Check out our photo gallery showing the rocket on the eve of launch.
The classified mission to launch a surveillance satellite for the National Reconnaissance Office was postponed from last week to verify the deployable nozzle system on the vehicle's upper stage. Once the first stage drops away just over four minutes into flight, the carbon-carbon extendible nozzle that is stowed during liftoff slides down into place on the RL10B-2 engine for ignition.
During pre-launch closeouts of the barrel-like interstage compartment between the two stages, where the nozzle is housed, an operation caused some unplanned movement on the nozzle system. Inspections and assessments conducted the past few days have ensured there was no harm done to the flight hardware.
At a Monday afternoon gathering of launch officials, the final review was completed and approval granted to move into the countdown activities overnight.
Retraction of the mobile service gantry from the Delta 4 to reveal the rocket will occur before dawn. The Terminal Count commences at 9:42 a.m. local (12:42 p.m. EDT) and cryogenic fueling of liquid hydrogen and liquid oxygen into the two-stage booster begins a short time later.
Tuesday appears to offer the best weather conditions for the next several days. Forecasters are predicting a 60 percent chance of allowable winds for the rocket to fly.
The launch forecast time predicts a broken deck of stratus clouds in the marine layer at 500 feet, scattered high cirrus clouds at 25,000 feet, visibility of 5 to 7 miles with mist, a temperature between 58-63 degrees F and northwesterly winds of 12 to 18 knots, gusting to 22 knots. Upper level winds will max out at 80 knots from the west around 37,000 feet.
For the backup launch shot on Wednesday, winds will grow stronger and gust as high as 35 knots, dropping the probability of acceptable launch weather conditions to just 10 percent.
The previous upper stage engine problem that slipped the last from last week has not yet been cleared after all, and the team needs more time to work the concern.
"The team is continuing to review data from an observation on the upper stage engine and requires additional time to complete its final assessment prior to launch," ULA said in a statement.
Launch is scheduled for 4:04 p.m. local time (7:04 p.m. EDT; 2304 GMT).
"This will be the first ever launch of the Delta 4 Medium+ (5,2) configuration, which houses the payload in a five-meter-diameter payload fairing and is boosted by two added solid rocket motors," said Lt. Col. Brady Hauboldt, the 4th Space Launch Squadron commander and the Air Force launch director.
It is the lone member of the five-configuration Delta 4 family that hasn't been used in the program's 18 previous launches from Florida and California. The most recent launch in January flew a close comparison, but it had the maximum number of four strap-on boosters for extra thrust off the pad instead of just two needed for this upcoming mission.
The payload's size likely drove the mission planners to pick a Delta 4 with the roomier nose cone size of five meters versus the other option of four meters in diameter.
Read our overview story on the rocket configuration and Delta 4 family.
"Vandenberg has a long history launching our most precious payloads and this launch is no exception. The men and women of the 30th Space Wing have been incredibly focused on mission assurance over the past year to prepare for a successful orbital insertion of this NRO satellite," said Hauboldt.
The rocket will fire away from the planet to propel the payload into an unusual retrograde orbit around the Earth. It's smokey contrail should be visible throughout the area, and spectators are invited to see the liftoff from Surf Beach.
The official Notices to Airmen, or NOTAMs, released to the public before launch to establish clear warning zones around the rocket's flight path indicate the booster's trek downrange is headed southwestward from Vandenberg Air Force Base.
This distinct trajectory out over the Pacific Ocean will result in delivering its classified satellite cargo into a retrograde orbit flying against Earth's natural rotation.
One other recent Vandenberg launch took such a course in September 2010 when an Atlas 5 rocket deployed the first in a new-generation of radar surveillance spacecraft that provide imaging of targets in daylight or darkness, can see through cloud cover and resolve structures beneath the ground's surface.
The National Reconnaissance Office, the customer for this launch and operator of the country's fleet of spy satellites, does not announce the purpose of the spacecraft being sent up on each launch. The flight is known only as the NROL-25 mission.
"From launching and operating the most technically-capable systems to continued operations of legacy satellites the NRO remains the premier space reconnaissance organization in the world," Betty Sapp, the NRO's principal deputy director, said in recent testimony before Congress.
Read our our full pre-launch story that looks at the NRO's work.
Watch this page for continuing pre-flight status and live coverage throughout Monday's countdown.
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The forecast at launch time predicts scattered high cirrus clouds at 25,000 feet, visibility of 7 miles, a temperature between 58-63 degrees F and west-northwesterly winds of 12 to 18 knots, gusting to 22 knots. Upper level winds will max out at 60 knots from the west around 35,000 feet.
For the backup launch shot on Tuesday, similar winds are expected and the odds are 60 percent for out-of-limit winds or 40 percent for favorable conditions.
Check out our photo gallery.
