Launching. Just launching. That's been the mantra for the United Launch Alliance Atlas 5 rocket program, which successfully completed its fourth flight in four months on Tuesday by boosting a surveillance satellite into orbit for the Pentagon.

Read our launch story.

And here's our launch photo gallery.

"I am extremely proud of the hard work and dedication of the launch team," said Col. Jim Planeaux, director of the Infrared Space Systems Directorate at the Space and Missile Systems Center, Los Angeles Air Force Base, Calif. "Today's launch of GEO 2 culminates years of preparation by a broad team of government and industry professionals."

"The ULA team is honored to serve a pivotal role in placing this critical capability in orbit for our women and men serving around the world and protecting our freedom," said Jim Sponnick, ULA vice president for mission operations.

"From nearly two years ago when we began production of the launch vehicle, through today's successful mission delivery, this very strong and well-integrated government and industry team has ensured that mission success remains the highest priority at every step in the process."

The next Atlas rocket launch is scheduled for May 15 from Cape Canaveral to deploy a replenishment satellite into orbit the Global Positioning System. The target liftoff time is 5:39 p.m. EDT.

T+plus 43 minutes, 20 seconds. SPACECRAFT SEPARATION! The Centaur upper stage has deployed the second Space Based Infrared System Geosynchronous satellite into orbit following today's launch from Cape Canaveral.

T+plus 43 minutes, 8 seconds. Centaur has begun a one-degree-per-second roll for spacecraft deploy.

T+plus 42 minutes, 20 seconds. One minute away from releasing the payload.

T+plus 42 minutes. The upper stage is despinning from the coast phase's thermal conditioning roll.

T+plus 40 minutes. Deployment of the SBIRS GEO 2 spacecraft to complete today's launch sequence is expected at 6:04 p.m. EDT.

T+plus 37 minutes. The vehicle has crossed Africa. It's now soaring above the Indian Ocean while climbing away from the planet. Currently 900 miles in altitude.

T+plus 35 minutes. The concept of having infrared satellites orbiting Earth to detect and provide early warning of a nuclear attack on the U.S. dates back to 1960s. Today's launch of the Air Force's SBIRS GEO 2 satellite will take the next step in improving and modernizing the constellation of observatories that serve as the nation's safety net.

"That's still the primary mission. Folks think of that as a Cold War mission, but the reality is it's a dangerous world still today and these infrared-detectable missiles are even more numerous in type and capability than they have ever been," said Col. Jim Planeaux, director of the Infrared Space Systems Directorate at the Air Force's Space and Missile Systems Center.

GEO 2 is built by Lockheed Martin with scanning and staring sensors to provide wide and narrow views of the globe below the spacecraft.

"They work a little bit like night vision goggles in that infrared is very visible to anything that is much warmer than its surroundings. Things like missiles and very bright events are extremely visible in the infrared. So the sensor is tuned and set up so that it's highly sensitive to that radiation," Planeaux said.

"These sensors are like a big eyeball. They have a telescope that let the light in, focuses it on the focal plane, which is a lot like your retina of the eyeball, that is tuned to the infrared frequencies, then that shows up a very detectable signature when anything bright is happening down on the Earth. There's also mechanisms inside the sensor that allow very agile movement to look at different points on the Earth through scanning mirrors."

The satellite should be checked out and certified for missile warning alert duty by the end of this year.

"We really hope it never has to do its primary mission -- to warn the president and secretary of defense if there is an attack on the United States homeland," Planeaux said.

T+plus 34 minutes, 30 seconds. Centaur is operating well with good battery voltages.

T+plus 32 minutes. Although the Centaur has finished firing, the rocket won't immediately deploy the payload. That milestone moment will wait about 15 minutes as the rocket crosses Africa and Madagascar, eventually flying within communications range of the Diego Garcia tracking station on an island in the Indian Ocean for live telemetry coverage of the critical event.

Release of the payload from the rocket to complete the launch is expected at T+plus 43 minutes.

