Hoisting a clandestine prime payload -- likely a pair of formation-flying ocean surveillance satellites -- and a menagerie of micro missions hitchhiking into space, the Atlas 5 rocket reeled off another successful performance Thursday in a rare mid-day launch from California.

Read our launch story.

6:17 p.m. local (9:17 p.m. EDT; 0117 GMT)
Today's Atlas 5 rocket launch has been declared a success, deploying its National Reconnaissance Office payload and cubesat secondary cargo.

"Today's successful launch of the NROL-36 mission occurred on the same day as the national memorial service honoring American hero Neil Armstrong. The scientists and engineers developing and operating these remarkable current-day launch and spacecraft systems reflect Neil's incredible legacy to mankind," said Jim Sponnick, ULA vice president, Mission Operations.

"Today's launch marks the fourth and final EELV mission for the NRO's Road to Launch 2012 accomplished in the last five months. This launch tempo is a tribute to all of the mission partners' dedication and continued focus on mission success -- one launch at a time."

4:20 p.m. local (7:20 p.m. EDT; 2320 GMT)
The Atlas 5' official liftoff time today was 2:39:00.242 p.m. local time, and officials report the vehicle's flight from the Western Range's perspective was performed successfully.

"It's an honor to work alongside the men and women of Team Vandenberg, United Launch Alliance, the National Reconnaissance Office and our mission partners," said Col. Nina Armagno, 30th Space Wing commander at Vandenberg. "These synergistic relationships are what guide Vandenberg to continual mission success and lead us into the next generation of spaceport excellence."

