0905 GMT (1:05 a.m. PST; 4:05 a.m. EST)
United Launch Alliance has declared this launch a success.

The Colorado-based launch services company, formed in 2006 by merging the rocket divisions of Boeing Co. and Lockheed Martin Corp., announced the mission was a success in a press release.

"Today's successful launch of the NROL-39 mission is a testament to the tremendous government-industry partnership. We greatly appreciate the teamwork with the NRO Office of Space Launch and our many mission partners," said Jim Sponnick, ULA vice president of Atlas and Delta programs, in the press release. "We are honored to be entrusted to launch these one-of-a-kind national assets to orbit to protect our national security and to support the many brave men and women serving around the world."

Still to come later this morning is the deployment of 12 CubeSat secondary payloads.

The next Atlas 5 launch is set for Jan. 23 from Cape Canaveral with NASA's next Tracking and Data Relay Satellite.

0729 GMT (11:29 p.m. PST; 2:29 a.m. EST)
We expect a press release from United Launch Alliance later tonight once the mission is complete, and NASA says there will be updates on the status of the 12 CubeSat payloads in about three hours.
0719 GMT (11:19 p.m. PST; 2:19 a.m. EST)
The Atlas 5 has flown into a scheduled news blackout with jettison of the rocket's payload shroud. The veil of secrecy surrounding the launch of this classified satellite means no further information about the progress of the ascent nor release of the payload will be announced in real-time.
0718:15 GMT (11:18:15 p.m. PST; 2:18:15 a.m. EST)
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.
0717:30 GMT (11:17:30 p.m. PST; 2:17:30 a.m. EST)
T+plus 3 minutes. RD-180 is performing well as the rocket climbs away from the planet on its south-southwesterly trajectory.

Altitude is 39 miles, downrange distance is 29 miles, and velocity is 3,235 mph.