T+plus 30 minutes. The Centaur's orbit is right on target with an apogee of approximately 22,233 statute miles, perigee of 115 statute miles and inclination of 22.2 degrees.

T+plus 28 minutes, 8 seconds. MECO 2. Main engine cutoff confirmed. Centaur has completed its second burn of the day.

T+plus 27 minutes, 10 seconds. Engine performance still reported nominal.

T+plus 26 minutes, 15 seconds. About two minutes are left in the burn to reach the planned geosynchronous transfer orbit.

T+plus 25 minutes, 45 seconds. The engine is burning well. This is a planned four-minute firing by the Centaur's single Pratt & Whitney Rocketdyne RL10 engine.

T+plus 25 minutes, 10 seconds. Bus and battery voltages, tank pressures and other system measurements look good.

T+plus 24 minutes, 55 seconds. Centaur is accelerating at 0.82 g's.

T+plus 24 minutes, 12 seconds. Ignition and full thrust! The Centaur's single RL10 engine has re-ignited to accelerate the SBIRS GEO 2 payload into the planned deployment orbit.

T+plus 23 minutes. Centaur is getting pressurized again in preparation for the next engine burn.

T+plus 22 minutes, 30 seconds. Centaur is de-spinning out of its thermal roll.

T+plus 20 minutes. The flight path is taking the vehicle over the equatorial Atlantic Ocean, just off the western coast of Africa.

T+plus 19 minutes, 45 seconds. Centaur's onboard systems are stable in this coast period continues.

T+plus 17 minutes, 30 seconds. That first burn by Centaur inserted the rocket into the correct orbit an orbit with a high point of 658 statute miles, a low point of 109 statute miles and inclination of 26.77 degrees.

T+plus 16 minutes, 45 seconds. Centaur is performing a one-degree roll during this coast for thermal conditioning.

T+plus 16 minutes. The rocket is performing its turn to the proper position for the next engine firing.

T+plus 15 minutes, 24 seconds. MECO 1. Centaur's main engine has shut down following its first burn today, achieving a preliminary orbit around Earth. The rocket will coast in this orbit for about 9 minutes before the RL10 engine re-ignites.

T+plus 15 minutes. Orbital velocity has been achieved.

T+plus 14 minutes, 45 seconds. Centaur systems continue to be in good shape as the rocket nears orbit.

T+plus 14 minutes, 30 seconds. Everything looking normal with one minute to go in this burn.

T+plus 13 minutes, 35 seconds. Centaur is 171 miles in altitude, 2,130 miles downrange from the launch pad, traveling at 15,992 mph.

T+plus 13 minutes. Centaur remains on course and looking good.

T+plus 12 minutes, 30 seconds. RL10 engine parameters still look good.

T+plus 11 minutes, 40 seconds. The rocket traveling at 14,540 mph.

T+plus 11 minutes, 30 seconds. About four minutes are left in this burn of Centaur.

T+plus 10 minutes. All systems reported stable as the Centaur fires to reach an initial Earth orbit.

T+plus 8 minutes. The RL10 continues to perform well, burning liquid hydrogen and liquid oxygen propellants.

T+plus 7 minutes, 45 seconds. The rocket is 179 miles in altitude, some 856 miles downrange and traveling at 12,129 mph.

T+plus 6 minutes, 30 seconds. Centaur performance right on target.

T+plus 5 minutes, 45 seconds. The rocket is tracking right down the planned flight path.

T+plus 5 minutes, 28 seconds. The rocket is 123 miles in altitude and traveling over 11,262 mph already.

T+plus 5 minutes. Centaur engine readings look good as this burn gets underway.

T+plus 4 minutes, 25 seconds. Centaur has ignited! The RL10 engine is up and running at full thrust for its first of two planned firings today.

T+plus 4 minutes, 14 seconds. The Atlas 5's Common Core Booster first stage has been jettisoned, and the Centaur upper stage's liquid hydrogen and liquid oxygen systems are being readied for engine start.

T+plus 4 minutes, 5 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.

