BY JUSTIN RAY

Follow the countdown and first California launch of Boeing's Delta 4 rocket. Reload this page for the very latest on the mission.

2354 GMT (7:54 p.m. EDT)

The first stage liquid oxygen tank is one-third full as fast-fill continues.

2349 GMT (7:49 p.m. EDT)

The latest episode of Spaceflight Now's free Mission Report Minute video podcast is now available on iTunes. You can download the bulletin here.

2347 GMT (7:47 p.m. EDT)

First stage liquid hydrogen loading has transitioned to "fast-fill" mode.

2335 GMT (7:35 p.m. EDT)

The hydrogen system's cold gas chilldown conditioning has been accomplished. Liquid hydrogen propellant will begin to flow into the first stage in "slow-fill" mode. That is sped up to "fast-fill" after a small portion of the tank is loaded.

Chilled to Minus-423 degrees Fahrenheit, the liquid hydrogen will be consumed by the RS-68 main engine along with liquid oxygen during the first four minutes of the launch.

2324 GMT (7:24 p.m. EDT)

The first stage liquid oxygen chilldown is complete. Fueling will begin in the "slow-fill" mode to load a small percentage of the tank. The process then speeds up to the "fast-fill" mode until the tank is nearly full.

2321 GMT (7:21 p.m. EDT)

Weather has been deemed acceptable for pressing forward with the countdown. Thick clouds and winds will be the two areas of concern at launch time. For now, conditions are within limits.

2314 GMT (7:14 p.m. EDT)

Now inside the final four hours of the launch countdown.

2310 GMT (7:10 p.m. EDT)

The cold gas chilldown conditioning of the first stage liquid hydrogen system has started.

2305 GMT (7:05 p.m. EDT)

Chilldown of the first stage liquid oxygen system is starting. This prepares the tank and pumping to guard against shock when the supercold oxidizer begins flowing into the rocket a short time from now.

2256 GMT (6:56 p.m. EDT)

The launch team has been given a "go" to begin fueling the Delta 4 rocket with supercold liquid hydrogen and liquid oxygen!

2244 GMT (6:44 p.m. EDT)

Now four-and-a-half hours from the planned launch time. Another weather update is expected shortly.

2210 GMT (6:10 p.m. EDT)

Mission managers are continuing to monitor the situation. No decision has been made to begin fueling the Delta 4 rocket for tonight's launch.

2150 GMT (5:50 p.m. EDT)

A weather briefing is expected in about 10 minutes. Officials will decide after that how to proceed with the countdown. Fueling remains on hold.

2120 GMT (5:20 p.m. EDT)

We have posted a photo gallery featuring some great shots of the Delta 4 rocket standing atop Space Launch Complex 6. See the images here.

2109 GMT (5:09 p.m. EDT)

Countdown clocks are running for today's Delta 4 launch. But strong winds at Space Launch Complex 6 are violating the allowable limit during the time crews load propellants into the rocket. So the fueling operations are on hold pending improvements with the weather.

2050 GMT (4:50 p.m. EDT)

Launch activities are proceeding at Vandenberg. However, winds have become very strong at the pad. The situation is under discussion among mission managers before approval is given to start fueling the rocket this afternoon.

1745 GMT (1:45 p.m. EDT)

The official launch time has been publicly announced by the Air Force. The Delta 4 rocket is scheduled for liftoff at 8:14 p.m. PDT (11:14 p.m. EDT; 0314 GMT).

1630 GMT (12:30 p.m. EDT)

It is a beautiful morning at Vandenberg Air Force Base where the mobile service tower has been wheeled away from the Delta 4 rocket for tonight's liftoff. The 20-story white and orange vehicle stands ready for the start of the countdown and fueling operations this afternoon.

MONDAY, JUNE 26, 2006

The inaugural West Coast launch of Boeing's Delta 4 rocket passed the final readiness reviews Monday, affirming plans for liftoff Tuesday evening carrying a classified national security spy satellite from Vandenberg Air Force Base, California.

