T+plus 3 minutes, 45 seconds. The two-halves of the Atlas 5 rocket nose cone encapsulating the spacecraft have separated. Also jettisoned was the Forward Load Reactor, a two-piece deck that rings the Centaur stage to support the bulbous fairing during launch.

T+plus 3 minutes. RD-180 is performing well as the rocket climbs away from the planet on its south-southwesterly trajectory.

T+plus 2 minutes, 30 seconds. The rocket now weighs half of what it did at liftoff.

T+plus 2 minutes. Atlas is looking good as the engine eases back to 95 percent throttle.

T+plus 85 seconds. Mach 1 as the main engine fires at full throttle.

T+plus 80 seconds. The post-liftoff roll maneuver is complete, putting the rocket in the correct orientation for the flight downrange.

T+plus 60 seconds. One minute into the ascent.

T+plus 30 seconds. The RD-180 main engine is burning bright as the Atlas majestically rises into the evening sky.

T+plus 15 seconds. And the vehicle has cleared the tower at Space Launch Complex 3.

LIFTOFF! Liftoff of the Atlas 5 rocket on the NROL-41 mission, launching a new reconnaissance satellite to oversee and protect the nation.

T-minus 20 seconds. "Go Atlas" and "Go Centaur" was just called by launch team during a final status check.

T-minus 40 seconds. Centaur's liquid oxygen and liquid hydrogen tanks are stable at flight pressures.

T-minus 1 minute. Now 60 seconds from launch at America's western spaceport.

T-minus 90 seconds. The safety system has been armed.

T-minus 1 minute, 50 seconds. Liquid oxygen and liquid hydrogen propellant topping to the Centaur upper stage is being secured.

T-minus 1 minute, 55 seconds. The launch sequencer has been commanded to start.

T-minus 2 minutes. The Atlas first stage and Centaur upper stage are now switching from ground power to internal batteries.

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.

T-minus 3 minutes. The Atlas first stage liquid oxygen replenishment is being secured so the tank can be pressurized for launch.

T-minus 3 minutes, 50 seconds. The ground pyrotechnics have been enabled.

T-minus 4 minutes and counting. Clocks have resumed for the final minutes of tonight's countdown to launch the Atlas 5 rocket carrying a clandestine payload for the National Reconnaissance Office. Liftoff is set to occur at 9:03:30 p.m. local.

Countdown clocks will resume in one minute.

The launch director and the mission director each have given their approval to press onward with the countdown.

All systems are reported "go" to continue with the countdown for liftoff at 9:03:30 p.m.

The launch team is being polled for a "go" or "no go" to proceed with the count.

Now 8 minutes away from the scheduled launch time. The final readiness polls will be performed shortly.

Officials are optimistic that this latest target launch time is going to happen. Atlas it aiming for 9:03:30 p.m.

There are no technical problems being worked in the countdown. It's just a matter of ensuring the safety of all personnel before proceeding with the liftoff tonight.

NEW LAUNCH TIME. Officials have retargeted liftoff for 9:03:30 p.m.

HOLD EXTENDED AGAIN. Five more minutes added, pushing launch to no sooner than 8:56:30 p.m.

HOLD EXTENDED AGAIN. At least five further minutes have been added to this delay. Officials are working to determine if the restricted safety area beneath the rocket's course is clear.

HOLD EXTENDED AGAIN. Five more minutes are being added to the hold, pushing liftoff to 8:46:30 p.m.

NEW LAUNCH TIME. An additional five minutes have been inserted into this hold. That puts liftoff time at 8:41:30 p.m., if the clearance is granted.

The hold is expected to be at least five minutes. The SLC 8 pad is used by the Minotaur rockets, which has a liftoff planned on Saturday night carrying the Space Base Space Surveillance satellite.

HOLD EXTENDED. Officials have announced to reporters that a sweep revealed a private vehicle at the SLC 8 pad south of the Atlas complex. That has prompted a launch hold to ensure no personnel are in the restricted area.

Here's a look at some stats about today's mission. This will be:

• The 605th launch for Atlas program since 1957
• The 287th Atlas to occur from Vandenberg AFB since 1959
• The 36th Atlas to use Space Launch Complex 3
• The 23rd launch of an Atlas 5 since 2002
• The third Atlas 5 to occur from Vandenberg
• The 15th Atlas 5 under United Launch Alliance
• The second Atlas 5 to fly in the 501 configuration
• The 10th Department of Defense mission on Atlas 5
• The fourth National Reconnaissance Office use of Atlas 5

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 8:31:30 p.m. PDT.

T-minus 5 minutes. Standing by to go into the final built-in hold.

All three cryogenic tanks are reported at flight level.

Weather conditions are acceptable for liftoff at 8:31:30 p.m. PDT. There remains some low overcast clouds and light fog, but all of the weather rules are "go" for liftoff.

Twenty minutes from liftoff now. The countdown clocks are heading to the T-minus 4 minute mark where a planned 12-minute, 30-second hold will occur. Launch of Atlas 5 remains scheduled for 8:31:30 p.m. PDT, following a slight adjustment for safety reasons.