T+plus 3 minutes, 50 seconds. The vehicle is accelerating at 5 g's.

T+plus 3 minutes, 30 seconds. Atlas is 42 miles in altitude, 89 miles downrange, traveling at 6,909 mph.

T+plus 3 minutes, 25 seconds. The rocket now weighs only a quarter of what it did at liftoff.

T+plus 3 minutes. RD-180 is performing well as the rocket climbs away from the planet.

T+plus 2 minutes, 45 seconds. Reaction control system has been activated.

T+plus 2 minutes. The rocket is tracking on course.

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.

T+plus 95 seconds. Maximum dynamic pressure.

T+plus 85 seconds. Mach 1.

T+plus 60 seconds. One minute into the ascent. It will take 43 minutes for the two-stage rocket to deploy the Space Based Infrared System Geosynchronous 2 satellite into the desired orbit.

T+plus 40 seconds. Good engine performance is reported from the rocket as the booster's thunder crackles across the Cape.

T+plus 15 seconds. The vehicle has cleared the towers at Complex 41 on 860,000 pounds of thrust from the RD-180 main engine.

Main engine start, release and LIFTOFF! Liftoff of the Atlas 5 rocket and the SBIRS GEO 2 missile warning satellite, improving the nation's safety net in space. And the vehicle has cleared the tower!

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 55 seconds. Range is green.

T-minus 1 minute. Now 60 seconds away from launching the SBIRS GEO 2 satellite that will bolster and upgrade the U.S. military's infrared surveillance system that continuously scans the world looking for enemy missile launches and sounding the alarm on impending threats.

T-minus 90 seconds. The rocket's safety system has been armed.

T-minus 1 minute, 45 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 are enabled.

T-minus 4 minutes and counting. Clocks have resumed for the final minutes of today's countdown to launch the Atlas 5 rocket with SBIRS surveillance satellite.

Standing by to pick up the count.

The GEO 2 spacecraft is running on internal power for launch.

ULA and the Air Force have given their respective final approvals to resume the countdown as scheduled.

Polling of the team by Atlas launch conductor just occurred. All systems are "go" for a liftoff today at 5:21 p.m. EDT.

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.

The SBIRS GEO 2 satellite is switching to internal power for launch.

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.)

And check out Spaceflight Now's Facebook page!

The countdown remains on target for liftoff at 5:21 p.m. EDT (2121 GMT) when today's 40-minute launch opportunity opens.

The kerosene-fueled first stage will be burn for the first four minutes of flight before separating to allow the cryogenic Centaur upper stage to ignite for an 11-minute initial burn to reach a preliminary parking orbit. The nose cone is jettisoned four-and-a-half minutes after launch.

The rocket settles into a 9-minute coast across the equatorial Atlantic before restarting the RL10 upper stage engine 24 minutes into flight for a four-minute burn that accelerates SBIRS to the intended highly elliptical orbit.

The satellite is deployed 43 minutes after liftoff into an orbit with an apogee of 22,237 statute miles, perigee of 115 statute miles and inclination of 22 degrees.

Upper level winds are good for launch today, ULA says.

Twenty minutes from liftoff. Weather is GO for launch.

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

• The 619th launch for Atlas program since 1957
• The 329th Atlas to occur from Cape Canaveral
• The 208th mission for the Centaur upper stage
• The 185th use of Centaur by an Atlas rocket
• The 37th launch of an Atlas 5 since 2002
• The 58th Evolved Expendable Launch Vehicle flight
• The 12th Atlas 5 dedicated to the Defense Department
• The 31st Atlas 5 to occur from the Cape
• The 25th daytime Atlas 5 from the Cape
• The 29th Atlas 5 under United Launch Alliance
• The 25th 400-series flight of the Atlas 5
• The 17th Atlas 5 to fly in the 401 configuration
• The fourth SBIRS program launch
• The third SBIRS program launch on Atlas 5
• The second SBIRS GEO spacecraft
• The third Atlas launch of 2013

The first stage liquid oxygen tank and Centaur's liquid oxygen and liquid hydrogen tanks are topped at flight level.