0717:00 GMT (11:17:00 p.m. PST; 2:17:00 a.m. EST)
T+plus 2 minutes, 30 seconds. The rocket now weighs half of what it did at liftoff.
0716:30 GMT (11:16:30 p.m. PST; 2:16:30 a.m. EST)
T+plus 2 minutes. Atlas is looking good as the engine eases back to 95 percent throttle.
0715:55 GMT (11:15:55 p.m. PST; 2:15:55 a.m. EST)
T+plus 85 seconds. Mach 1 as the main engine fires at full throttle.
0715:50 GMT (11:15:50 p.m. PST; 2:15:50 a.m. EST)
T+plus 80 seconds. The post-liftoff roll maneuver is complete, putting the rocket in the correct orientation for the flight downrange.
0715:30 GMT (11:15:30 p.m. PST; 2:15:30 a.m. EST)
T+plus 60 seconds. One minute into the ascent.
0715:00 GMT (11:15:00 p.m. PST; 2:15:00 a.m. EST)
T+plus 30 seconds. The RD-180 main engine is burning bright as the Atlas majestically rises into the evening sky.
0714:45 GMT (11:14:45 p.m. PST; 2:14:45 a.m. EST)
T+plus 15 seconds. And the vehicle has cleared the tower at Space Launch Complex 3.
0714:30 GMT (11:14:30 p.m. PST; 2:14:30 a.m. EST)
LIFTOFF! Liftoff of the Atlas 5 rocket on the NROL-39 mission, delivering a national security satellite to orbit for the U.S. government.
0714:10 GMT (11:14:10 p.m. PST; 2:14:10 a.m. EST)
T-minus 20 seconds. "Go Atlas" and "Go Centaur" was just called by launch team during a final status check.
0713:50 GMT (11:13:50 p.m. PST; 2:13:50 a.m. EST)
T-minus 40 seconds. Centaur's liquid oxygen and liquid hydrogen tanks are stable at flight pressures.
0713:30 GMT (11:13:30 p.m. PST; 2:13:30 a.m. EST)
T-minus 1 minute. Now 60 seconds from launch at America's western spaceport.
0713:00 GMT (11:13:00 p.m. PST; 2:13:00 a.m. EST)
T-minus 90 seconds. The safety system has been armed.
0712:40 GMT (11:12:40 p.m. PST; 2:12:40 a.m. EST)
T-minus 1 minute, 50 seconds. Liquid oxygen and liquid hydrogen propellant topping to the Centaur upper stage is being secured.
0712:35 GMT (11:12:35 p.m. PST; 2:12:35 a.m. EST)
T-minus 1 minute, 55 seconds. The launch sequencer has been commanded to start.
0712:30 GMT (11:12:30 p.m. PST; 2:12:30 a.m. EST)
T-minus 2 minutes. The Atlas first stage and Centaur upper stage are now switching from ground power to internal batteries.
0712:00 GMT (11:12:00 p.m. PST; 2:12:00 a.m. EST)
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.
0711:30 GMT (11:11:30 p.m. PST; 2:11:30 a.m. EST)
T-minus 3 minutes. The Atlas first stage liquid oxygen replenishment is being secured so the tank can be pressurized for launch.
0710:40 GMT (11:10:40 p.m. PST; 2:10:40 a.m. EST)
T-minus 3 minutes, 50 seconds. The ground pyrotechnics have been enabled.
0710:30 GMT (11:10:30 p.m. PST; 2:10:30 a.m. EST)
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 11:13 p.m. PST.
0709 GMT (11:09 p.m. PST; 2:09 a.m. EST)
The launch director and the mission director each have given their approval to press onward with the countdown. Clocks will resume in one minute.
0708 GMT (11:08 p.m. PST; 2:08 a.m. EST)
All systems are reported "go" to continue with the countdown for liftoff at 11:14:30 p.m. PST (0714:30 GMT; 2:14:30 a.m. EST)
0707 GMT (11:07 p.m. PST; 2:07 a.m. EST)
The launch team is being polled for a "go" or "no go" to proceed with the count.
0706 GMT (11:06 p.m. PST; 2:06 a.m. EST)
Now 8 minutes away from the scheduled launch time. The final readiness polls will be performed shortly.
0705 GMT (11:05 p.m. PST; 2:05 a.m. EST)
The NROL-39 payload is transitioning to internal power at this time.
0703 GMT (11:03 p.m. PST; 2:03 a.m. EST)
Here's a look at some stats about today's mission. This will be:
0700 GMT (11:00 p.m. PST; 2:00 a.m. EST)
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 11:14:30 p.m. PST.
0659 GMT (10:59 p.m. PST; 1:59 a.m. EST)
T-minus 5 minutes. Standing by to go into the final built-in hold.

All three cryogenic tanks are reported at flight level, and the RD-180 engine's fuel fill sequence is complete.

0655 GMT (10:55 p.m. PST; 1:55 a.m. EST)
Weather conditions are acceptable for liftoff at 11:14:30 p.m. PST. Despite cold temperatures, all of the weather rules are "go" for liftoff.
0654 GMT (10:54 p.m. PST; 1:54 a.m. EST)
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 11:14:30 p.m. PST (0714:30 GMT; 2:14:30 a.m. EST).

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

0647 GMT (10:47 p.m. PST; 1:47 a.m. EST)
The fuel-fill sequence for the first stage main engine is underway.
0644 GMT (10:44 p.m. PST; 1:44 a.m. EST)
With a half-hour to go until launch time, there are no issues reported that could prevent a liftoff at 11:14:30 p.m. PST (0714:30 GMT; 2:14:30 a.m. EST).
0638 GMT (10:38 p.m. PST; 1:38 a.m. EST)
The Centaur's liquid hydrogen tank is reported at flight level.
0636 GMT (10:36 p.m. PST; 1:36 a.m. EST)
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.

The workhorse Centaur upper stage has flown in various configurations for decades and will be making its 207th mission with tonight's launch. For this launch, the stage will use one Aerojet Rocketdyne 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.

0629 GMT (10:29 p.m. EST; 1:29 a.m. EST)
The version of the Atlas 5 flying tonight is known as the "501" variant with no solid rocket boosters, a five-meter diameter payload fairing and a single-engine Centaur upper stage.