2:44 p.m. local (5:44 p.m. EDT; 2144 GMT)
The United Launch Alliance Atlas 5 rocket has flown into a pre-arranged news blackout following jettison of the rocket's payload shroud. The veil of secrecy surrounding the launch of this clandestine satellite cargo means no further information about the progress of the ascent, upper stage engine firings or release of the payload will be announced in real-time.
2:43 p.m. local (5:43 p.m. EDT; 2143 GMT)
T+plus 4 minutes, 45 seconds. The two halves of the four-meter-diameter Atlas 5 rocket nose cone encapsulating the spacecraft have separated.
2:43 p.m. local (5:43 p.m. EDT; 2143 GMT)
T+plus 4 minutes, 25 seconds. Centaur has ignited! The RL10 engine is up and running at full thrust to power the vehicle into orbit.
2:43 p.m. local (5:43 p.m. EDT; 2143 GMT)
T+plus 4 minutes, 12 seconds. The Atlas 5's Common Core Booster has been jettisoned, completing the first stage of flight, and the Centaur upper stage's liquid hydrogen and liquid oxygen systems are being readied for engine start.
2:43 p.m. local (5:43 p.m. EDT; 2143 GMT)
T+plus 4 minutes, 4 seconds. BECO. Booster Engine Cutoff is confirmed as the RD-180 powerplant on the first stage completes its burn. Standing by to fire the retro thrusters and separate the spent stage.
2:42 p.m. local (5:42 p.m. EDT; 2142 GMT)
T+plus 3 minutes, 45 seconds. Atlas now weighs just a quarter of what it did at liftoff.
2:42 p.m. local (5:42 p.m. EDT; 2142 GMT)
T+plus 3 minutes, 30 seconds. Vehicle performance remains nominal.
2:42 p.m. local (5:42 p.m. EDT; 2142 GMT)
T+plus 3 minutes. The main engine is burning well as the rocket climbs away from the planet.
2:41 p.m. local (5:41 p.m. EDT; 2141 GMT)
T+plus 2 minutes, 30 seconds. The vehicle is right on course.
2:41 p.m. local (5:41 p.m. EDT; 2141 GMT)
T+plus 2 minutes, 20 seconds. Atlas now weighs half of what it did at liftoff.
2:40 p.m. local (5:40 p.m. EDT; 2140 GMT)
T+plus 1 minutes, 45 seconds. The RD-180 main engine continues to fire normally, burning a mixture of highly refined kerosene and liquid oxygen.
2:40 p.m. local (5:40 p.m. EDT; 2140 GMT)
T+plus 95 seconds. Now passing through the region of maximum aerodynamic pressure on the vehicle as its accelerates through the dense lower atmosphere.
2:40 p.m. local (5:40 p.m. EDT; 2140 GMT)
T+plus 85 seconds. Mach 1. All looks good aboard the 19-story-tall rocket.
2:40 p.m. local (5:40 p.m. EDT; 2140 GMT)
T+plus 60 seconds. One minute into this afternoon ascent by the Atlas 5 from the Central Coast of California.
2:39 p.m. local (5:39 p.m. EDT; 2139 GMT)
T+plus 30 seconds. Pitch, yaw and roll maneuvers have been performed, putting Atlas 5 on the proper heading. The rocket is riding atop 860,000 pounds of thrust from the RD-180 main engine.
2:39 p.m. local (5:39 p.m. EDT; 2139 GMT)
T+plus 15 seconds. The launcher is maneuvering to its southerly trajectory to deliver a secret surveillance payload into orbit and dispatch 11 tiny cubesats.
2:39 p.m. local (5:39 p.m. EDT; 2139 GMT)
LIFTOFF! Liftoff of the Atlas 5 rocket for the National Reconnaissance Office, defending America through intelligence from above.
2:38 p.m. local (5:38 p.m. EDT; 2138 GMT)
T-minus 20 seconds. "Go Atlas" and "Go Centaur" was just called by launch team during a final status check.
2:38 p.m. local (5:38 p.m. EDT; 2138 GMT)
T-minus 40 seconds. Centaur's liquid oxygen and liquid hydrogen tanks are stable at flight pressures.
2:38 p.m. local (5:38 p.m. EDT; 2138 GMT)
T-minus 1 minute. Heading for a rare mid-day launch of the Atlas rocket from America's western spaceport.
2:37 p.m. local (5:37 p.m. EDT; 2137 GMT)
T-minus 90 seconds. The safety system has been armed.
2:37 p.m. local (5:37 p.m. EDT; 2137 GMT)
T-minus 1 minute, 50 seconds. Liquid oxygen and liquid hydrogen propellant topping to the Centaur upper stage is being secured.
2:37 p.m. local (5:37 p.m. EDT; 2137 GMT)
T-minus 1 minute, 55 seconds. The launch sequencer has been commanded to start.
2:37 p.m. local (5:37 p.m. EDT; 2137 GMT)
T-minus 2 minutes. The Atlas first stage and Centaur upper stage are now switching from ground power to internal batteries.
2:36 p.m. local (5:36 p.m. EDT; 2136 GMT)
T-minus 2 minutes, 30 seconds. The first stage RP-1 kerosene fuel tank and the liquid oxygen have stepped up to proper flight pressure levels.
2:36 p.m. local (5:36 p.m. EDT; 2136 GMT)
T-minus 3 minutes. The Atlas first stage liquid oxygen replenishment is being secured so the tank can be pressurized for launch.
2:35 p.m. local (5:35 p.m. EDT; 2135 GMT)
T-minus 3 minutes, 50 seconds. The ground pyrotechnics have been enabled.
2:35 p.m. local (5:35 p.m. EDT; 2135 GMT)
T-minus 4 minutes and counting. Clocks have resumed for the final minutes of today's countdown to launch the Atlas 5 rocket carrying a clandestine payload for the National Reconnaissance Office. Liftoff is set to occur at 2:39 p.m. local.
2:34 p.m. local (5:34 p.m. EDT; 2134 GMT)
A direct link to text updates.
2:34 p.m. local (5:34 p.m. EDT; 2134 GMT)
Countdown clocks will resume in one minute.
2:33 p.m. local (5:33 p.m. EDT; 2133 GMT)
The launch director and the mission director each have given their approval to press onward with the countdown as well.
2:32 p.m. local (5:32 p.m. EDT; 2132 GMT)
All systems are reported "go" to continue with the countdown for liftoff at 2:39 p.m. The clocks will resume from this hold at 2:35 p.m.
2:31 p.m. local (5:31 p.m. EDT; 2131 GMT)
Standing by for the final readiness check to be conducted. The launch team will be polled for a "go" or "no go" to proceed with the count.
2:29 p.m. local (5:29 p.m. EDT; 2129 GMT)
Here's a look at some stats about today's mission. This will be:
2:28 p.m. local (5:28 p.m. EDT; 2128 GMT)
Weather conditions are acceptable for liftoff at 2:39 p.m. PDT. It is very foggy and misty at Space Launch Complex 3, but there are no constraints for winds and clouds.
2:26 p.m. local (5:26 p.m. EDT; 2126 GMT)
All three cryogenic tanks are reported at flight level.
2:25 p.m. local (5:25 p.m. EDT; 2125 GMT)
T-minus 4 minutes and holding. The countdown has entered the planned hold to give the launch team a chance to review all systems before pressing ahead with liftoff. Today's launch time is aimed for 2:39:00 p.m. PDT.
2:24 p.m. local (5:24 p.m. EDT; 2124 GMT)
T-minus 5 minutes. Standing by to go into the final built-in hold that will last for 10 minutes.
2:22 p.m. local (5:22 p.m. EDT; 2122 GMT)
Upper level wind conditions are within allowable limits for launch.
2:19 p.m. local (5:19 p.m. EDT; 2119 GMT)
Twenty minutes from liftoff now. The countdown clocks are heading to the T-minus 4 minute mark where a planned 10-minute hold will occur. Launch of Atlas 5 remains scheduled for 2:39 p.m. PDT.
2:14 p.m. local (5:14 p.m. EDT; 2114 GMT)
The workhorse Centaur upper stage has flown in various configurations for decades and will be making its 204th mission with NROL-36. For this launch, the stage will use one Pratt & Whitney-built RL10A-4-2 liquid oxygen/liquid hydrogen engine that develops a thrust of about 22,300 pounds.