All weather conditions remain GO at the current time. There is a cumulus cloud being watched to the northwest of the pad over the Shuttle Landing Facility, however it is not violating any launch rules at this time.

T-minus 4 minutes and holding. The countdown has entered the planned 25-minute hold to give the launch team a chance to review all systems before pressing ahead with liftoff.

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

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.)

And check out Spaceflight Now's Facebook page!

Today marks the 37th flight for Atlas 5, born of the Air Force's competition to develop next-generation Evolved Expendable Launch Vehicles. In its previous 36 missions since debuting in August 2002, the tally shows 11 flights dedicated to the Defense Department, 9 commercial missions with communications spacecraft, 9 for NASA and 7 with spy satellites for the National Reconnaissance Office.

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

Just 45 minutes until liftoff time of the SBIRS GEO 2 satellite that continues the next generation of geosynchronous spacecraft to continuously look for missile launches and alert the national leadership and battlefield commanders.

The SBIRS GEO 2 spacecraft, which weighs about 10,000 pounds at launch, was built by Lockheed Martin using the company's commercial A2100 satellite design. It features a pair of power-generating solar arrays, two communications antenna wings that unfold and a deployable light shade to shield its sensitive infrared instruments.

Unlike the previous generation of Defense Support Program missile warning satellites that have only scanning sensors, the SBIRS GEO spacecraft has both scanning and staring instruments to increase the amount of reconnaissance that can be collected.

The SBIRS system will augment and gradually replace the heritage Defense Support Program satellites.

Today's launch will "take another big step in delivering the next generation of Air Force infrared surveillance capability," said Col. Jim Planeaux, director of the Infrared Space Systems Directorate at the Air Force's Space and Missile Systems Center.

"While our current constellation of Defense Support Program satellites has served the nation and our allies well for over 40 years, SBIRS is bringing unprecedented new capabilities into service to support our mission areas, which are missile warning, missile defense, technical intelligence and battlespace awareness."

Weather odds just jumped up to 80 percent favorable for today's launch window as the concerns about clouds ease for meteorologists.

The outlook for the launch window now predicts scattered clouds at 2,500 and 25,000 feet, isolated showers in the area, good visibility, northwesterly winds of 12 gusting to 16 knots, a temperature of 75 degrees F and humidity level of 80 percent.

Now 60 minutes from liftoff. Fueling of the Atlas rocket with cryogenic liquid oxygen and liquid hydrogen has moved into topping mode as the countdown continues as planned for a liftoff at 5:21 p.m. EDT. Weather remains GO and no technical problems are being reported.

If you are heading out to the beach or Port Canaveral to watch the launch, sign up for our Twitter feed to get occasional countdown 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 if you are need tips on picking a good viewing spot, check out this authoritative guide on where to go.

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

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

Centaur's liquid hydrogen tank is 50 percent full. The cryogenic propellant will be consumed with liquid oxygen by the stage's Pratt & Whitney Rocketdyne-made RL10 engine.

Chilldown of the liquid hydrogen system has been accomplished. The launch team has received the "go" to begin filling the Centaur upper stage with the supercold fuel.

First stage liquid oxygen tank is 50 percent full thus far. 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 four minutes of flight today. The 25,000 gallons of RP-1 were loaded into the rocket after rollout yesterday.

The Centaur engine chilldown sequence is being initiated.

The first stage liquid oxygen tank has reached the 30 percent mark.

Centaur liquid oxygen is now at flight level.

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

The first stage liquid oxygen loading is transitioning from slow-fill to fast-fill mode.

The chilldown conditioning of liquid hydrogen propellant lines at Complex 41 is starting to prepare the plumbing for transferring the Minus-423 degree F fuel into the rocket. The Centaur holds about 12,300 gallons of the cryogenic propellant.

Centaur liquid oxygen is 50 percent loaded.

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 48,750 gallons of cryogenic oxidizer for the RD-180 main engine.

Passing the 20 percent level on the Centaur upper stage's liquid oxygen tank.

