A Lockheed Martin Titan 4B rocket, America's most powerful unmanned launcher, will haul a top-secret spy satellite into orbit this week from Vandenberg Air Force Base in California. Experts believe the cargo is some sort of imaging spacecraft, possibly a powerful eye-in-the-sky KeyHole satellite.

A Titan 4B rocket sitting atop Vandenberg's SLC-4E pad awaiting liftoff last August. Photo: Justin Ray/Spaceflight Now

Blastoff from Space Launch Complex-4 East is scheduled for approximately 2119 GMT (5:19 p.m. EDT; 2:19 p.m. PDT). The duration of the available launch window remains classified. However, the Air Force says the launch won't occur after 2300 GMT (7 p.m. EDT; 4 p.m. PDT).

Although the identity of the Titan 4B rocket's cargo is classified, three available clues have allowed observers to make a good guess.

Following liftoff, the Titan 4B rocket will head southward on a planned 189.9-degree flight azimuth. Such a trajectory would place the cargo into a sun-synchronous orbit just like image-taking spy satellites use, such as KeyHole spacecraft.

The $350 million rocket -- a two-stage, liquid-fueled core vehicle with two solid-propellant motors -- features a 66-foot long nose cone, or payload fairing, sources say. This fairing size has been used during the previous launches of the KeyHole photo-electronic reconnaissance satellites.

Thirdly, Ted Molczan, an experienced hobbyist satellite observer from Toronto, Canada, determined that the orbit of an aging KeyHole satellite passes over Vandenberg at Monday's launch time. That means in all likelihood the Titan rocket will launch into the orbital plane of the KeyHole known as USA 116 to replace the six-year-old craft.

Thought to have been built by Lockheed Martin and TRW, KeyHole satellites resemble the Hubble Space Telescope with a long barrel and solar wings. They have a propulsion module with engines and fuel on back for orbital maneuvering.

These powerful orbiting eyes are likely capable of seeing objects as tiny as a couple inches across in visible light, and probably have some sort of infrared and thermal imaging capabilities, experts say.

The KeyHoles are important national security tools, providing the U.S. government and its allies with daily global coverage.

Other possibilities for the payload's identity include a sister-satellite to the mysterious spacecraft launched by a Titan 4 in 1999 or the Lacrosse radar imaging craft carried aloft from Vandenberg. But those two theories are squashed by the fact that the mystery mission featured a much shorter 50-foot nose cone and Lacrosse satellites don't fly in sun-synchronous orbits.

Of course, there is always the chance this is a first-of-its-kind satellite. But the trajectory, fairing size and launch time all point to KeyHole for this Titan 4 mission.

Watch our Mission Status Center for play-by-play live coverage during the countdown and launch!