The stage is 41.5 feet in length and 10 feet it diameter. It also houses the navigation unit that serves as the rocket's guidance brain.

2:13 p.m. local (5:13 p.m. EDT; 2113 GMT)
The fuel-fill sequence for the first stage main engine is underway.
2:04 p.m. local (5:04 p.m. EDT; 2104 GMT)
The Atlas 5 rocket's rigid body first stage is known as the Common Core Booster. The CCB replaced the "balloon" pressure-stabilized stage used by previous Atlas vehicles.

It is equipped with the RD-180 liquid-fueled main engine. This liquid oxygen/kerosene powerplant is a two-thrust chamber, two-nozzle engine.

As the CCB's name suggests, the stage is common and is used in all the various configurations of the Atlas 5 family. The booster stage is 106.6 feet long and 12.5 feet diameter.

1:59 p.m. local (4:59 p.m. EDT; 2059 GMT)
This flight builds on the legacy of the Atlas vehicle's 401 configuration, which has flown over a dozen times in the past decade with the combination of a four-meter payload fairing, no solid rocket boosters and a single-engine Centaur upper stage.

Depending on a payload's weight and desired orbit, mission planners add strap-on solid boosters to the United Launch Alliance-made rocket to incrementally increase the vehicle's performance.

But this cargo, a hush-hush cargo for the NRO known only as the NROL-36 mission, is satisfied with the lifting power of the basic Atlas 5 design.

The vehicle will fly south-southeastward away from Vandenberg, according to the notice to mariners issued today to warn the public of the impending rocket flight over the Pacific. Details about the ascent, its duration and the number of Centaur fairings are not revealed given the secretive payload, which appears to be targeting an orbital inclination tilted 63 degrees to the equator based on the flight path in the NOTAMs.

The Russian RD-180 first stage main engine will ignite at T-minus 2.7 seconds, roaring to full power while undergoing a check to ensure its vital signs are healthy.