Filling of the Centaur upper stage with about 4,100 gallons of liquid oxygen has begun at Cape Canaveral's Complex 41 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 Centaur will perform two firings to propel the SBIRS GEO 2 satellite into its intended orbit today.

The Centaur liquid oxygen 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.

T-minus 120 minutes and counting! The launch countdown has resumed for today's flight of the Atlas 5 rocket on a mission to deploy the Air Force's second Space Based Infrared System Geosynchronous satellite, known as SBIRS GEO 2 for short.

Clocks have one more built-in hold planned at T-minus 4 minutes. That pause will last 25 minutes during which time the final "go" for launch will be given. All remains targeted for liftoff at 5:21 p.m. EDT from Cape Canaveral's Complex 41.

In the next couple of minutes, chilldown thermal conditioning of the mobile launch platform upon which the rocket stands will begin. This is meant to ease the shock on equipment when supercold cryogenic propellants start flowing into the rocket.

All console operators have reported GO status during the pre-fueling readiness poll. The ULA launch director also voiced his approval for moving forward with the countdown as scheduled today.

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

The ULA launch conductor at the Atlas Spaceflight Operations Center is briefing his team on procedures before entering into the final two hours of the countdown.

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.)

And check out Spaceflight Now's Facebook page!

T-minus 2 hours and holding. The countdown has just entered the first of two planned holds over the course of the morning that will lead to the 5:21 p.m. EDT launch of the Atlas 5 rocket.

This initial pause was designed to give the team some margin in the countdown timeline to deal with technical issues or any work that could fall behind schedule before fueling starts. But all is going smoothly today, with officials not reporting any troubles in the count.

The final hold will occur at T-minus 4 minutes.

The final hands-on work has wrapped up at the launch pad and technicians have departed the complex. Safety officials just confirmed that the surrounding danger area has been cleared of all workers for the remainder of the countdown.

Hold-fire checks were just performed with the Eastern Range to ensure safety personnel can hold the countdown if necessary.

The weather status board shows all launch criteria still green and GO. The forecast for the 5:21 to 6:01 p.m. EDT opportunity continues to reflect a 70 percent chance of favorable conditions, with the main concern being cumulus clouds later in the launch window.

The outlook for the launch window now predicts scattered clouds at 2,500, a broken deck at 25,000 feet, isolated showers and thunderstorms in the area, good visibility, northerly winds of 10 gusting to 14 knots, a temperature of 75 degrees F and humidity level of 80 percent.

Guidance system testing has been accomplished as the countdown goes smoothly today at Cape Canaveral.

The hazard area roadblocks around the launch site's safety perimeter have been established. Also, the launch team has configured the pad's water deluge system.

Countdown activities are progressing as planned and right on schedule for liftoff of the Atlas 5 in just over four hours. The team has completed the liquid oxygen system preparations and internal battery checks over the past little while.

Hit reload to activate live streaming video.

Now entering the final five-and-a-half hours of the countdown to the United Launch Alliance Atlas 5 rocket flight carrying the Air Force's second Space Based Infrared System Geosynchronous satellite.

Weighing about 10,000 pounds at launch, the SBIRS GEO 2 satellite will be injected by the Atlas 5 into a standard geosynchronous transfer orbit with a high point of 22,237 statute miles, low point of 115 miles and inclination of 22 degrees to the equator. Separation of the spacecraft from the rocket occurs about 43 minutes after liftoff.

From there, ground controllers will spend about 9 days performing 6 firings of the craft's main engine to circularize the orbit before deploying the appendages and commencing the testing campaign.

Built around Lockheed Martin's commercial A2100 satellite platform, with a 12-year design life, the craft is a clone of the GEO 1, having been built at the same time using the same parts. The birds are equipped with RH-32 radiation-hardened single board computers with reloadable flight software.

After deploying its pair of power-producing solar arrays for 2,800 watts, GEO 2 will have a wing span of 49 feet. The craft also extends a light shade for the payload and unfolds gimbaled communications antennas for secure, spot beam transmissions.