After the upcoming hold begins, officials and the launch team will conduct a series of final readiness checks before resuming the countdown.

"We'll move into a final poll for the terminal count, at which point most of the issues and anomalies need to be worked out, and team vouch for their readiness to proceed into terminal count," Lt. Col. Brady Hauboldt, the 4th Space Launch Squadron commander and Air Force launch director, explained in a recent interview.

"That final four minutes is an exciting time. Everybody is watching anxiously for anything to pop up. Thousands of telemetry points are being monitored and the system continues its final count for a nominal T-0."

The fuel-fill sequence for the first stage main engine is underway.

Today's launch culminates what has been a smooth pre-flight campaign for this Atlas 5 rocket at Vandenberg Air Force Base.

Comments by Lt. Col. Brady Hauboldt, the 4th Space Launch Squadron commander, from an interview last week:

"We have spent roughly the last three years preparing for this mission. We assigned a launch mission manager to integrate the entire launch campaign with the launch vehicle and space community, as well as the 30th Space Wing team. We've had trailblazer activities, really to dry-run bringing the launch vehicle and spacecraft here, checking out all of the procedures and system modifications that have gone into Space Launch Complex 3 well before the launch vehicle got here in March of this year.

"Since then, it went very smoothly. We successfully completed receipt inspection here at Vandenberg and then put the booster on (the pad) in early May.

We'll have live streaming video coverage of the launch. If you hit reload to this page, the video will load into the right-hand side. Space shuttle Discovery's rollout is being showing at the moment, but we'll switch the Atlas shortly.

The liquid hydrogen tank in the Centaur upper stage just reached the 97 percent level. Topping is now beginning.

The Centaur liquid hydrogen tank is two-thirds loaded so far. The cryogenic propellant will be consumed with liquid oxygen by the stage's Pratt & Whitney-made RL10 engine.

Fast-filling of the first stage liquid oxygen tank has been completed. Topping mode is now underway.

Now 60 minutes from liftoff. All activities are proceeding toward a liftoff at 8:31:30 p.m. Pacific Time.

A reminder that if you will be away from your computer but would like to receive countdown updates, sign up for our Twitter feed to get text messages on your cellphone. U.S. readers can also sign up from their phone by texting "follow spaceflightnow" to 40404. (Standard text messaging charges apply.)

Chilldown of the liquid hydrogen system is now complete, allowing the super-cold rocket fuel to begin filling the Centaur upper stage.

Centaur's liquid oxygen is reported at flight level.

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.

The Centaur liquid oxygen tank reached the 95 percent level and the topping off process is starting.

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.

The first stage liquid oxygen loading is switching from slow-fill to fast-fill mode as the countdown continues as planned.

Half of the Centaur liquid oxygen tank has been filled already.

ULA says the 2-minute, 30-second change to the launch time was driven by a COLA, or Collision Avoidance period that ensures the rocket doesn't fly on a trajectory that would take it too close to another object in space. Again, the new target T-0 is the precise moment of 8:31:30 p.m. PDT.

The extra time will be added into the scheduled hold at T-minus 4 minutes to sync up the clocks.

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 pumped into the stage a little later in the countdown.

The conditioning of the systems for the first stage liquid oxygen tank have been completed. And a "go" has been given to begin pumping 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.

NEW LAUNCH TIME. The target liftoff time has been adjusted slightly to 8:31:30 p.m.

T-minus 120 minutes and counting! The launch countdown has resumed for this evening'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 8:29 p.m. local time (11:29 p.m. EDT; 0329 GMT) from Vandenberg Air Force Base, California.

The launch team and all systems are "go" to proceed with the countdown this evening as planned.

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

Check out some great photos of the Atlas 5 rocket taken during tower rollback today.

T-minus 2 hours and holding. The countdown has just entered the first of the planned holds over the course of the evening that will lead to the 8:29 p.m. PDT (11:29 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 wrapped up all of their hands-on activities at the launch pad and cleared the area in advance of this evening's propellant loading and launch of the Atlas 5 rocket.

The fog situation at the launch pad is getting better and visibility has improved remarkably.

The forecast for liftoff time three hours from now expects a 60 percent chance of meeting the launch weather rules. Gusty ground winds are the only concern.

The mobile service tower has been secured in its launch position. The ground crew will get its doors closed, plus finish final buttoning up of pad equipment over the next hour before all workers clear the pad for the remainder of the countdown.

Today's launch will be the third Atlas 5 rocket to fly from Vandenberg's Space Launch Ccomplex 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:

• 30-foot extension to mobile service tower
• 20-foot exhaust duct depth increase
• 250-ton Fixed Launch Platform installed
• Significant ambient and cryogenic fluid ground system modifications
• Complete replacement of the Ground Command/Control/Communication system
• New and refurbished Launch Control/Mission Support Centers

Preps for the Atlas first stage liquid oxygen system and pneumatics, as well as Centaur liquid oxygen and liquid hydrogen systems have been completed. Other work that occurs at this portion of the countdown include internal battery checks and testing of the C-band system used to track the rocket as it flies downrange.