Rising off the pad in a slow, majestic fashion, the 19-story Atlas vehicle will deliver nearly a million pounds of ground-shaking thrust for the mid-day departure.

Information about the launch will be available though the first stage of flight, initial ignition of the Centaur and jettison of the nose cone about five minutes after liftoff. At that point, the mission will go into the now-standard news blackout for NRO launches.

Watch this page for live updates during the count and launch!

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1:48 p.m. local (4:48 p.m. EDT; 2048 GMT)
The liquid hydrogen tank in the Centaur upper stage just reached the 97 percent level. Topping is now beginning.
1:43 p.m. local (4:43 p.m. EDT; 2043 GMT)
Fast-filling of the first stage liquid oxygen has been completed at the tank's 97.5 percent mark. Topping mode is now underway.
1:40 p.m. local (4:40 p.m. EDT; 2040 GMT)
The Centaur liquid hydrogen tank is half-way loaded so far. The cryogenic propellant will be consumed with liquid oxygen by the stage's Pratt & Whitney-made RL10 engine.
1:39 p.m. local (4:39 p.m. EDT; 2039 GMT)
Now 60 minutes from launch. All activities are proceeding smoothly toward a liftoff at 2:39 p.m. Pacific Time.

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1:33 p.m. local (4:33 p.m. EDT; 2033 GMT)
Now beyond the three-quarters level of liquid oxygen on the first stage.
1:28 p.m. local (4:28 p.m. EDT; 2028 GMT)
Chilldown of the liquid hydrogen system is now complete, allowing the super-cold rocket fuel to begin filling the Centaur upper stage.
1:23 p.m. local (4:23 p.m. EDT; 2023 GMT)
First stage liquid oxygen tank is passing the half-full mark. Chilled to Minus-298 degrees F, the liquid oxygen will be used with RP-1 kerosene by the RD-180 main engine on the first stage during the initial minutes of flight today. The 25,000 gallons of RP-1 were loaded into the rocket earlier.
1:19 p.m. local (4:19 p.m. EDT; 2019 GMT)
Centaur engine chilldown sequence is being initiated.
1:16 p.m. local (4:16 p.m. EDT; 2016 GMT)
Liquid oxygen on Centaur has reached flight level.
1:11 p.m. local (4:11 p.m. EDT; 2011 GMT)
The Centaur liquid oxygen tank reached the 95 percent level and the topping off process is starting.
1:09 p.m. local (4:09 p.m. EDT; 2009 GMT)
Now 90 minutes from liftoff. There are no reports of technical troubles from the launch team and the weather is within limits for today's countdown. Fueling operations remain in work for the launch time of 2:39:00 p.m. local.
1:06 p.m. local (4:06 p.m. EDT; 2006 GMT)
The first stage liquid oxygen loading is switching from slow-fill to fast-fill mode as planned.
1:05 p.m. local (4:05 p.m. EDT; 2005 GMT)
The chilldown conditioning of liquid hydrogen propellant lines is starting to prepare the plumbing for transferring the Minus-423 degree F fuel into the rocket. The Centaur holds about 13,000 gallons of the cryogenic propellant.
1:04 p.m. local (4:04 p.m. EDT; 2004 GMT)
The Centaur liquid oxygen tank is 75 percent full now.
12:59 p.m. local (3:59 p.m. EDT; 1959 GMT)
Half of the Centaur liquid oxygen tank has been filled so far.
12:55 p.m. local (3:55 p.m. EDT; 1955 GMT)
The Centaur liquid oxygen tank is nearing one-third full already.
12:45 p.m. local (3:45 p.m. EDT; 1945 GMT)
The conditioning of the systems for the first stage liquid oxygen tank have been completed. And a "go" has been given to begin flowing supercold liquid oxygen into the Atlas 5's first stage.

The Common Core Booster stage's liquid oxygen tank is the largest tank to be filled today. It holds about 50,000 gallons of cryogenic oxidizer for the RD-180 main engine.