Its scanner and starer instruments, amounting to 1,000 pounds of hardware, have highly agile pointing mirrors and dual infrared sensors that feature short wave, mid wave and see-to-ground spectral band assemblies.

In today's first weather briefing to mission managers, all current conditions are observed GO for launch of the Atlas 5 rocket and odds for the 5:21 p.m. EDT liftoff time are 70 percent favorable.

After a day of overcast skies, the main area of cloudiness associated with the passing cold front has moved south of the Cape this morning.

The outlook for the launch window now predicts scattered clouds at 2,500 feet and a broken deck at 25,000 feet, isolated showers and thunderstorms in the area, good visibility, northerly winds of 10 gusting to 14 knots, a temperature of 75 degrees F and humidity level of 80 percent.

If the launch should slip to Wednesday for some reason, there is a 40 percent chance of acceptable wether.

The countdown begins now for today's launch of the Atlas 5 rocket to deploy the Air Force's second next-generation Space Based Infrared System Geosynchronous satellite. The surveillance system provides the U.S. and its allies with early-warning of incoming missile launches, giving warfighters the necessary alert to intercept the weapon.

The SBIRS GEO 2 satellite launching today will work to expedite the time-critical sequence by identifying faint missiles faster and allowing forces to engage sooner.

Clocks are picking up the seven-hour, 15-minute sequence of work that will prepare the booser, payload and ground systems for blastoff at 5:21 p.m. EDT (2121 GMT).

Soon the launch team will begin powering up the rocket to commence standard pre-flight tests. Over the subsequent few hours, final preps for the Centaur's liquid oxygen and liquid hydrogen systems will be performed, along with a test of the rocket's guidance system and the first stage propulsion and hydraulic preps, internal battery checks and testing of the C-band system used to track the rocket as it flies downrange, plus a test of the S-band telemetry relay system. The Complex 41 site will be cleared of all personnel at 2:11 p.m.

A planned half-hour hold begins at 2:26 p.m. when the count reaches T-minus 120 minutes. Near the end of the hold, the team will be polled at 2:53 p.m. to verify all is in readiness to start fueling the rocket for launch.

Supercold liquid oxygen begins flowing into the Centaur upper stage around 3:13 p.m., followed by the first stage filling around 3:26 p.m. 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 starting at 4:52 p.m. That 25-minute pause will give everyone 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.

The launch window extends from 5:21 to 6:01 p.m. EDT (2121-2201 GMT).

Here's our photo gallery showing the Atlas 5's trip to the pad on this morning and the ULA Tweetup participants enjoying the show.

The weather outlook continues to call for a 70 percent chance of acceptable conditions to launch the Atlas 5 rocket on Tuesday afternoon.

A cold front pushing through Central Florida during the countdown poses a slight concern for violating one of the cumulus cloud rules.

The launch time forecast calls for scattered low- and broken high-level clouds, isolated showers in the area, good visibility, southerly winds at 12 gusting to 16 knots and a temperature of 75 degrees F.

On the pad! Completing the first third-of-a-mile on the trek to orbit, the United launch Alliance Atlas 5 rocket has arrived at the Complex 41 pad for Tuesday's ascent to deploy the Air Force's SBIRS GEO 2 surveillance satellite for missile warning.

"United Launch Alliance is very proud to support the United States Air Force in the launch of this really important SBIRS GEO 2 mission," said Tony Taliancich, United Launch Alliance's customer program office director. "The ULA team is honored to serve a pivotal role in placing the critical capability into orbit for our men and women who serve around the world protecting our freedoms."

The Air Force's second Space Based Infrared System Geosynchronous satellite, known as SBIRS GEO 2 for short, will further improve the sensitivity and speed at which the nation can detect, track and stop enemy missiles launched at the U.S. and its allies around the globe.

"When GEO 2 is launched...it will join GEO 1 in delivering an unprecedented global, persistant and taskable infrared surveillance capability to our nation for decades to come," said Dave Sheridan, Lockheed Martin's SBIRS program director.