The forecast at launch time calls for scattered high cirrus clouds at 25,000 feet, visibility of 7 miles, a temperature between 58-63 degrees F and west-northwesterly winds of 12 to 18 knots, gusting to 22 to 25 knots. Upper level winds will max out at 55 knots from the west around 45,000 feet.
For the backup launch shot on Tuesday, the odds are 60 percent for out-of-limit winds or 40 percent for favorable conditions.
A low pressure system will be entering the California launch base area tomorrow, bringing rain showers and strong winds. It should clear out Sunday, but the winds will persist into next week.
The outlook at launch time calls for scattered high cirrus clouds at 25,000 feet, visibility of 7 miles, a temperature between 60-65 degrees F and west-northwesterly winds of 12 to 18 knots, gusting to 22 to 25 knots. Upper level winds will max out at 45 knots from the west around 47,000 feet.
For the backup launch shot on Tuesday, the high pressure will continue to dominate over the base with winds expected to decrease by early morning and then pick up again in the afternoon.
Although fog is not a constraint for launch, it could ruin the view for spectators at the base.
The concern for the 3:38 p.m. local time liftoff opportunity will be gusty ground winds, giving only a 30 percent chance of conditions being within limits.
The outlook at launch calls for the marine layer creating a ceiling at 500 feet, visibility of 5 to 7 miles, a temperature between 60-65 degrees F and west-northwesterly winds of 12 to 18 knots, gusting to 22 to 25 knots. Upper level winds will max out at 40 knots from the southwest around 20,000 feet.
For the backup launch shot on Saturday, a low pressure system will be moving into the area. The result is a 100 percent chance of weather preventing the launch due to rain, winds and cumulus clouds.
Check out our photo gallery from around the launch site today.
News of the delay arrived while photographers were at the pad setting up their sound-activated cameras to capture the blastoff.
Crews at Space Launch Complex 6 have retracted the 27-story mobile assembly shelter away from the rocket. This 9-million-pound 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 shelter remains in use for the Delta 4 program to provide pleasant launch pad work conditions inside what amounts to a large building, instead of an open-air complex where technicians battle the weather.
In the predawn hours on Friday, the 32-story, 13-million-pound mobile service tower that actually wraps around the Delta 4 to provide workers full access to the rocket will be rolled back to fully unveil the space launcher.
This will be the Delta 4 rocket's fourth flight from Vandenberg and follows up the family's first Heavy mission from California last January. It will be the National Reconnaissance Office's sixth use of the rocket, including missions performed from Florida's Cape Canaveral.
Also see this photo gallery of the payload being mounted atop the rocket earlier this month.
Although windy conditions could be a problem for a 3:38 p.m. local (6:38 p.m. EDT) launch Friday, the public is encouraged to witness the blastoff from Surf Beach, a mere seven miles north of the pad. The rocket will thunder skyward on a pillar of fire and smoke, headed to the southwest on the trek to reach a retrograde orbit around Earth to deploy a surveillance satellite for the NRO.
"During close out for launch, the team observed an upper stage engine condition that will require additional time for review. The launch is set for Friday, March 30 from Space Launch Complex 6 at 3:38 p.m. PDT."
Check back later for more details.
The outlook at launch calls for some mostly cloudy skies with high cirrus at 25,000 feet, 7 miles of visibility, a temperature between 60-65 degrees F and northwesterly winds of 12 to 18 knots, gusting to 22 to 25 knots. Upper level winds will max out at 50 knots from the west around 40,000 feet.
For the backup launch opportunity on Friday, meteorologists are predicting much of the same and identical odds.
Read our full story.
The outlook for the 3:30 p.m. local time launch calls for some broken high cirrus clouds at 25,000 feet, 7 miles of visibility, a temperature between 59-64 degrees F and the northwesterly winds of 12 to 18 knots, peaking to 22 to 25 knots. Upper level winds will max out at 50 knots from the west around 37,000 feet.
For the backup launch opportunity on Friday, meteorologists have improved the odds of acceptable winds from a mere 10 percent in yesterday's forecast to 30 percent in today's prediction.
Read our full story.
Eventually, the whole idea of launching the shuttle from there was cancelled, but the pad lives on and now supports the unmanned Delta 4.
So check out these comprehensive photo collections that document Enterprise's time at Vandenberg beginning with a tour of the shuttle inside its hangar on North Vandenberg.
The shuttle then took a road trip over Vandenberg's hilly terrain and reached Space Launch Complex 6 for attachment to the external fuel tank and solid rocket motors at the pad.
Once the vehicle was put together, the pad structures were retracted to unveil the fully assembled shuttle. There's even a page of nighttime views with Enterprise.
The launch control center was located right at the pad, too. Take a look inside.
And one last page with more beautiful views of the West Coast space shuttle.
The photos are a mixture from William G. Hartenstein and the U.S. Air Force. Please check the captions on each page for credit information.