12:43 p.m. local (3:43 p.m. EDT; 1943 GMT)
Filling of the Centaur upper stage with about 4,300 gallons of liquid oxygen is beginning at Vandenberg's Space Launch Complex 3 following the thermal conditioning of the transfer pipes.

The liquid oxygen -- chilled to Minus-298 degrees F -- will be consumed during the launch by the Centaur's single RL10 engine along with liquid hydrogen to be loaded into the stage a little later in the countdown.

12:34 p.m. local (3:34 p.m. EDT; 1934 GMT)
The Centaur liquid oxygen system's pad storage area has been prepped. The next step is conditioning the transfer lines, which is now beginning to prepare the plumbing for flowing the cryogenic oxidizer.
12:29 p.m. local (3:29 p.m. EDT; 1929 GMT)
T-minus 120 minutes and counting! The launch countdown has resumed for today's flight of the Atlas 5 rocket following the planned half-hour built-in hold.

Clocks have one more hold scheduled at T-minus 4 minutes. That pause will last 10 minutes during which time the final "go" for launch will be given.

All remains targeted for liftoff at 2:39:00 p.m. local time (5:39:00 p.m. EDT; 2139:00 GMT) from Vandenberg Air Force Base, California.

12:26 p.m. local (3:26 p.m. EDT; 1926 GMT)
The launch team and all systems are "ready" to proceed with the countdown and begin fueling the Atlas 5 rocket this afternoon as planned.

Loading of cryogenic liquid oxygen and liquid hydrogen into the Atlas 5 rocket will be getting underway a short time from now.

12:24 p.m. local (3:24 p.m. EDT; 1924 GMT)
The Atlas launch conductor is briefing his team on procedures before entering into the final two hours of the countdown. A readiness check of the team members is next.
12:20 p.m. local (3:20 p.m. EDT; 1920 GMT)
Atlas 5 represents the culmination of evolution stretching back several decades to America's first intercontinental ballistic missile. At the dawn of the space age, boosters named Atlas launched men into orbit during Project Mercury and became a frequent vehicle of choice to haul civil, military and commercial spacecraft to orbit.

Topped with the high-energy Centaur upper stage, Atlas rockets have been used since the 1960s to dispatch ground-breaking missions for NASA, including the Surveyors to the Moon, Mariner flights to Mars, Venus and Mercury, and the Pioneers that were the first to visit Jupiter and beyond.

In its newest era, the Atlas 5 rocket sent the Mars Reconnaissance Orbiter to the red planet in 2005, propelled the New Horizons probe toward Pluto and the solar system's outer fringes in 2006, doubled up with the dual Lunar Reconnaissance Orbiter and LCROSS impactor to the Moon in 2009, hurled Juno to Jupiter last August and dispatched the car-sized Curiosity rover on the Mars Science Lab mission in November.

Today marks the 33rd flight for Atlas 5, born of the Air Force's competition to develop next-generation Evolved Expendable Launch Vehicles. In its previous 32 missions since debuting in August 2002, the tally shows 10 flights dedicated to the Defense Department, 9 commercial missions with communications spacecraft, seven for NASA and six with spy satellites for the National Reconnaissance Office.

12:09 p.m. local (3:09 p.m. EDT; 1909 GMT)
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12:00 p.m. local (3:00 p.m. EDT; 1900 GMT)
And SLC-3 is clear of all personnel for the remainder of the count.
11:59 a.m. local (2:59 p.m. EDT; 1859 GMT)
T-minus 2 hours and holding. The countdown just entered the first of the planned holds over the course of the day that will lead to the 2:39 p.m. PDT (5:39 p.m. EDT) launch of the Atlas-Centaur rocket.

This initial pause lasts 30 minutes, giving the team some margin in the countdown timeline to deal with technical issues or any work that is running behind. The final hold is scheduled to occur at T-minus 4 minutes.

Workers have left the pad area in advance of this afternoon's propellant loading and launch of the Atlas 5 rocket.

