The final Lockheed Martin Titan 4 rocket to launch from Cape Canaveral will soar up the U.S. eastern seaboard Friday night carrying a mysterious military payload.

Clogs in launch pad fuel lines caused by corrosion put a hold on the launch in early April, forcing technicians to replace a pump and install additional filters to overcome the problem. At last, the nitrogen tetroxide was loaded into the booster's first and second stages on Sunday and Monday to clear the way for this week's liftoff.

Launch from Complex 40 will happen sometime between 8:00 and 10:30 p.m. EDT (0000-0230 GMT), beginning a 9-minute, 30-second ascent to deliver the secret spy satellite payload into Earth orbit for the U.S. National Reconnaissance Office.

The exact target launch time remains under wraps.

Air Force weather forecasters predict better than a 90 percent chance of acceptable conditions for launch with only a slight possibility of coast showers. The outlook predicts a few clouds at 3,000 feet and scattered at 28,000 feet, good visibility, southeasterly winds at 10 gusting to 15 knots and a temperature of 70 to 72 degrees F.

Unlike virtually all past Titan 4s flown from the Cape that were fitted with Inertial Upper Stages or Centaurs, this $411 million rocket has no upper stage.

But Titan 4s lacking such an additional kick motor are typical for missions launched from Vandenberg Air Force Base, California. And, in fact, the Pentagon acknowledges moving this mission from the West Coast to Cape Canaveral about two years ago.

Given the classified nature of the payload, officials will not comment on the satellite's purpose or the exact orbit it is destined to occupy.

Space watchers have speculated that the clandestine cargo nestled inside the rocket's 66-foot long nose cone could be the fifth in a series of radar imaging spacecraft, commonly called LACROSSE.

The sophisticated intelligence-gathering craft probably use a synthetic aperture radar system to observe strategic targets around the globe in both daylight and darkness from orbital perches 420 miles above Earth. The eyes-in-the-sky can pierce clouds, detect objects a few feet across and even reveal underground structures like military bunkers.

These satellites have been launched from the Cape aboard shuttle Atlantis in 1988 and from Vandenberg on three unmanned Titan 4s in 1991, 1997 and 2000.

The first and third satellites were placed into 57-degree inclination orbits, which means the craft fly as far north and south of the equator as 57 degrees latitude. The second and fourth LACROSSEs were placed into 68-degree orbits to cover more of the planet.

Although the targeted inclination for Friday's launch has not been disclosed, hazard warnings issued to mariners and Canadian oil platforms confirm the Titan 4 is headed up the Atlantic seaboard.

"The northeast trajectory will result in a quasi 60-degree orbital inclination, similar to those of the LACROSSEs," said Ted Molczan, an experienced and respected hobbyist satellite observer from Toronto, Canada. "LACROSSE was the only Vandenberg Titan 4 payload to combine a quasi 60-degree orbital inclination and a 66-foot fairing."

There are other possibilities for the payload's identity. Two of 26 previous Titan 4s from the Cape featured no upper stages and flew into high inclination orbits, but those flights in 1990 and 1996 lofted ocean surveillance and data relay satellite cargos not thought to be strong candidates for Friday's launch.

"Both missions followed trajectories similar to the upcoming launch, but their payloads have long since been replaced by newer generations, which employed Atlas 2 and 3 boosters," Molczan noted.

Space experts are relying on hobbyists watching the skies to spot the new object following launch to determine what the Titan 4 has really carried aloft.

This liftoff ends the Titan era at Cape Canaveral after five decades of flights, including Titan 1 missiles, Titan 2 boosters launching Gemini astronauts, Titan 3s with Viking and Voyager, Titan 34D carrying critical military satellites and the past 16 years of Titan 4.

Vandenberg hosts the final Titan 4 launch in July when another hush-hush payload is deployed for the NRO. The satellite will be shrouded inside a "modified version of a standard Titan 76-foot payload fairing," according to the Air Force. Such a nose cone has never been used on the previous 11 Titan 4s from the West Coast.

The Titan name is fading into history as the Atlas 5 and Delta 4 rocket families in the Evolved Expendable Launch Vehicle program provide the U.S. government's primary heavy-lifting needs for the foreseeable future.