Meanwhile, space shuttle Discovery is journeying from Kennedy Space Center's Vehicle Assembly Building to launch pad 39A for its final spaceflight before retirement. You can watch our live streaming video coverage of the shuttle's rollout.

The tower is clear of the vehicle as it continues to slowly roll away. Atlas 5 has been unveiled for launch, although the incredibly dense fog is keeping the rocket hidden this afternoon.

At the Space Launch Complex 3 pad, the mobile service gantry has been configured for its retraction away from the Atlas 5 rocket this afternoon. Technicians are ready to wheel the 8-million-pound tower to its launch position a short distance from the 19-story-tall booster.

The structure and its internal crane was instrumental in assembling the rocket and payload during the pre-flight campaign.

Gantry rollback should be getting underway shortly in the incredibly foggy conditions at Vandenberg. Forecasters continue to hope that some of this smothering fog will dissipate in the next few hours before launch, but time will tell.

The launch site is completely blanketed with dense fog, but the hope is conditions will clear during the next few hours. Countdown activities are proceeding as planned and the rocket's guidance system is being tested right now.

The formal countdown sequence is underway and the Atlas-Centaur rocket has been powered for tonight's launch from Vandenberg Air Force Base in California. The launch team is running through routine testing and preps, as well as configuring pad systems as the clocks tick toward the 8:29 p.m. local time liftoff.

And a reminder that 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.)

Out at Space Launch Complex 3-East, the pad crew has begun its procedures to ready the mobile service tower for rollback later this afternoon. The gantry is the massive building on wheels that surrounds the Atlas during the pre-flight campaign, then retracts a few hours before liftoff time to reveal the rocket.

In their latest forecast, meteorologists are predicting a 60 percent chance of favorable conditions for tonight's launch of the Atlas 5 rocket from Vandenberg Air Force Base, California.

Those odds have decreased slightly because of strong winds expected at the launch base behind the passage of a cold front.

The launch time outlook includes scattered-to-broken low clouds and a few high clouds, no widespread fog and 7 miles of visibility, sustained northwesterly winds between 22 and 28 knots and a temperature around 53 degrees F.

As the countdown gets underway this afternoon, the launch team will power up the rocket and begin standard pre-flight tests to ready the Atlas 5 for tonight's clandestine satellite deployment mission.

Rollback of the mobile service tower from around the rocket is scheduled to start about five hours before launch.

Crews at the pad will make preparations to systems and equipment before the site is cleared of all personnel before fueling can start.

A planned hold begins when the count reaches T-minus 120 minutes. With five 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, 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 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 is scheduled for 8:29 p.m. local time (11:29 p.m. EDT; 0329 GMT). The duration of the launch window remains a secret for this classified National Reconnaissance Office mission.

Whether it's launching a national security payload with an Atlas 5 rocket Monday night or rehearsing for the West Coast debut of the mighty Delta 4-Heavy, these are active times for Vandenberg Air Force Base's 4th Space Launch Squadron.

"That's right. We perform better and are happiest when we are busy," Lt. Col. Brady Hauboldt, the squadron commander, said in an interview Friday.

The squadron's mission is overseeing the Evolved Expendable Launch Vehicles flown from Vandenberg carrying satellites into highly-inclined and polar orbits. Situated 150 miles northwest of Los Angeles, the base is the primary U.S. site to launch spacecraft into such advantageous vantage points.

Read our full story.

A United Launch Alliance Atlas 5 rocket carrying a classified cargo for the nation's spy satellite agency plans a primetime blastoff Monday evening from California's Vandenberg Air Force Base.

Liftoff is targeted for 8:29 p.m. local (11:29 p.m. EDT; 0329 GMT) from Space Launch Complex 3-East. The 19-story rocket should be visible across the region as a flame in the night sky, streaking southward to orbit.

"I'm incredibly proud of all the hard work of the men and women of the 4th Space Launch Squadron team in preparing for this Atlas 5 mission. We've worked side-by-side with United Launch Alliance and the entire 30th Space Wing to pull this campaign together over the last few months and are now 'go' for launch," said Lt. Col. Brady Hauboldt, 4th SLS commander.

"We're all excited to have the opportunity to launch our newest Atlas 5 and demonstrate Team Vandenberg's continued contributions to space power."

The Launch Readiness Review was held Friday morning and gave approval to proceed with the mission, known as NROL-41, for the National Reconnaissance Office.

Weather forecasters are predicting a 70 percent chance of acceptable conditions for the evening launch opportunity. Gusty winds are the only concern.

The outlook includes some scattered low- and high-level clouds, clear visibility of 7 miles, north-northwesterly winds of 20-25 knots with gusts to 30 knots and a temperature of 52 to 55 degrees F.

Meteorologists predict a cold front to move over Vandenberg around noontime Monday, stirring up the atmosphere and preventing fog from blanketing the launch pad. That's good news considering the last Atlas flight from Vandenberg a year ago was nearly invisible to even close-up cameras.

"For those that supported the DMSP launch last fall, seven miles (of visibility) is substantially better," Hauboldt said.

Similar weather is expected on Tuesday night, if the launch happens to slip for some reason.

Watch this page for continuing mission reports and live launch coverage!

And a reminder that 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.)