The historic launch -- likely an emotional moment for aerospace workers -- is scheduled between 9 a.m. and 1 p.m. from Space Launch Complex 4-East, a visible beacon on the base's southern shoreline.

Lockheed-Martin's Titan 4-B, for years the nation's largest and most powerful unmanned rocket, is being retired to make way for a new brand of booster known as the EELV, or Evolved Expendable Launch Vehicle. Those rockets, Boeing's Delta 4 and Lockheed's Atlas 5, will also be launched from Vandenberg.

Titans began their storied history at Vandenberg in 1961, with the launch of a Titan 1 from an underground silo, as opposed to the standard above-ground launchpads now used for the rocket. Since then, 199 Titan boosters have gone into space from Vandenberg. If weather and technology cooperate, Wednesday's launch will be the 200th Titan locally.

As with each of the previous Titan 4 launches from the coastal base, this final mission is classified, so the true nature of its payload remains closely held by the military. Even the exact launch time will not be revealed to the media until shortly before blastoff.

But the massive rocket, some 200 feet tall, will not go quietly into the history books. The cacophony from the powerful engines has been known to rattle windows as far away as the Santa Ynez Valley, and its ascent skyward will be easily visible around the county if skies are clear.

"This will be a historic day for Vandenberg Air Force Base and the Central Coast of California," said Col. Jack Weinstein, commander of the 30th Space Wing, in a statement. "The Titan program has been critically important for our nation and we are fortunate to be able to close out this chapter with a final launch of this important system."

The rocket will carry a payload for the National Reconnaissance Office.

After the initial Titan 1 launch as a converted two-stage intercontinental ballistic missile some 44 years ago, the family of boosters expanded over the years to include the Titan 2, Titan 3, Titan 34D and several versions of Titan 4's. The so-called "heritage" rockets have all been phased out. The last East Coast Titan 4 was launched in April this year.

Initially envisioned as a backup for the space shuttle fleet, the Titan 4 is estimated to cost about $400 million and routinely carries payloads believed to cost about $1 billion each -- making it the most expensive rocket to fly from the base.

Since the first West Coast Titan 4's ear-shattering engine noise boomed through the night silence at 4:03 a.m. on March 8, 1991, there have been 11 Titan 4 launches from Vandenberg. The booster's heyday peaked in the 1990s with nine launches, including one explosion offshore 101 seconds after liftoff on Aug. 2, 1993. The most recent Titan 4 from Vandenberg was in October 2001.

In those years, which saw a total of 38 Titan 4 launches between Vandenberg and Cape Canaveral, hundreds of workers locally had some involvement on the rocket program -- readying the highly visible launchpad near Surf Beach, installing delicate satellite systems, guarding classified information about the launch or taking part in the highly orchestrated launch countdown.

The system also meant employment for about 400 Lockheed workers at Vandenberg. While some are likely to remain to work on the new rocket system or to phase the old system out, many workers will certainly be out of jobs once the Titan ascends to space.

Initially, the entire Titan 4 program was shrouded in mystery. Envisioned in 1986, its first launch came five years later under cloak of darkness. The classified nature of the Titan missions -- and finding out when one was planned -- became a cat-and-mouse game between the military and space enthusiasts. In those early years, when the government refused to even acknowledge a launch was happening, space watchers would look for clues in everything from mariners' warnings to suddenly full hotels, to hastily rescheduled sports games on base. Jalama Beach is also routinely evacuated during Titan launches.

Although the government kept liftoff times heavily guarded, popular viewing sites around the Lompoc Valley would suddenly become swamped with military and aerospace workers about a half-hour before a Titan flight.

Finally, in 1993, the Defense Department lifted the Cold War veil of secrecy, easing security restrictions.

Vandenberg's Titan payloads have been highly used, space experts estimate, for gleaning international data for the Central Intelligence Agency and other top-level government agencies. Satellite trackers around the world could look at a Titan's trajectory and make educated guesses about its payload. The Titans usually carried satellites known as Lacrosse, Keyhole and White Cloud.

In the past, a Keyhole provided the U.S. with its first evidence of the Iraqi invasion of Kuwait, while others could see through clouds, observe objects at night, make images of objects as small as 4 inches and provide tracking of ships, planes and troop movements around the clock.

In the early 1990s, the Air Force had hoped to convert Vandenberg's $3.5 billion Space Launch Complex-6, the former space shuttle pad, into another Titan 4 launch spot. But no defense funds were allocated for the conversion, ending expectations for two Titan 4 launches annually.

For as much excitement that it has prompted in the space world, the final Titan mission has also disrupted two other rocket launches. The private firm Space Exploration Technologies moved its inaugural Falcon rocket launch from Vandenberg to Kwajalein in the Marshall Islands after repeated delays with the Titan mission created problems with its launch timeline.

Boeing's debut Delta 4 launch, set initially for late September, has been pushed to November because of various booster issues. The classified Delta payload had to be moved into a safe area at Space Launch Complex-6 in case the Titan 4 explodes after liftoff from the nearby pad and puts it in harm's way.