The Atlas 5 rocket will deliver the 10,000-pound, Lockheed Martin-built satellite into a highly elliptical transfer orbit, and the spacecraft will spend about 9 days maneuvering itself into a circular orbit 22,300 miles above the planet for testing and checkout.

Its operational location is classified.

"This is a very exciting time for the SBIRS program, for the entire government and industry team...We will launch our second GEO satellite into the SBIRS constellation, greatly enhancing its resilient space-based infrared surveillance capabilities for our nation and allies," Sheridan said.

The two mobile trailers connected to the launching platform, which were part of the convoy during this morning's rollout, soon will be hooked up to power and communications systems at the pad. These trailers provide conditioned air to the payload and communications with the rocket during the rollout and the countdown. They are protected from the blast of launch by a concrete structure on the north-side of the platform.

Within the next hour, the auto couplers between the pad and platform will be engaged to route umbilical connections from the ground to the rocket for tomorrow's fueling of the booster with cryogenic propellants.

Later today, the undercarriages used to move the mobile platform will be disconnected and the "trackmobiles" pulled free.

And the first stage will be loaded with its supply of highly refined kerosene fuel, called RP-1.

Ground crews will secure the rocket for a quiet evening leading into the launch opportunity tomorrow. The countdown will start with powerup of the Atlas-Centaur vehicle just after 10 a.m.

The 40-minute launch window opens at 5:21 p.m. and closes at 6:01 p.m. EDT.

We will have live updates and streaming video on this page.

If you will be away from your computer but would like to receive occasional status checks, sign up for our Twitter feed to get text message updates sent to 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 United Launch Alliance Atlas 5 rocket has emerged from its vehicle assembly building for the journey to the pad, a milestone move for the program about to make its 37th trip to space.

Riding atop a mobile launching platform, the 189-foot-tall satellite booster is moving to Cape Canaveral's Complex 41 for launch of SBIRS GEO 2.

The rocket's rollout to the pad has begun!

This slow half-hour drive from the 30-story Vertical Integration Facility to Cape Canaveral's Complex 41 pad uses a pair of specially-made "trackmobiles" to carry the rocket's 1.4-million pound mobile launching platform along rail tracks for the 1,800-foot trip.

The two-stage rocket and the SBIRS GEO 2 spacecraft were put together inside the assembly building over the past month in preparation for this rollout event. The Atlas 5 is designed to spend minimal time at the launch pad, which does not include a service gantry like other sites.

From a beach mound overlooking Cape Canaveral's Complex 41 -- the home of United Launch Alliance Atlas 5 rocket -- participants of the social media Tweetup are enjoying special access to watch rollout of the 19-story booster from its assembly building to the pad this morning.

ULA is sharing the message of what the company does to support security from space for the U.S. and its allies, boosting critical communications, navigation, reconnaissance and weather spacecraft into orbit for the Department of Defense and intelligence community.

Tomorrow's Atlas 5 launch will deploy a vital upgrade the nation's safety shield that detects enemy missile launches and alerts the military to intercept the weapon.

The firm also provides launch services for NASA's wide array of scientific spacecraft, ranging from the Mars rovers and probes to visit nearby planets, telescopes to peer deep into the cosmos and observatories to study Earth.

The future commercial endeavor to resume launching American astronauts from U.S. soil would rely on the Atlas 5 for the boost into space, if NASA selects Boeing's CST-100 capsule or Sierra Nevada's Dream Chaser spaceplane in the ongoing competition to ferry crews to and from the International Space Station.

Two dozen members of the public are attending this two-day behind-the-scenes event, which will include tours of ULA's Atlas Spaceflight Operations Center where the countdown is controlled, the Air Force's Morrell Operations Center that serves as the nerve center for the Eastern Range, receive briefings on the importance of tomorrow's launch of SBIRS and enjoy a front row seat to watch the thunderous blastoff.