11:54 a.m. local (2:54 p.m. EDT; 1854 GMT)
With all the hands-on work now finished at the pad, technicians are clearing Space Launch Complex 3 for the remainder of the countdown.
11:41 a.m. local (2:41 p.m. EDT; 1841 GMT)
Testing of the vehicle's guidance system is complete.
11:25 a.m. local (2:25 p.m. EDT; 1825 GMT)
At this point in today's countdown, final preps for the Centaur's liquid oxygen and liquid hydrogen systems are underway. The C-band and S-band system testing has been accomplished.
11:05 a.m. local (2:05 p.m. EDT; 1805 GMT)
Completed in the countdown over the past few minutes, engineers conducted checks of the vehicle's internal batteries and finished final preps to the Atlas pneumatics, hydraulics and propulsion systems.
11:03 a.m. local (2:03 p.m. EDT; 1803 GMT)
The pad crew reports the gantry is parked, closed up and ready for launch.
10:40 a.m. local (1:40 p.m. EDT; 1740 GMT)
The mobile service tower is being secured in its launch position. The ground crew will get its doors closed, plus finish the final buttoning up of pad equipment over the next half-hour before all workers clear the pad for the remainder of the countdown.

Today's launch will be the fifth Atlas 5 rocket to fly from Vandenberg's Space Launch Complex 3-East pad. The site underwent an extensive overhaul, with construction occurring in 2004 and 2005, to accommodate the larger and more powerful Atlas 5 family of rockets. Some of the major modifications included:

This is the 38th Atlas to fly from SLC 3 throughout Vandenberg history.

10:15 a.m. local (1:15 p.m. EDT; 1715 GMT)
Standard countdown tests are getting started on the C-band system used to track the rocket as it flies downrange and the S-band system used for telemetry relay from vehicle.
10:09 a.m. local (1:09 p.m. EDT; 1709 GMT)
Now 4 hours, 30 minutes from blastoff.

Today's mission will deliver to orbit a payload for the National Reconnaissance Office, the agency that operates the country's fleet of spy satellites. This is the fourth of four launches that the NRO has planned this year, a batch of missions that began in April with the Delta 4 from Vandenberg Air Force Base and continued with an Atlas 5 and a Delta 4-Heavy, both in June from Cape Canaveral.

"Last year we executed the most aggressive launch campaign in over 25 years. We successfully launched six satellites in seven months and this year with the same determination we're scheduled to launch four more in five months," Betty Sapp, now the NRO's director, said in testimony before Congress this spring.

"These successful launches are a very important and visible reminder of the space reconnaissance mission the NRO started over 50 years ago, and continues with such great success today. We are committed to smart acquisition investments and practices to ensure the continued coverage and availability of our vital national security systems and we work tirelessly to deliver these systems on time and within budget."

Last year's remarkable launch surge used various types of Atlas and Delta rockets to launch replacement satellites into virtually all of the NRO's networks of imaging, eavesdropping, surveillance and data-relay spacecraft, plus the small Minotaur booster lofted a research and development payload.

"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," said Sapp.

10:06 a.m. local (1:06 p.m. EDT; 1706 GMT)
The tower is clear of the vehicle as it continues to slowly roll away. The structure's internal crane was instrumental in bringing the rocket stages and payload together. And now the fully assembled Atlas 5 has been unveiled for its 33rd launch, the fifth to originate from Vandenberg.
9:55 a.m. local (12:55 p.m. EDT; 1655 GMT)
GO FOR ROLL. At the Space Launch Complex 3 pad, the mobile service gantry has been configured for its retraction away from the Atlas 5 rocket this morning. Approval has been radioed to the team to wheel the 8-million-pound tower to its launch position a short distance from the 19-story-tall booster.
9:39 a.m. local (12:39 p.m. EDT; 1639 GMT)
Now entering the final five hours in this countdown to launch. Centaur's helium bottles have been charged. The flight control preps are complete and the operational test is underway.