In advanced of the first Delta 4-Heavy rocket launch last year, we published a special feature story about the history of Space Launch Complex 6 from manned spaceflight projects to the nation's unmanned heavy-lifter.
A low pressure system will sweep through the area Wednesday, bringing some rain showers to the launch site, but that weather should push out and take low-level clouds with it by Thursday morning. As the launch time approaches, high pressure will assume control with only high cirrus clouds. However, northwesterly winds of 12 to 18 knots, gusting to 22 knots are expected for launch.
With that outlook, Air Force meteorologists say there's a 70 percent chance the winds will be too strong to permit a launch.
The forecast for the 3:30 p.m. local time launch calls for some broken high cirrus clouds at 25,000 feet, 7 miles of visibility, a temperature between 58-63 degrees F and the northwesterly winds of 12 to 18, peak 22 knots. Upper level winds will max out at 50 knots from the west around 37,000 feet.
For the backup launch opportunity on Friday, the winds will be a bit stronger and the odds of violating the weather rules jumps to 90 percent. High pressure will remain in place over Vandenberg, but surface winds will be out of the northwest at 15 to 20 knots with gusts up to 25 knots.
Watch this page for continuing pre-flight status and live coverage throughout Thursday's countdown.
And if you will be away from your computer but would like to receive occasional countdown updates, sign up for our Twitter feed to get text message updates on your cellphone. U.S. readers can also sign up from their phone by texting "follow spaceflightnow" to 40404. (Standard text messaging charges apply.)
And check out Spaceflight Now's Facebook page!
The Air Force has unveiled a target liftoff time of 3:30 p.m. local (6:30 p.m. EDT; 2230 GMT) for the towering white-and-orange rocket to haul a critical national security satellite into space. The exact duration of the day's available launch window remains a secret.
"Today, the 4th Space Launch Squadron in final preparations for Thursday's Delta 4 launch, along with our mission partners from United Launch Alliance, the Aerospace Corp., Space and Missile Systems Center and the National Reconnaissance Office," Lt. Col. Brady Hauboldt, the 4th Space Launch Squadron commander, said in an interview Friday.
"I'm incredibly proud of the men and women of the 4th Space Launch Squadron team in preparing for this Delta 4 mission. We've worked side by side with United Launch Alliance and the entire 30th Space Wing to pull this campaign together over the past few months and are approaching a final 'go' for launch. We're all excited to have the opportunity to launch another Delta 4 and NRO spacecraft, demonstrating once again 30th Space Wing's continued contributions to space power," said Hauboldt, who also serves as the Air Force launch director.
The Delta 4 mission has passed the 14th Air Force's operational readiness review and the Space and Missile Systems Center's flight worthiness review earlier this week as the scheduled launch date approaches.
Out at the launch pad, activities currently underway include final compartment closeouts on the rocket and removal of ground support equipment. Rollback of the building-like mobile assembly shelter is planned for Tuesday, followed by final safe-and-arm connections and main engine ignitor electrical connections. Retraction of the pad's mobile service tower from around the rocket occurs before dawn Thursday.
"Our final technical reviews and inspections will continue into the weekend and next week, leading to 30th Space Wing's launch readiness review Wednesday afternoon," Hauboldt said.
The United Launch Alliance-made rocket arrived at Vandenberg on October 28 inside the ocean-going Delta Mariner cargo vessel after a 23-day voyage from the production facility in Decatur, Alabama. The ship had waited offshore while a Delta 2 rocket soared over the Pacific from the base on a NASA launch before coming into the harbor for unloading.
The first and second stages were taken to the Horizontal Integration Facility at Space Launch Complex 6 where they were checked out and then mated together last fall in preparation for rollout to the pad. The vehicle was hydraulically erected onto the launch table January 5.
"And since January, we've had a remarkably smooth processing flow...During that time, we've completed 1,400 individual technical risk assessments are part of our overall flight worthiness certification process," said Hauboldt.
"It's gone exceedingly well, really the best I've seen since I've been here the last three years."
A simulated launch day to put the Delta 4 rocket and the ground systems through a fueling exercise occurred successfully about a month ago, giving the team confidence going into the real countdown next week.
The dress rehearsal "went very well and we're looking forward to a relatively quiet count," said Hauboldt.
(For more insight about 4th Space Launch Squadron, see our earlier feature story written during a previous mission)
Liftoff is scheduled for Thursday, March 29 on the NROL-25 mission to deploy a hush-hush payload for the U.S. National Reconnaissance Office, the secretive government agency that designs and operates the country's fleet of orbiting spy satellites.
Although the exact launch time hasn't been revealed, officials say the liftoff will happen some time between 2 and 5:15 p.m. local time (5-8:15 p.m. EDT; 2100-0015 GMT).
Read our full story.