This launch will mark the 37th launch of an Atlas 5 booster in the past decade, the 17th in the 401 configuration with a four-meter nose cone and no strap-on solid motors and the fourth Atlas flight in four months. It also represents United Launch Alliance's 69th mission in six years and the 29th using an Atlas 5.

The previous SBIRS GEO 1 was entrusted to fly on Atlas and GEO 2 is the rocket's 12th launch for the Defense Department. Including NRO satellite deployments, this is the vehicle's 19th in service to U.S. national security payloads.

The Atlas 5 rocket will be hauled out to the launch pad this morning, riding the rails from its assembly building to Complex 41 for the final day-and-a-half of preparations to fly the Space Based Infrared Systems Geosynchronous 2 satellite into orbit tomorrow.

First motion for the rollout is targeted for 10 a.m. EDT (1400 GMT). Watch this page for updates and photos!

Highly sophisticated infrared eyes to spot incoming enemy missiles against the United States and its allies will rocket into space Tuesday atop a United Launch Alliance Atlas 5 booster from Cape Canaveral.

Read our launch preview story.

The weather outlook for Tuesday afternoon's liftoff of the Atlas 5 rocket calls for a 70 percent chance of acceptable conditions during the 5:21 to 6:01 p.m. EDT launch window.

"On launch day, a cold front is expected in North Florida in the morning and pushing into Central Florida through the afternoon and evening. Moisture is sufficient for isolated showers during the afternoon and evening. Upper level dynamics are weak with a small threat of isolated thunderstorms as the boundary pushes into Central Florida through the count and window," Air Force meteorologists say.

The primary threat for violating the launch weather rules will be cumulus clouds.

The launch time outlook calls for scattered low-, mid- and high-level clouds, isolated showers in the area, good visibility, southerly winds at 10 gusting to 15 knots and a temperature of 74 degrees F.

United Launch Alliance will give members of the public an unprecedented glimpse behind the scenes during one of its Atlas 5 rocket flights in support of national security and a front-row seat to the blastoff next week.

ULA is inviting "avid space social media" users to apply for the two-day gathering next Monday and Tuesday at Cape Canaveral Air Force Station. The participants will receive tours of ULA and 45th Space Wing facilities, briefings on the mission to place a missile-warning satellite into orbit and watch the 19-story Atlas 5 booster launch on 860,000 pounds of thrust.

The rocket is scheduled for rollout to the Complex 41 pad from its assembly building on Monday. The launch is planned for Tuesday at 5:21 p.m. EDT (2121 GMT).

If you are interested, send an email to tweetup@ulalaunch.com with the following information:

• Full name as it appears on your driver's license
• Mailing address
• Links to publicly viewable social media feeds/profiles that you use (Twitter, Facebook, blog, etc.)
• Any special accommodations (i.e., dietary needs or mobility accommodations)
• Date of birth
• City and state of birth

But hurry the registration deadline is Wednesday night at 11:59 p.m. EDT. Winners selected from the emails will be contacted Friday.

Additional details from ULA:

You must have a social media account to register and you may only register one person (you). Any friends or family interested in participating must register separately, and the registration is non-transferable.

Participants must be 18 years or older and a U.S. citizen. Registration indicates you are willing to travel to the launch and understand you are responsible for your own travel, accommodation, food outside of programmed activities and other amenities. If the launch schedule changes, participants will be responsible for any additional costs they may incur.

ULA assumes no liability for any direct or consequential damages or costs incurred by participants as a result of this event and all decisions regarding eligibility and participation remain the exclusive right of ULA.

An Air Force surveillance satellite and its United Launch Alliance Atlas 5 booster were joined together today as the duo targets a March 19 blastoff from Cape Canaveral.

Fitting a payload atop a towering rocket is the sure sign a launch is right around the corner, and that milestone was achieved this morning at Complex 41.

Read our full story.

After successfully carrying out two high-profile satellite launches for NASA in the opening weeks of the year, the Atlas 5 rocket program has stacked its third booster of 2013 to deploy a critical missile-warning spacecraft for the Pentagon.

Read our full story.