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9:23 a.m. local (12:23 p.m. EDT; 1623 GMT)
Preparations for moving the mobile service tower are reported complete, and the pad crew has been given approval to start jacking up the gantry for retraction.
8:39 a.m. local (11:39 a.m. EDT; 1539 GMT)
Six hours from liftoff! Out at Space Launch Complex 3-East, the pad crew is working through its checklist to ready the mobile service tower for rollback later this morning. The gantry is the massive building on wheels that is used for assembling the rocket and payload, provides workers full access to the vehicle during the pre-flight campaign, then retracts a few hours before liftoff time to reveal the Atlas.
8:18 a.m. local (11:18 a.m. EDT; 1518 GMT)
The Atlas first stage and Centaur upper stage have been powered up as the early portion of the countdown activities rolls onward this morning at Vandenberg. Guidance system testing is about to begin next.
6:59 a.m. local (9:59 a.m. EDT; 1359 GMT)
COUNTDOWN UNDERWAY! Clocks have begun ticking for today's flight by the Atlas 5 rocket from America's western spaceport on the NROL-36 mission for the National Reconnaissance Office.

As the countdown gets started, the launch team will power up the rocket to conduct standard pre-flight tests and ready the vehicle for this national security satellite deployment mission.

Rollback of the mobile service tower from around the rocket is expected around 10 a.m. local (1 p.m. EDT). Once the gantry is removed, crews at the pad will make preparations to systems and equipment before the site is cleared of all personnel for fueling.

A planned 30-minute hold begins when the count reaches T-minus 120 minutes at 11:59 a.m. local time (2:59 p.m. EDT). With a few minutes remaining in the hold, the team will be polled to verify all is in readiness to start loading propellant into the rocket for launch.

Supercold liquid oxygen begins flowing into the Centaur upper stage around 12:46 p.m. local (3:46 p.m. EDT), followed by the Atlas first stage. Liquid hydrogen fuel loading for Centaur will be completed a short time later.

A final hold is scheduled at 2:25 p.m. when clocks hit the T-minus 4 minute mark. That will give the team a chance to finish any late work and assess the status of the rocket, payload, Range and weather before proceeding into the last moments of the countdown.

Liftoff remains targeted for 2:39 p.m. local time (5:39 p.m. EDT; 2139 GMT).

After a six-week delay to unravel a puzzling Range software glitch, United Launch Alliance's Atlas 5 rocket will try again Thursday afternoon to deploy a classified national security payload and batch of hitchhiking cubesats into space from California.

The final launch readiness review was conducted Wednesday and gave approval for starting the countdown operations Thursday morning, with clocks picking up at 7 a.m. local. Retraction of the gantry to unveil the 19-story rocket occurs at 10 a.m. and fueling commences at 12:45 p.m.

Liftoff is targeted for 2:39 p.m. local (5:39 p.m. EDT; 2139 GMT). The steady march of the payload orbit moved the launch about 10 hours earlier than the original blastoff time from the late-night try Aug. 2.

Read our launch preview story.

The weather outlook appears near-perfect for Thursday afternoon's Atlas 5 rocket launch from Vandenberg Air Force Base in California at 2:39 p.m. local time.

Vandenberg's trademark marine layer will burn off during the morning, leaving only a few lingering low-level clouds by launch time, Air Force meteorologists predict.

At launch time, the forecast calls for a few stratus clouds at 500 feet with tops at 800 feet and 2/8ths sky coverage, 7 miles of visibility, northwesterly winds of 12 to 18 knots and a temperature in the mid 60s F.

With high pressure controlling the stable weather picture, forecasters say none of the launch rules will be threatened. There is a 100 percent chance of weather permitting a launch on Thursday, as well as Friday for the backup opportunity.

The target launch time for next Thursday's Atlas 5 rocket has been announced to the public. Liftoff from Vandenberg Air Force Base is scheduled for 2:39 p.m. local (5:39 p.m. EDT; 2139 GMT).

The exact duration of the daily launch window hasn't been disclosed. But officials previously said the liftoff would not occur after 4:15 p.m. local.

Read our earlier status center coverage.