Liftoff will happen sometime between 7 and 9 p.m. PDT (10 p.m. and midnight EDT; 0200-0400 GMT). The actual launch time and the available window to get the rocket airborne fall within that two-hour period. The real liftoff timing remains hush-hush for now.

Crews at Space Launch Complex 6 retracted the 27-story tall Mobile Assembly Shelter on Monday. This structure, which moves along rails, was built in the 1980s when the pad was being prepared for space shuttle launches. It was designed to shield the shuttle from the weather during on-pad assembly of the vehicle's solid rocket boosters, external tank and mating of the orbiter.

The rocket now stands partially visible at the pad, like it would at the Delta 4 complex at Cape Canaveral that does not feature a mobile shelter. But the Mobile Service Tower remains in position next to the vehicle with its 360-degree access platforms still in place. That 425-foot tall tower, heavily renovated from the shuttle era design, will be rolled back for launch on Tuesday morning.

Countdown clocks will enter an hour-long hold at the T-minus 5 hour, 15 minute mark in the early afternoon. During this time, the entire launch team and mission managers will report for duty at their consoles and verify all systems are ready for proceeding with the Terminal Countdown.

Fueling of the Delta 4's first and second stages with supercold liquid oxygen and liquid hydrogen propellants will get underway in the final five hours prior to launch time. It takes roughly three hours to fully load all four tanks on the rocket.

Next, testing of communications links between the rocket and Air Force Western Range will occur. Steering checks of the first stage RS-68 engine and second stage RL10 powerplant are performed about an hour before liftoff.

A 15-minute build-in hold is slated for T-5 minutes, during which time teams will go through final polling for launch clearance. As the count resumes, rocket systems will transition to internal power, the ordnance armed and all four cryogenic fuel tanks secured for flight.

At T-minus 22 seconds, the steering system for the twin strap-on solid rocket motors will be activated. About 8 seconds later, sparklers are fired beneath the RS-68 main engine to burn away excess hydrogen before ignition.

The computer sequencer assumes control of the countdown at T-minus 8.5 seconds. Command to light the main engine comes at T-minus 5.5 seconds, creating a large fireball around the base of the rocket from the hydrogen-rich ignition of the powerplant. The engine begins to produce thrust at T-minus 3.3 seconds and should reach full power by T-minus 1 second to undergo the computer-run check of key parameters.

The solid motors fire to life at T-0 as four hold-down bolts pop apart to release the 20-story rocket for its liftoff from Space Launch Complex 6.

1645 GMT (12:45 p.m. EDT)

The final launch readiness reviews will be conducted today to confirm everything is prepared to begin the countdown tomorrow morning at Vandenberg Air Force Base, California.

Meanwhile, the latest weather forecast issued this morning is slightly worse than the previous outlook. There is a 70 percent chance of acceptable conditions for launch. Clouds and winds are the two worries. See the full forecast here.

SUNDAY, JUNE 25, 2006

The final piece of Boeing's new generation Delta 4 rocket fleet makes its long-awaited debut this week when a booster blasts off from the U.S. West Coast for the first time, punctuating the company's work to develop a line of launchers and build pads in Florida and California.

Conceived in the Air Force's Evolved Expendable Launch Vehicle program to field new American rockets to carry government payloads for the next two decades, the Delta 4 has flown five times in its medium- and heavy-lift configurations from Cape Canaveral, Florida. Now, the maiden mission from California's Vandenberg Air Force Base is poised for launch Tuesday evening.

The payload for Tuesday's mission is a classified spacecraft for the U.S. National Reconnaissance Office, the secretive government agency responsible for operating the nation's network of spy satellites. The Delta 4 will haul the craft into a highly inclined, highly elliptical orbit.

Liftoff will happen sometime between 7 and 9 p.m. PDT (10 p.m. and midnight EDT; 0200-0400 GMT). The actual target launch time has not been revealed.

The weather outlook predicts an 80 percent chance of meeting the launch rules. But low clouds and fog could ruin the view of liftoff. See the full forecast here.

Read our launch preview story.