The secret NROL-49 spy satellite blasts off from Vandenberg Air Force Base in California atop a huge Delta 4-Heavy rocket in this photo taken at 1:10 p.m. PST on Jan. 20, 2011.

The clandestine cargo carried into polar orbit Thursday aboard the first California-launched Delta 4-Heavy rocket was a crucial replacement satellite for the nation's surveillance and security network, amateur sky-watchers say.

The sophisticated imaging bird follows a long line of Keyhole-type spacecraft that provide ultra-high resolution imagery for the U.S. intelligence community, according to hobbyists who track orbiting satellites with remarkable precision.

Ever since the Delta 4-Heavy rocket fired away from Space Launch Complex 6 at Vandenberg Air Force Base, the amateur observers have been hunting for the new satellite to figure out its identity. The conventional wisdom before the launch said the payload would fly into the Keyhole satellite constellation, and observations from the past few days proved the guess correct.

The government agency responsible for operating the country's fleet of spy satellites, the National Reconnaissance Office, doesn't publicly reveal the characteristics of the spacecraft being launched. But the satellite trackers, located around the globe, can easily determine the payload by the type of orbit each new craft reach.

Built by Lockheed Martin, the telescope-like Keyhole satellites are electro-optical imaging craft that circle the planet in elliptical polar orbits and collect pictures for U.S. national security uses. There have now been 15 such launches since 1976, all but one successfully reaching orbit, said Ted Molczan, a respected observer who keeps tabs on orbiting spacecraft. [Gallery: Spotting Satellites From Earth]

The newest satellite joins the two primary Keyholes in space today, having launched in 2001 and 2005 from Vandenberg aboard Titan 4 rockets, plus an older one deployed in 1996 that's still running in a backup role. The Delta 4-Heavy flight targeted the orbit of the 2001 satellite, apparently to assume the lead observation duties from the aging craft that's now achieved the record for the longest prime mission duration, Molczan says.