Get to know the United Launch Alliance rocket with this cutaway view of the launcher.

Rising off the pad in a slow, majestic fashion, the 20-story Atlas vehicle will deliver nearly a million pounds of ground-shaking thrust for the late-night departure.

The rocket will surpass the speed of sound less than 90 seconds after liftoff, and the five-meter diameter nose shroud built by RUAG Space of Switzerland will be jettisoned a little over three minutes into flight.

With that milestone, the flight is expected to enter a news blackout with no real-time information on the mission's progress, including events such as staging and firings of the Centaur upper stage's RL10 engine.

ULA and the National Reconnaissance Office plan to issue press releases once the mission is complete Friday morning.

0626 GMT (10:26 p.m. EST; 1:26 a.m. EST)
The liquid hydrogen tank in the Centaur upper stage just reached the 97 percent level. Topping is now beginning.
0622 GMT (10:22 p.m. PST; 1:22 a.m. EST)
Fast-filling of the first stage liquid oxygen tank has been completed. Topping mode is now underway.
0619 GMT (10:19 p.m. PST; 1:19 a.m. EST)
The Centaur liquid hydrogen tank is 60 percent loaded so far. The cryogenic propellant will be consumed with liquid oxygen by the stage's Aerojet Rocketdyne RL10 engine.
0617 GMT (10:17 p.m. PST; 1:17 a.m. EST)
Liquid oxygen on the Centaur upper stage has reached flight level.
0614 GMT (10:14 p.m. PST; 1:14 a.m. EST)
Now 60 minutes from liftoff. All activities are proceeding toward a liftoff at 11:14:30 p.m. PST (0714:30 GMT; 2:14:40 a.m. EST).
0610 GMT (10:10 p.m. PST; 1:10 a.m. EST)
The first stage liquid oxygen tank is passing 80 percent full.
0605 GMT (10:05 p.m. PST; 1:05 a.m. EST)
Chilldown of the liquid hydrogen system is now complete, allowing the super-cold rocket fuel to begin filling the Centaur upper stage.
0558 GMT (9:58 p.m. PST; 12:58 a.m. EST)
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.
0555 GMT (9:55 p.m. PST; 12:55 a.m. EST)
The Centaur engine chilldown sequence is being initiated.
0547 GMT (9:47 p.m. PST; 12:47 a.m. EST)
The Centaur liquid oxygen tank reached the 96.4 percent level and the topping off process is starting.

The Atlas first stage liquid oxygen tank is now 20 percent full.

0544 GMT (9:44 p.m. PST; 12:44 a.m. EST)
Now 90 minutes from liftoff. There are no reports of technical troubles from the launch team and the weather is beautiful but chilly for tonight's countdown. Fueling operations remain in work for launch at 11:14:30 p.m. local time.
0541 GMT (9:41 p.m. PST; 12:41 a.m. EST)
The Centaur liquid oxygen tank is 75 percent full now.

And 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.

0540 GMT (9:40 p.m. PST; 12:40 a.m. EST)
The first stage liquid oxygen loading is switching from slow-fill to fast-fill mode as the countdown continues as planned.
0535 GMT (9:35 p.m. PST; 12:35 a.m. EST)
Half of the Centaur's liquid oxygen tank has been filled.
0533 GMT (9:33 p.m. PST; 12:33 a.m. EST)
NEW LAUNCH TIME. The countdown will be adjusted at the T-minus 4 minute hold to target a new launch time of 11:14:30 p.m. PST (0714:30 GMT; 2:14:30 a.m. EST).
0511 GMT (9:26 p.m. PST; 12:26 a.m. EST)
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 Centaur liquid oxygen is now at the 10 percent level.

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.

0503 GMT (9:03 p.m. PST; 12:03 a.m. EST)
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 11:13 p.m. PST local time (0713 GMT; 2:13 a.m. EST) from Vandenberg Air Force Base, Calif.

