-southeast from the Central California coastline over the Pacific Ocean. Within five minutes the Atlas stage and solid boosters had jettisoned, leaving the high-energy Centaur upper stage to deliver the U.S. National Reconnaissance Office cargo into a transfer orbit around Earth.

Just under 11 minutes after liftoff, the Centaur had completed its first of two firings and settled into a quiet coast.

The Atlas rocket punches through the clouds over the launch pad. Photo: Tech. Sgt. Rodney Jones/USAF SEE LARGER IMAGE

After cruising over the Pacific for about a half-hour, the vehicle swooped over the southern tip of South America before ascending across the Atlantic and then the Indian Ocean on a northeasterly track.

Sixty-four minutes into the flight saw the twin Centaur engines re-ignite for a short 12-second burst to achieve the planned orbit to release the cargo. Separation occurred at T+plus 68 minutes.

"Everything has been as expected on this flight," Lockheed Martin launch commentator Don Spencer announced. "All the folks here at Vandenberg Air Force Base are thrilled."

While virtually nothing is officially known about the payload's identity, experts and hobbyists alike agree the cargo is most likely associated with the current orbiting network called the Space-Based Wide Area Surveillance System.

Civilians more commonly refer to the network as the Naval Ocean Surveillance System, or NOSS. These clusters of spacecraft, which launched together in one package only to split apart in orbit, are used to spy on ships at sea by picking up radar and radio transmissions. The clusters fly in formation like the points of a triangle to accurately determine the locations of enemy vessels.

Based upon the available launch information such as the rocket's planned trajectory and the daily liftoff window, the payload will likely fill a gap in the surveillance system constellation, said Ted Molczan, an experienced hobbyist satellite observer from Toronto, Canada.

See our launch preview story for a full explanation of the system, its current satellites and reasoning behind the experts' guess of the payload's identity.

Another aerial view shows the ground-lit solid rocket boosters burning out and the air-lit motors igniting on the Atlas as it streaked downrange with the NRO payload. Photo: ILS TV/Spaceflight Now

Unraveling the mystery now rests on the shoulders of satellite observers around the world who are looking for the spacecraft. Acting as detectives, the network of watchers will gather clues about the payload by studying its behavior, specifically when it maneuvers from the current orbit achieved during launch into an operational altitude.

If it takes up residence in the current surveillance system, then the mystery will be solved.

The remaining key will be watching for the payload to separate into the triplet cluster. In the past, that event has occurred a few weeks after launch. The triangle is readily visible in binoculars, and sometimes is bright enough to be seen with the naked eye as the three objects pass overhead in perfect formation.

But it is suspected this may be the first of a new generation of spacecraft for the Space-Based Wide Area Surveillance System, giving rise to the possibility of a new satellite design that might scale down the triad into one craft.

The NRO has two more launches planned in the next 32 days -- a heavy-lift Titan 4 rocket from Vandenberg Air Force Base on October 1 and another Atlas 2AS rocket from Cape Canaveral Air Force Station in Florida on October 10.

Today marked the third of five Atlas 2AS launches for the NRO, all of which have been successful. In addition to the one next month, the final mission in the series on the books is slated for next September from Vandenberg.

Overall, Saturday's launch was the 568th for the venerable Atlas rocket and the 23rd to fly in the Atlas 2AS configuration using strap-on solid rocket motors. It was the 57th consecutive successful Atlas flight dating back to 1993.

This mission was managed under the auspices of International Launch Services, the joint U.S. and Russian endeavor formed in 1995 to commercially market the Atlas and Proton rockets.