0501 GMT (9:01 p.m. PST; 12:01 a.m. EST)
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.

0458 GMT (8:58 p.m. PST; 11:58 p.m. EST Thurs.)
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.
0450 GMT (8:50 p.m. PST; 11:50 p.m. EST Thurs.)
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 in August 2011, dispatched the car-sized Curiosity rover on the Mars Science Lab mission in November 2011, and deployed a pair of hardened probes in the heart of Earth's radiation belts in August 2012.

Most recently, an Atlas 5 rocket launched from Cape Canaveral sent NASA's MAVEN atmospheric research probe on the way to Mars on Nov. 18.

Today marks the 42nd flight for Atlas 5, born of the Air Force's competition to develop next-generation Evolved Expendable Launch Vehicles. In its previous 41 missions since debuting in August 2002, the tally shows 15 flights dedicated to the Defense Department, 10 missions for NASA, nine flights with commercial payloads, and seven with spy satellites for the National Reconnaissance Office.

0435 GMT (8:35 p.m. PST; 11:35 p.m. EST Thurs.)
The SLC-3 launch pad is clear of all personnel for the remainder of the count.
0433 GMT (8:33 p.m. PST; 11:33 p.m. EST Thurs.)
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 11:13 p.m. PST (0713 GMT; 2:13 a.m. EST) 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.

0424 GMT (8:24 p.m. PST; 11:24 p.m. EST Thurs.)
The launch team is clearing the pad in preparation for fueling of the two-stage Atlas 5 rocket.
0413 GMT (8:13 p.m. PST; 11:13 p.m. EST Thurs.)
The forecast for liftoff time three hours from now predicts a 60 percent chance of meeting the launch weather rules. The only concern is cold temperatures during tanking of the rocket with cryogenic propellants.
0406 GMT (8:06 p.m. PST; 11:06 p.m. EST Thurs.)
The C-band tracking and S-band telemetry system testing has been accomplished.
0350 GMT (7:50 p.m. PST; 10:50 p.m. EST Thurs.)
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.
0328 GMT (7:28 p.m. PST; 10:28 p.m. EST Thurs.)
The mobile service tower has been secured in its launch position. The ground crew has closed its doors and will 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 seventh 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:

0313 GMT (7:13 p.m. PST; 10:13 p.m. EST Thurs.)
Now entering the final four hours in the countdown to launch at 11:13 p.m. local time.

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.)

0242 GMT (6:42 p.m. PST; 9:42 p.m. EST Thurs.)
The tower is clear of the vehicle as it slowly rolls away from the rocket. Atlas 5 has been unveiled for its 42nd launch.
0229 GMT (6:29 p.m. PST; 9:29 p.m. EST Thurs.)
At the Space Launch Complex 3 pad, the mobile service gantry has been configured for its retraction away from the Atlas 5 rocket this evening. Technicians are ready to wheel the 8-million-pound tower to its launch position a short distance from the 20-story-tall booster.

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

0150 GMT (5:50 p.m. PST; 8:50 p.m. EST Thurs.)
Check out photos of the Atlas 5 rocket on the launch pad this morning.
0105 GMT (5:05 p.m. PST; 8:05 p.m. EST Thurs.)
Countdown activities are proceeding as planned and the rocket's guidance system is being tested right now.
0015 GMT (4:15 p.m. PST; 7:15 p.m. EST Thurs.)
The formal countdown has begun with power-up of the rocket to 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 11:13 p.m. local time (0713 GMT; 2:13 a.m. EST). The duration of the launch window remains a secret for this classified National Reconnaissance Office mission.

DECEMBER 5, 2013

2155 GMT (1:55 p.m. PST; 4:55 p.m. EST)
In a turnaround from yesterday's forecast, there is now a 60 percent chance temperatures will cooperate for launch of an Atlas 5 rocket tonight at 11:13 p.m. PST (0713 GMT; 2:13 a.m. EST).

The main threat continues to be cold temperatures that could violate constraints for fueling of the 206-foot-tall rocket with cryogenic propellants. That activity is set to get underway about two hours before launch.

Officials from United Launch Alliance and the U.S. government gave approval today to press ahead with final preparations for Thursday night's liftoff of an Atlas 5 rocket from Vandenberg Air Force Base, but meteorologists predict cold temperatures along California's Central Coast will likely prevent launch.

The launch readiness review today gave a green light for Thursday's countdown, which is targeting liftoff at 11:13 p.m. PST Thursday (0713 GMT; 2:13 a.m. EST Friday) from Space Launch Complex 3-East at Vandenberg.

There is an 80 percent chance cold weather will keep the launch team from even fueling the rocket, according to Air Force forecasters.

There is a temperature limit of 42 degrees Fahrenheit during the final phase of the countdown Thursday evening. Cryogenic tanking of the Atlas 5 with liquid oxygen and liquid hydrogen is due to begin at 9:20 p.m. PST (0520 GMT; 12:20 a.m. EST).

Temperatures are forecast to be about 45 degrees Fahrenheit at 9 p.m. PST, dropping to around 41 degrees by launch time.

Otherwise, clear skies are predicted for the launch with northerly winds of 10 knots gusting to 15 knots. Drying conditions will prevent the formation of fog in the area.

The forecast for Friday evening, in the event of a 24-hour delay, calls for slightly warmer conditions but thick clouds move over Vandenberg. There is an overall probability of weather violation of 60 percent Friday.

The Atlas 5 is flying in the "501" configuration with a five-meter payload fairing, no solid rocket boosters and a single-engine Centaur upper stage. The medium-length version of the five-meter fairing is being employed for this launch, making the rocket's total height approximately 206 feet.

The customer for the mission is the National Reconnaissance Office, the U.S. government's spy satellite agency. The payload's specific purpose is top secret, but it is likely a low-orbiting surveillance satellite.

Check out images of the payload's attachment to the Atlas 5 rocket.

An Atlas 5 rocket is scheduled to launch a classified satellite for the National Reconnaissance Office on Thursday, and the preliminary forecast shows cold temperatures at the California launch site will be the only weather concern.

The United Launch Alliance rocket is set to launch at 11:13 p.m. PST Thursday (0713 GMT; 2:13 a.m. EST Friday) from Space Launch Complex 3-East, or SLC-3E at Vandenberg Air Force Base on California's Central Coast.

Located between Los Angeles and San Francisco, Vandenberg is the primary launch base on the U.S. West Coast.

The launch should be visible across the Central Coast as the 19-story booster ascends into the night sky southwest from Vandenberg.

The mission is codenamed NROL-39, and the U.S. government's spy satellite agency is not releasing any details on the payload.

The flight will mark the eighth and final Atlas 5 launch of the year, and the second from California. The last liftoff from the SLC-3E complex was the Feb. 11 launch of the Landsat 8 Earth observation satellite for NASA and the U.S. Geological Survey.

"Vandenberg has successfully made it through many obstacles to get to this point of launch preparations," said Col. Keith Balts, commander of the Air Force's 30th Space Wing garrisoned at Vandenberg. "With the last Minuteman 3 test launch postponed due to the [government] shutdown, we are excited to come back from Thanksgiving break and showcase our unique mission."

"A launch like this takes teamwork and dedication," said Lt. Col. James Bodnar, commander of the 4th Space Launch Squadron, which is responsible for mission assurance and safety for Thursday's Atlas 5 launch. "Our mission assurance technicians and engineers have worked hand in hand with United Launch Alliance going over critical procedures and tasks to ensure this week's launch is a safe and successful one."

The first official weather forecast issued by the Air Force on Monday shows generally favorable conditions for Thursday's launch, but a cold air mass is expected to take hold later this week.

"Temperatures will be a concern," the forecast said. "A high of 55 F during the day will continue to drop through the night, reaching 42 F by T-0. Drying conditions will prevent development of the marine layer over land."

There is a 30 percent chance the cold temperatures will prevent launch Thursday night. Temperatures must be above 40 degrees Fahrenheit during fueling of the two-stage rocket with cryogenic propellants.

The forecast calls for clear skies, northwest winds at 10 to 15 knots, a temperature of 42 degrees Fahrenheit and a visibility of 7 miles.

Meteorologists predict slightly warmer temperatures Friday night, with no chance of a weather violation.